Carbapenem resistance and hypervirulence in Klebsiella pneumoniae are raising global public health concerns in the 21^st century

Summary

Hypervirulence and antibiotic resistance in Klebsiella pneumoniae toward carbapenem antibiotics have raised global public health concerns, with increased incidence of community-acquired infections causing an unprecedented rise in mortality and morbidity in the 21^st century. Carbapenem-resistant Klebsiella pneumoniae (CRKp) and hypervirulent Klebsiella pneumoniae (hvKp), in general, harbor significant virulence factors such as siderophores, lipopolysaccharide (LPS), adhesive fimbriae, and capsules that contribute to its pathogenicity. The determinants of hvKp are usually found cardinal among larger virulence plasmids, which may also harbor carbapenem resistance genes, thereby making management of hypervirulent-carbapenem resistance strain nearly impossible. These virulence factors allow K. pneumoniae to escape phagocytosis; therefore, attempts to counter using immune cells such as neutrophils and macrophages fail. Further, immune-mediated attempts are frequently hampered by bacterial resistance to drugs, resulting in severe infections, particularly in immunocompromised patients. Although there are several management options for K. pneumoniae infections, there is still a need to find innovative approaches to control the illness in light of the increasing drug resistance pattern reported worldwide. As a result, the purpose of this review is to highlight the latest information regarding the resistance and virulence mechanisms that let bacteria evade human immune factors, as well as how to best manage them before the antibiotic regimen runs out.

Graphical abstract

Medical microbiology; Public health; Disease

Introduction

Klebsiella pneumoniae, Gram-negative opportunistic pathogens, are a serious case of concern to public health when they acquire resistance to β-lactam antibiotics, as they result in remarkably higher mortality and morbidity.¹ As a last resort, carbapenem, a beta-lactam antibiotic, is used to treat bacteria that are resistant to multi-drugs.² The growing prevalence of carbapenem-resistant Enterobacterales (CRE), particularly K. pneumoniae and Escherichia coli, has imposed a substantial burden on global healthcare systems due to limited therapeutic options and increased treatment failure. These strains exhibit several intrinsic, adaptive, and acquired ways to develop resistance to these carbapenems, including: acquiring carbapenemase genes through the mobile genetic element (MGE), decreasing cell wall permeability, or overexpressing β-lactam-specific efflux pumps, which makes managing infections caused by them challenging.^2,3

Globally, the clinical burden of CRKp has seen a steep increase in the 21^st century, particularly with bloodstream infections with more than 50% fatality outcomes in nosocomial settings.⁴ Additionally, K. pneumoniae infections were the primary cause of 29% of the recent sharp increase in mortality among patients with secondary infections during the COVID-19 pandemic.⁵ It is noted that about 50% of the patients suffering from pneumonia caused by K. pneumoniae result in mortality.⁴ Additionally, there has been a global worry as seen by the increasing incidence rates of CRKp in China, where cases have increased from 9.2% to 27.1% between 2010 and 2021, and in European countries, from 6.2% to 8.1% between 2012 and 2015.^6,7 Further, every year, CRKp causes over 90,000 infections and over 7,000 fatalities in Europe alone.^8,9

Taiwan and other Asian nations were the first to report cases of hypervirulent K. pneumoniae (hvKp) in the 1980s and 1990s, but recently, the infections caused by the hvKp variants have been spreading across various nations.^10,11,12 hvKp frequently exhibits a hyper-mucoviscosus phenotype, which arises from the development of two distinct virulence determinants, magA and rmpA. It is interesting to note that the virulence determinants of hypervirulent K. pneumoniae were found chromosomally in the early findings; however, virulence plasmids with distinct hypervirulence determinants (such as rmpA) have recently been found.¹⁰ Because hypervirulence determinants can propagate through horizontal gene transfer (HGT), which leads to increased dispersion, this is an important trait.^10,11 The vulnerability of hvKp to antibiotics is another intriguing characteristic. Acquired resistance determinants, such as beta-lactamases, are the primary cause of multidrug resistance in the majority of currently isolated hvKp strains.^11,13 Furthermore, because of their hypermucoviscosity, infections with hvKp bacteria that are resistant to carbapenem (CR-hvKp) result in a significant death rate. CR-hvKp is capable of escaping phagocytosis by host macrophages and is killed by serum. By changing from hypermucoviscosity to hypomucoviscosity, CR-hvKp can increase its persistence in the human host.¹⁴

Notably, Carbapenem resistance and hypervirulence in K. pneumoniae converge through two distinct evolutionary routes. CR-hvKp emerges when classical hypervirulent lineages (e.g., ST23, K1/K2) acquire carbapenemase or resistance plasmids, thereby gaining multidrug resistance while retaining hypermucoviscosity and invasive potential. Conversely, hv-CRKp arises from hospital-adapted, carbapenem-resistant clones (e.g., ST11, ST258, especially ST11-KL64 in East Asia) that subsequently acquire virulence plasmids harboring rmpA, iuc, and iro loci. These trajectories differ in origin, genetic mechanism, and epidemiological consequence: CR-hvKp is typically community-associated and causes severe, therapy-refractory infections, whereas hv-CRKp drives nosocomial outbreaks with combined resistance and virulence. The global dissemination of ST11-KL64 hv-CRKp and pK2044-like hybrid plasmids exemplifies how HGT accelerates this resistance-virulence convergence, posing an escalating challenge to treatment and infection control.^15,16,17

This dual convergence of resistance and virulence underscores the growing threat of CR-hvKp as a “perfect storm” pathogen capable of both extensive antibiotic resistance and enhanced pathogenicity. Hence, CRKp is one of the top-priority antimicrobial-resistant infection-causing bacteria identified by the World Health Organisation (WHO), for which novel treatment approaches are desperately needed. Additionally, the rise in CR-hvKp is extremely concerning, and until their resistance mechanisms, virulence factors, transmission patterns, and epidemiological relationships are better understood, managing infections will become practically impossible.

Epidemiology of hypervirulent K. pneumoniae

The epidemiology of K. pneumoniae has shifted dramatically from largely antibiotic-susceptible strains to a spectrum that includes multidrug-resistant (MDR) and hvKp pathotypes, culminating in the recent emergence of CR-hvKp that now represents a global public health concern. Antibiotic resistance in K. pneumoniae is generally well documented, with reports of resistance to aminoglycosides, beta-lactams, and fluoroquinolones having been made worldwide.¹⁸ The synthesis of various beta-lactamases, specifically carbapenemases and extended-spectrum beta-lactamases (ESBLs) by K. pneumoniae, is the primary cause of resistance to beta-lactam antibiotics.

Early epidemiological observations indicated that hypervirulent strains were largely antibiotic-susceptible^10,19; however, accumulating evidence now demonstrates that several hvKp sequence types (STs) have acquired antimicrobial resistance. Although the bulk of hypervirulent and multidrug-resistant K. pneumoniae strains have been found in China, their subsequent global dissemination has been increasingly documented.^20,21 Several high-risk, multidrug-resistant K. pneumoniae clones, including ST11, ST147, and ST307, have been identified to carry a hypervirulent pathotype.²⁰ This convergence of resistance and virulence represents a major epidemiological turning point, with previously antibiotic-susceptible hvKp lineages (e.g., ST23, ST1265, and ST1797) acquiring resistance plasmids that enable the emergence of multidrug-resistant hypervirulent strains.²⁰

One of the most commonly found hypervirulent and multidrug-resistant K. pneumoniae strains is ST11. A well-known high-risk clone that is resistant to antibiotics, ST11 has also been shown to demonstrate hypervirulence. Colistin-resistant hvKp ST11 strains were identified in a Chinese study. mgrB mutations (ISKpn26 or Q30Stop), phoQ mutations (D150G), pmrD mutations (D80G), and pmrB mutations (R256G and D313N) were the primary mediators of colistin resistance in these strains.²¹ Ventilator-associated pneumonia was caused by hypervirulent (CRKp) during an outbreak in China in 2016. Each of the five strains that were found carried a pLVPK-like virulence plasmid (∼170 kbp) belonging to the ST11 clone.²² Between 2016 and 2017, a total of 163 cases of pyogenic liver abscess caused by K. pneumoniae were examined in China, of which 12 strains were multidrug resistant. Among these isolates, hypermucoviscosity and virulence genes such as iroN, magA, rmpA, ybtA, and the aerobactin operon were common, with serotypes K1 and K2 accounting for nearly half of the cases. ST23 was the predominant lineage; newly emerging STs (ST3507, ST3508, and ST3509) were also detected, highlighting genetic diversification within hvKp populations.²³

In China, a hypermucoviscosus K. pneumoniae that produces NDM-5 carbapenemase was identified in a sputum sample. The rmpA virulence gene, specifying the hypervirulent phenotype, was located on a pVir-SCNJ1 plasmid, while blaNDM-5 resided on a conjugative IncX3 plasmid; the strain belonged to ST29.²⁴ Similarly, two hypervirulent K. pneumoniae that produce carbapenem-resistant IMP-4 were identified from sputum and blood samples in a Chinese intensive care unit (ICU). Both isolates of K. pneumoniae were of the K2 capsular serotype and ST65. These bacteria also exhibited virulence factors that indicate hypervirulence pathotype, specifically rmpA/rmpA2, iucA, and iroN. Tigecycline non-susceptibility was linked to a frameshift mutation in the TetR/AcrR regulator.²⁵

Further reports from across China underscore the growing heterogeneity of CR-hvKp. A carbapenem-resistant hvKp ST592 strain (K57 serotype) harboring blaKPC-2 and virulence determinants (rmpADC, rmpA2, iucABCD-iutA, and iroBCDN) was isolated from a patient with ventilator-associated pneumonia.²⁶ Another case involved a K2 ST25 strain with dual plasmids, one encoding the aerobactin operon (iucABCD-iutA) and another carrying ^blaKPC-2, that were both transferable via conjugation, suggesting simultaneous HGT of resistance and virulence.²⁷ Further, a multidrug-resistant ST464 strain from a hematological patient’s blood sample co-produced NDM-1 and KPC-2 carbapenemases and displayed resistance to multiple antibiotic classes such as fosfomycin, tigecycline, fluoroquinolones, and tetracycline.²⁸

Beyond Asia, the epidemiology of hvKp and CR-hvKp is increasingly global. A survey in Qatar identified carbapenem-resistant hvKp strains (ST231 and ST383) harboring blaNDM on IncHI1B plasmids co-carrying aerobactin (iucABCD-iutA), rmpA, and rmpA2.²⁹ Similarly, two hvKp ST420 strains identified in a German study chromosomally integrated virulence plasmids with enhanced siderophore production. In Ireland, a two-year surveillance study detected 28 carbapenem-resistant hvKp ST23 strains carrying OXA-48 and virulence markers (rmpA2, iuc, iro, and ybt).³⁰ Comparable CR-hvKp isolates have been documented in France, Italy, India, and Iran, emphasizing the global scale of this emerging hybrid pathotype (Figure 1).

Figure 1.

Geographic distribution of major antimicrobial resistance determinants in Klebsiella pneumoniae across selected countries in Europe and Asia

Two-part map (Europe, Asia) highlighting countries where key resistance mechanisms have been reported in K. pneumoniae. Countries are colour-coded and callout bubbles indicate representative determinants detected in the highlighted settings, including carbapenemases (OXA-48, NDM/NDM-1, VIM, KPC, and OXA-181) and extended-spectrum β-lactamases (ESBLs; TEM, SHV, and CTX-M), as well as plasmid-mediated colistin resistance. The figure is intended to illustrate the transregional spread and co-circulation of ESBLs and carbapenemases; labels are representative rather than exhaustive and reflect evidence summarized in the main text and references

Collectively, these observations reveal a dynamic epidemiological landscape in which hypervirulent and multidrug-resistant K. pneumoniae lineages increasingly converge. The emergence of CR-hvKp reflects the intersection of plasmid-mediated resistance and virulence, geographic expansion from East Asia to multiple continents, and ongoing genetic diversification of hvKp populations. Continuous genomic surveillance, molecular characterization of plasmid transfer, and international data sharing are critical to monitor the evolution of these high-risk clones and to guide effective infection-control strategies globally.

Carbapenem resistance mechanisms in Klebsiella pneumoniae

The β-lactam group of carbapenems, such as imipenem and ertapenem, are often used as the first line of treatment for treating non-urinary tract infection (UTI) infections caused by ESBL-producing K. pneumoniae, and their clinical overuse has led to the development of carbapenem resistance mechanisms in gram-negative bacteria such as Acinetobacter baumannii, Pseudomonas aeruginosa, and K. pneumoniae.^31,32 Over a period of time, K. pneumoniae has developed resistance mechanisms against carbapenem antibiotics via intrinsic resistance traits and those acquired through HGT.³³ The acquisition of plasmids that encode carbapenemase production is one of the primary carbapenem-resistance mechanisms of K. pneumoniae.³⁴ Some K. pneumoniae strains produce low levels of intrinsic β-lactamases, which can provide baseline resistance to carbapenems, although this is not the primary factor in clinically significant resistance.³⁵ Enzymes such as ESBLs, which hydrolyze penicillin, cephalosporins, and monobactams, are poorly effective against carbapenems as their molecular structure makes them relatively stable against the hydrolytic action of ESBLs, making carbapenem an effective treatment for ESBL infections. The overexpression and production of hydrolytic enzymes such as carbapenem-hydrolysing β-lactamases (CHDLs) and AmpC cephalosporinases is usually how K. pneumoniae acquires resistance to carbapenems.³⁶ These enzymes break down the carbapenem ring structure, rendering the antibiotic ineffective.³¹ The antibiotic activity of most β-lactam antibiotics can be suppressed by the hydrolysis of the β-lactam ring by β-lactamases. The characteristic location of side chains in the trans-position of carbapenem antibiotics makes them less susceptible to β-lactamases, in contrast to other β-lactam antibiotics that otherwise possess side chains in the cis-position. The following sections provide a brief outline of the carbapenem resistance mechanisms facilitated by efflux systems, downregulation of outer membrane porins, production of Ambler carbapenemases, and genetic transfer of carbapenem resistance genes.

Efflux pumps and outer membrane modifications

K. pneumoniae possesses a set of non-carbapenemase-producing mechanisms that facilitate carbapenem resistance. Multi-drug efflux pumps in the inner and outer membranes actively expel various antibiotics, including carbapenems, from the bacterial cell. The mechanism of efflux pumps is promoted by plasmids and has a baseline of three proteins produced by oxidative phosphorylation and respiratory enzymes. These pumps reduce intracellular β-lactam antibiotic concentrations, rendering the drug ineffective. Resistance-nodulation-division (RND) efflux pumps are found in CRKp. The tripartite complex AcrAB-TolC efflux pump belonging to RND is composed of antibiotic complexes AcrA/AcrB, which are periplasmic and inner membrane transporter proteins, respectively. The TolC component encodes the outer membrane transporter protein. The AcrB complex traps and effluxes antibiotics through the TolC channel after changing its conformation with the help of AcrA.³⁷ The upregulation of the AraC/XylS transcriptional activator RamA facilitates the expression of AcrAB and toxin contractile (TolC), translating AcrAB-TolC pump proteins and conferring multidrug resistance. The mutation of efflux pump regulators such as RamA could impact antibiotic resistance.³⁸ The overexpression of RamA and upregulation of AcrB/oqxB have been correlated with increased carbapenem resistance.³⁹ The overexpression of these efflux pumps decisively impacts carbapenem resistance.

The outer membrane of K. pneumoniae acts as a barrier to many β-lactam antibiotics, including carbapenems. The reduced permeability of the membrane, due to the absence or alteration of porin channels, limits the entry of carbapenems into the bacterial cell. Specific porins, such as outer membrane proteins OmpK35/OmpK36, are often downregulated or mutated in carbapenem-resistant strains, resulting in reduced uptake of carbapenems and enhanced carbapenem resistance. Downregulation of OmpK35 and OmpK36 diminishes the antibiotic resistance and desensitizes ESBL and AmpC cephalosporinases, respectively. Gene deletions, insertions, and mutations of the OmpK35/OmpK36 genes could suppress outer membrane function and decrease their susceptibility to antibiotics.^40,41 Downregulation of OmpA does not affect carbapenem resistance, in contrast to Omp37, which can potentially contribute to carbapenem resistance. The deficiency of Omps in CRKp is a secondary resistance mechanism that contributes to trivial carbapenem resistance in virulent strains.^42,43,44 The carbapenem resistance of non-carbapenemase-producing carbapenem-resistant K. pneumoniae (NC-CRKp) is mostly fueled by the acquisition of non-ESBL genes, efflux pump mutations, porin deficiency, and silent and missense mutations of OmpK35 and OmpK36, respectively.^45,46

Production of β-lactamases in carbapenem resistance

The clinical overuse of carbapenems has led to an increase in the production of β-lactamase enzymes, which are categorized into four Ambler classes (A–D). The production of these β-lactamases is chromosomally encoded, and classification is based on amino acid sequences. These enzymes function as carbapenemases by breaking down several β-lactam antibiotics, including carbapenems and cephalosporins, facilitating the development of resistance mechanisms. There are several types of carbapenemases mediated by plasmids that are responsible for these resistances. K. pneumoniae carbapenemase (KPC) enzymes were first isolated in the United States and are the most widespread and clinically relevant carbapenemases. They belong to the serine-based class-A β-lactamase group, which also contains ESBLs and can hydrolyze a broad range of β-lactams, including carbapenems. There are 54 KPC variants, among which KPC 1–13 are enzymes encoded by mobile elements that break down carbapenem antibiotics, rendering them ineffective and contributing to carbapenem resistance in K. pneumoniae and other gram-negative bacteria. KPC-1 was the original variant, while KPC-2 and KPC-3 are the most widespread, with KPC-3 being particularly clinically significant due to its higher resistance profile.⁴⁷ Variants like KPC-4 and KPC-5 also contribute to carbapenem resistance, while KPC-6, KPC-7, KPC-8, and KPC-9 are emerging forms with mutations that enhance resistance. KPCs are mostly resistant to multiple carbapenemase inhibitors. These variants play a critical role in hospital-associated outbreaks, complicating treatment options and leading to more severe and harder-to-treat infections in vulnerable patients.⁴⁸

New Delhi metallo-β-lactamase (NDM-1)-type enzymes were discovered in 2008 and belong to metallo-β-lactamases (MBLs), which are class-B β-lactamases that can hydrolyze nearly all β-lactams, including carbapenems, with the exception of the monobactam aztreonam. CRPK strains producing NDM have been reported in many regions, including Egypt, Europe, India, Pakistan, the United Arab Emirates, and Serbia, contributing to the global spread of carbapenem resistance.⁴⁹ Oxacillinase (OXA)-type carbapenemases are enzymes belonging to the serine-based class-D β-lactamases along with CHDLs that are capable of hydrolyzing carbapenems, although their activity is often less potent than that of KPC or NDM enzymes. OXA-type strains are known to produce class-D carbapenemases, particularly OXA-48, which was first isolated in Turkey and is frequently found in K. pneumoniae strains along with OXA-181 in regions where carbapenem resistance is prevalent. The carbapenemase activity of OXA-type carbapenemases is negligible except in instances of co-production with other carbapenemases.⁵⁰

Verona integron-encoded (VIM) and imipenemase (IMP) are class-B β-lactamases or MBLs that confer resistance to carbapenems and require zinc ions as active centers that activate the β-lactam ring.^51,52 The MBLs VIM and IMP are found in a range of K. pneumoniae isolates, though they are less common than KPC and NDM. These MBLs are classified into subgroups (B1/B2/B3) and are observed in integron structures, which form a network with MGEs such as transposons and plasmids, which help in transferring resistance genes through HGT between different K. pneumoniae strains.⁴⁴ Class-C β-lactamases, such as cephalosporinases and AmpC β-lactamases, are not considered carbapenemases but have low potential to exhibit carbapenem hydrolysis.³⁵ Multiple carbapenemases such as OXA-48/KPC-2, NDM-1/KPC-2, and VIM-1/KPC-2 can be produced simultaneously by CRKp strains to deliver a more pronounced resistance to carbapenems.^50,53,54 The distribution of carbapenemases among CR-hvKp isolates can vary with geography. The MBLs (NDM-1 and VIM-1) and OXA carbapenemases (OXA-48 and OXA-181) are found in Asia and Europe. The bla_KPC, bla_NDM, bla_OXA, bla_IMP, and bla_VIM plasmids are thoroughly disseminated in China, the United Kingdom, India, Japan, and Iran, respectively.^34,55 The co-production of multiple carbapenemases in CR-hvKp isolates also varies with region. The NDM-1/OXA-48 co-producing strains have been observed in Italy, whereas the KPC-2/OXA-48 strains are reported in Egypt.^56,57 The class-A carbapenemase Guiana extended-spectrum-β-lactamase (GES) is observed to decrease the hydrolytic properties of carbapenems.⁵⁸

Genetic mechanisms driving carbapenem resistance

K. pneumoniae acquires antibiotic resistance mechanisms through the HGT of resistance genes or genetic mutations. The transfer of MGEs, such as plasmids and transposons conferring carbapenem resistance among carbapenemases, is done through transduction, transposition, plasmid fusion, and HGT.⁵⁹ These mechanisms introduce carbapenem resistance to hvKp strains by carrying over carbapenem resistance genes.^56,60 The acquisition of bla_KPC by hvKp can increase the emergence of carbapenem-resistant hvKp (CR-hvKp).^42,55 Different strains possess an arsenal of carbapenemases encoded by their corresponding plasmids to help facilitate carbapenem resistance. The carbapenem resistance in K. pneumoniae strain ST11 is mediated by KPC-2, NDM-1, and OXA-48 carbapenemases encoded by bla_KPC-2, bla_NDM-1, and bla_OXA-8 plasmids, respectively.⁶¹ Most carbapenemases are encoded on MGEs such as plasmids, which can transfer resistance genes between different bacterial species. The presence of plasmids allows K. pneumoniae to acquire and spread resistance to carbapenems rapidly, making these bacteria highly adaptable to treatment pressures. K. pneumoniae may acquire mutations in target sites for carbapenem antibiotics. The binding of these target sites produces inhibitory effects, and the mutation of this mechanism can facilitate resistance. This could involve changes to penicillin-binding proteins (PBPs), which are the targets for β-lactam antibiotics. Acquired resistance can also occur through the upregulation of efflux pumps in K. pneumoniae. Efflux pumps contribute to carbapenem resistance by actively expelling carbapenems from the bacterial cell. Mutations in regulatory genes that control the efflux system cause the overexpression of these pumps (Figure 2).⁶²

Figure 2.

An illustration outlining the primary carbapenem resistance mechanisms in Klebsiella pneumoniae

(1) Downregulation and alteration of porin proteins decrease outer membrane permeability by altering the selectivity and size of porin channels, thereby limiting carbapenem uptake. (2) Overexpression of multidrug resistance efflux pumps (RND) actively effluxes carbapenem antibiotics from the cell. (3) Plasmid-mediated production of hydrolytic carbapenemases (β-lactamases) facilitates the breakdown of carbapenem antibiotics. (4) Enzymatic inactivation of carbapenem antibiotics through hydrolysis facilitated by carbapenemases (β-lactamases) reduces their antimicrobial activity. (5) Hydrolysis of carbapenems by AmpCs and ESBLs produced via bacterial plasmids transferred through horizontal gene transfer (HGT) mechanisms such as conjugation, transformation, or transduction diminishes their efficacy. (6) The binding of carbapenems to PBPs and their subsequent inactivation inhibits peptidoglycan synthesis, disrupting cell wall synthesis

Pathogenic impact of key K. pneumoniae virulence determinants and the host immune-regulatory circuitry

Virulence factors in CRKp

K. pneumoniae possesses a wide array of virulence factors encoded chromosomally and on horizontally acquired virulence plasmids, supporting its ability to cause both opportunistic and hypervirulent infections. A detailed schematic of these mechanisms is presented in Figure 3, representing major genetic loci and their associated functions contributing to bacterial pathogenesis. Its virulence is attributed to a combination of structural and biochemical factors that enhance its ability to colonize, evade host defenses, and cause damage. Virulence factors are the main tactics used by K. pneumoniae to proliferate and defend against the host immune system in various infection types.⁶³ Plasmids and integrative conjugal elements are genetic components that sustain the high virulence traits in hvKp strains. K. pneumoniae has four significant and potent virulence factors, such as siderophores, lipopolysaccharide (LPS), adhesive fimbriae (including type 1 and type 3 fimbriae), and capsules (Figure 3).¹⁷

Figure 3.

Schematic representation of major chromosomal and plasmid-borne virulence factors in Klebsiella pneumoniae

The diagram illustrates the principal genetic loci and their associated functions contributing to pathogenicity, including (i) capsule biosynthesis (CPS) genes (e.g., rmpADC and rmpA2) conferring protection against phagocytosis and complement-mediated killing; (ii) lipopolysaccharide (LPS) biosynthesis genes (rfb) contributing to immune evasion; (iii) fimbrial adhesins, such as type 1 fimbriae (fim operon) and type 3 fimbriae (mrk operon), enabling adhesion and biofilm formation; (iv) siderophore-mediated iron-acquisition systems—enterobactin (ent, fepA), yersiniabactin (ybt, ybtQ), salmochelin (iroN), and aerobactin (iutA)—allowing growth in iron-limited host environments; and (v) genotoxins such as colibactin (clb) involved in DNA damage. Virulence determinants are located both on the chromosome and on horizontally acquired virulence plasmids, supporting the ability of K. pneumoniae to cause opportunistic and hypervirulent infections. CPS, capsule polysaccharide; LPS, lipopolysaccharide; Fe³⁺, ferric iron; hvKp, hypervirulent K. pneumoniae

Capsule (K-antigen)

The polysaccharide capsule is the most critical virulence factor (Figure 3). It provides protection against phagocytosis and complement-mediated killing while also aiding in biofilm formation and adherence to host tissues. The cps gene cluster encodes the Wzx/Wzy-dependent polymerization pathway to produce the acidic polysaccharide capsule in K. pneumoniae as the outermost layer of a bacterial cell to interact with the host. Capsule antigens that mimic host glycans help K. pneumoniae to evade the host’s immune system.⁴⁴ Reduced mouse death rates and an inability to disseminate systemically are two results indicating that acapsular K. pneumoniae strains are substantially less virulent than their isogenic encapsulated counterparts in mice studies.⁶⁴ The chromosomal operon capsular polysaccharide (CPS) synthesis locus contains the genes wzi, wza, wzb, wzc, wzy, and wzx, which encode conserved machinery for the Wzy-dependent process (Figure 3).⁶⁵ Nonetheless, a wide variety of extremely different genes that are unevenly distributed across the K. pneumoniae population encode the sugar synthesis machinery at cps loci (KL). The created “Kaptive” tool found 134 KLs (serotypes) using the KL information from many genetic data.⁶⁶ The most common capsular serotypes linked to invasive infections are K1 and K2. Furthermore, K1 and K2 strains are often more virulent than those of other serotypes.⁶³ Thus, the K1 and K2 (capsular serotypes) are indicative of hvKp generally, and the serotype functions as a possible virulence marker.⁶⁷

A useful criterion for classifying and serotyping the pathogenic strain of K. pneumoniae is the K-antigen found in the capsule (Figure 3). The 5′ end of the cps gene cluster contains six genes as galF, orf2 (cpsACP), wzi, wza, wzb, and wzc. Sequencing of these six genes has revealed that they are highly conserved, whereas the mid zone of the cps loci contains a variable region of nucleotide sequences producing proteins that take part in capsule block assembly and polymerization. The K-typing approach is regarded as an efficient categorizing tool. According to the K-antigen capsule, more than 80 serotypes of pathogenic strains of K. pneumoniae have been identified to date.^44,68,69

The overproduction of the capsule is thought to be the main cause of the hypermucoviscosity (HMV) phenotype in K. pneumoniae (Figure 3). Notably, the reduction of CPS and HMV is dissociable yet coordinated.⁷⁰ The chromosome contains the gene clusters (wzy-K1, wzx, wzc, wza, wzb, wzi, gnd, wca, cpsA, cpsB, cpsG, and galF) that encode exopolysaccharide, whereas both chromosome and plasmid contain the rmpA, rmpB, and rmpA2 genes involved in capsule biosynthesis (Figure 3). Additionally, chromosomes also contain genes (kvrA, kvrB, rcsA, rcsB, c-rmpA, and c-rmpA2) involved in capsule biosynthesis. Plasmid-borne p-rmpA and p-rmpA2 genes are also involved in capsule biosynthesis.⁶⁴

Further, the cps gene of K. pneumoniae ST258 can be acquired for gain or loss of function to increase its pathogenicity.⁷¹ During bladder colonization by a bacterium, mutants without capsules are closely associated with UTIs, display relative benefits. They accomplish colonization by penetrating bladder epithelial cells and creating biofilms on catheters and bladder epithelium. Hypercapsule mutants, which are closely associated with bloodstream infections, carry wzc mutations that result in increased dissemination and resistance to phagocytosis. However, they cannot infiltrate bladder epithelial cells or create a biofilm.⁷¹

Lipopolysaccharide

The LPS plays a key role in resisting immune responses by protecting against complement activation and mediating inflammatory damage (Figure 3). LPS, consisting of lipid A, a core oligosaccharide, and an O antigen, is an essential component that decorates the outer membrane of K. pneumoniae and is encoded by lpx, waa, and rfb gene clusters, respectively.^72,73 LPS can function as a strong immune stimulator in addition to being an essential virulence component (Figure 3). The main defense against complement is LPS, and the O antigen component is essential to this strategy. By binding C3b, the O antigen protects bacteria against C3 by inhibiting pore formation and keeping them away from the bacterial membrane.⁷⁴ Moreover, LPS is recognized as a significant toll-like receptor 4 (TLR4) inducer biomolecule, which may stimulate the production and release of various cytokines and interleukins.^{75,76,77,78,79}

Nine distinct O-antigen serotypes and more than five subtypes have been found in K. pneumoniae, in contrast to K-antigen; serotypes O1 and O2 continue to be the most prevalent among clinical K. pneumoniae isolates.^72,80 Notably, the O1 antigen is the most prevalent O-antigen type found in hvKp strains and is typically linked to the K1 and K2 capsule types.¹³

Unlike isolates with smooth LPS (full-length O antigen), isolates with rough LPS (truncated or lacking O antigen) are susceptible to complement. K. pneumoniae without the O antigen could not spread systemically in animal pneumonic models. Lipid A of LPS is a strong mediator of the inflammatory cascade and septic shock because it is a strong ligand of TLR4.⁶⁴ Remarkably, strains of K. pneumoniae that express the K1, K10, and K16 antigens are able to use capsules to partially protect LPS from TLR recognition, while strains that express the K2 antigen are unable to do so.⁶⁴ Moreover, changes in lipid A may contribute to the development of bacterial resistance to antimicrobial peptides, such as polymyxin antibiotics.⁸¹ Different O-serotypes are produced by mutations in the rfb gene clusters. Among K. pneumoniae isolates in clinical settings, O1 and O2 are the most common serotypes among the nine LPS O serotypes.^72,82 O1 serotypes are differentiated from O2 serotypes by the presence of two wbb genes, wbbY and wbbZ, which are not within the rfb locus.⁸³ Notably, the O1 antigen is the most prevalent O serotype among hvKp strains, typically being linked to the K1 and K2 serotypes.¹⁷ Compared to non-tissue-invasive strains, the O1 serotype was more common in strains that caused pyogenic liver abscesses. Furthermore, as shown by mice models of septicemia and liver abscess, mutation of the O1 antigen decreased serum resistance and significantly diminished pathogenicity.⁸⁴ These suggested that O1 might be a possible indicator of K. pneumoniae pathogenicity. In ST11 CR-hvKp, the majority of strains were KL64:O2 and KL47:OL101, suggesting a link between K types and O types.^85,86 Nevertheless, it is unclear to what extent O2 and OL101 influence virulence and pathogenicity.

Siderophores/iron-acquisition systems

Iron-acquisition systems, including siderophores such as enterobactin, aerobactin, yersiniabactin, and salmochelin, allow the pathogen to thrive in iron-limited environments like the human body (Figure 3). Iron plays a critical role in the virulence and pathogenesis of pathogenic bacteria. The pathogenicity of microbial agents is impacted by the efficient connections between immune cells and iron metabolism. For harmful bacteria, such as K. pneumoniae, to survive within their host after a successful infection, iron molecules are known to be a competitive resource. Therefore, in the presence of immune cells, the pathogenic bacteria’s acquisition and recruitment of host iron metals is an efficient means of surviving and establishing infection within the host.^44,64

Pathogens that contain siderophores or iron carriers have a strong affinity for iron. It is interesting to note that human host iron-chelating proteins are effectively competed with by bacterial iron-binding proteins. Stealth iron carriers are present in certain bacterial infections, including K. pneumoniae. Thus far, several siderophores/iron scavengers with varying degrees of affinity for iron molecules have been identified in Gram-negative microbial pathogens. However, four iron transporters (enterobactin, aerobactin, yersinobactin, and salmochelin) can be frequently used by K. pneumoniae. The most prevalent siderophore released by around 90% of isolated Enterobacterales members is enterobactin, a highly conserved iron scavenger. The entABCDEF gene cluster on the chromosome encodes enterobactin, which is carried by fepABCDG and has the highest affinity for iron molecules among the previously identified iron carriers.^44,76,87,88

Enterobactin, yersiniabactin, aerobactin, and salmochelin are the four forms of siderophores secreted by K. pneumoniae. Enterobactin has a higher affinity for iron than do aerobactin and yersiniabactin.⁶³ The contribution of each siderophore to virulence varies, despite considerable functional overlap between them. Interestingly, in mouse models, three supplementary siderophores exhibit increased pathogenicity/virulence.^89,90 It has been shown by quantitative siderophore assays that hvKp strains generate more siderophores than cKp strains. Furthermore, a strong biomarker predictive of an hvKp isolate has been found to have a siderophore concentration greater than 30 μg/mL.^91,92,93 It is unlikely that enterobactin contributes to invasive infection because it is found in high levels in K. pneumoniae populations, and it is rendered inactive by the host protein lipocalin-2, which is produced by neutrophils and epithelial cells.¹³ More than 90% of all siderophore action is attributed to aerobactin, and in systemic and pulmonary infection models, only aerobactin considerably improves survival.⁸⁹ Salmochelin, aerobactin, and yersiniabactin are more common in hvKp strains than in classical K. pneumoniae (cKp) strains.^94,95 Approximately 40% of K. pneumoniae genomes have the ybt locus, which is strongly linked to infection isolates, especially those from invasive infections.⁹⁴ Although it is sometimes found on plasmids, ybt is usually found on a chromosomal ICE called ICEKp, integrated with the aspartate-tRNA gene.⁹⁴ There are 329 different types of yersiniabactin sequences (YbSTs) in all, and they have been grouped into 17 evolutionary lineages (Ybt 1–17).⁴⁴

Fimbriae and adhesins

These structures enable the bacterium to adhere to epithelial cells, initiating infection and facilitating biofilm formation (Figure 3), especially in devices like catheters. Fimbriae contribute to the pathogenesis of the bacteria by attaching to biological and non-biological surfaces to initiate the colonization, biofilm formation, and bacterial invasion processes.^96,97 Biofilms make bacteria more resistant toward the effects of numerous antibiotics and less vulnerable to innate host defense mechanisms. Type 1 (encoded by the fimBEAICDFGH operon) and type 3 (mrkABCDF/mrkABCDEF) fimbriae are two significant types of fimbriae that K. pneumoniae possesses (Figure 3). According to a study, type 1 fimbriae do not encourage K. pneumoniae to produce biofilms.⁹⁸ As a result, type 3 fimbriae are essential for the creation of biofilms, but type 1 fimbriae’s contribution to K. pneumoniae biofilm production remains debatable. However, K. pneumoniae’s production of fimbriae also boosts its binding to phagocytes, causing phagocytosis. This is expected to increase the pathogen’s clearance and compromise its pathogenicity.⁶³

The subunits FimA (body structure) and FimH (tip structure), which are responsible for adhesive qualities, are encoded by the fimA and fimH genes, respectively.⁶³ It is believed that fimK, a distinct gene in K. pneumoniae that is a member of the type 1 fimbriae gene cluster, regulates fimbrial expression.⁹⁹ Several Enterobacteriaceae members include type 1 fimbriae, which are responsible for attachment to mannose-containing structures on host cells and the extracellular matrix.⁹⁹ Furthermore, type 1 fimbriae are expressed in the urinary system but not in the lungs or gastrointestinal tract.⁹⁹

The mrkABCD gene cluster encodes type 3 fimbriae which are made up of an MrkD subunit at the tip and a MrkA subunit at the main structure.⁹⁷ Type 3 fimbriae have been demonstrated to bind extracellular matrix proteins, including collagens; however, they do not bind mannose like type 1 fimbriae do.^97,100 The mrk genes are typically found on conjugative plasmids and/or transposons in E. coli strains, and as a result, they are not commonly observed in this bacterium. In contrast, the genes encoding for type 3 fimbriae production in K. pneumoniae may be plasmid-borne or carried in the chromosome.^17,101

Host-pathogen interactions

An infection manifests as a result of specialized interactions occurring between the immune cells of the host and virulence factors of the pathogen in the host microenvironment. Infections caused by K. pneumoniae need to bypass mechanical barriers and overcome humoral and cell-mediated innate immunity. Mucociliary clearance is a mechanical defense strategy employed by the host for the trapping of pathogens in the mucus of the respiratory tract, shuttling them over from the distal to proximal alveolar airways. The acidic pH of urine in the genitourinary tract and digestive enzymes in the gastrointestinal tract, along with peristalsis and the mucus lining of the gut epithelium, limits adhesion and prevents colonization of pathogens in the bladder and other distant body sites.¹⁰²

K. pneumoniae eventually confronts the innate immune host defenses in the form of the chief complement system, which mediates angiogenesis, cytokine release, and immune cell proliferation while delivering opsonic and bactericidal action. The complement system can be activated through the classical, alternative, and mannose-binding lectin pathways. The activation of the classical pathway starts off with the recognition of the pathogen by C1q on apoptotic cells, which induces the formation of C3 convertase (C4b2b). The cleavage of C4b2b into C3a and C3b fragments is common to all pathways and occurs when C4b2b is activated by the binding of mannose-binding ficolins and lectins to carbohydrates. Deposition of C3b on bacterial surfaces binds factor-B, which cascades into the alternative C3 convertase (C3bBb) pathway, cleaving more C3 into C3b and subsequently amplifying the complement response. The association of C4b2b and C3b can produce C5 convertase, which dissociates C5 into C5a and C5b fragments. C5a plays a role in assembling membrane attack complexes (MACs) that facilitate cell lysis by creating pores in the bacterial membrane. Phagocytosis is induced through opsonization by C1q, C3b, and its degradation products, and complement factors (C3a and C5a) help in the mobilization of macrophages, monocytes, and neutrophils to the sites of infection by acting as chemoattractants. Inflammatory responses and T cell differentiation are also mediated by complement interactions, which are activated through pathogen recognition receptors (PRRs).^103,104

The following sections will focus on the role of cytokine signaling in mediating inflammatory reactions, the vast array of soluble immune effector cells involved in cellular clearance and immune cell mobilization, and a variety of countermechanisms employed by K. pneumoniae to evade innate immune host defenses.

Host immune signaling and pathogen recognition mechanisms

Cytokines released by immune cells are signaling protein molecules that facilitate cell communication, coordination, and host defense. Penetration of the first layer of host mechanical defenses elicits alarm signals from PRRs modulated by epithelial and immune cells. These signals stimulate the production of pro-inflammatory cytokines such as IL-1β, IL-6, IL-8, IL-17, and TNF-α that enhance phagocyte mobilization and innate immune response.¹⁰⁵ hvKp has been shown to produce outer membrane vesicles (OMVs), which activate nuclear factor κB (NF-κB) and facilitate the production of IL-8.¹⁰⁶ This response can be diminished by the release of anti-inflammatory cytokines such as IL-4 and IL-10. The PRRs that modulate the recognition of K. pneumoniae comprise nucleotide-binding and oligomerization domain-like receptors (NLRs), retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs), and TLRs. The TLR4/NLRP3 signaling pathway can be triggered by virulence factors like CPS and LPS for the production of pro-inflammatory cytokines. TLR2 signaling elicits an anti-inflammatory response during early infection to potentially balance out any overwhelming inflammatory responses made by other PRRs.¹⁰⁷

The production of pro-inflammatory chemokines and cytokines due to convergent activation of TLRs initiates host defense signaling pathways, MyD88 and toll/IL-1R (TIR) domain-containing adaptor protein (TIRAP), leading to the activation of NF-κB, mitogen-activated protein kinases (MAPKs), and transcriptional interferon-regulatory factors (IRF3/IRF7) for the production of cytokines and chemokines. TLRs also activate the TIR domain-containing adaptor-inducing IFN-β (TRIF) and TRIF-related adaptor molecule (TRAM) for the activation of NF-κB and type-I IFN. MyD88 is the central adapter for all TLRs, with TLR3 and endosomal TLR4 as exceptions, for which TRIF functions in a dual role by acting as an adapter for both TLR3 and TLR4 signaling. TLR9 detects DNA oligonucleotides with unmethylated CpG base pairs in the endosome and elicits Th1 cytokine responses. TLR (7/8) detects ssRNA, whereas TLR 3 recognizes dsRNA. The TLR5 pathway is solely activated in the case of flagellated Enterobacteriaceae like E. coli and Salmonella typhi through the pathogen-associated molecular pattern (PAMP) flagellin and has been elucidated for proper understanding of all the pathways which coincide with K. pneumoniae (Figure 4). The immunity conferred by the MyD88 pathway is mediated by hematopoietic and resident cells with the exception of endothelial cells, whereas TRIF immunity is solely mediated by hematopoietic cells. The TIRAP/MAL adaptor links MyD88 with activated TLR2 and TLR4 receptors, and MyD88-MAL also signals pathways modulated by IL-1β and IFNγ. The MyD88 signaling pathway modulates Th1 cytokine responses. The expression of these pro-inflammatory signaling cascades is regulated by the binding of different endogenous innate danger signals known as damage-associated molecular patterns and PAMPs facilitated by the multi-ligand receptor for advanced glycation end products (RAGE). IL-1 receptor-associated kinase-M (IRAK-M) inhibits TLR signaling delivered by epithelial cells and macrophages in the lungs, and its absence is marked by reduced bacterial spread.^108,109

Figure 4.

An illustration outlining all the TLR signaling pathways associated with the production of pro-inflammatory cytokines and type-1 IFNs

TLR5 is the only exception whose activation is not related to K. pneumoniae and only applies in the case of flagellated Enterobacteriaceae like Escherichia coli and Salmonella typhi through the PAMP flagellin. The activation of TLR3 and endosomal TLR4 is independent of MyD88, whereas the TIRAP/MAL adapter is involved in the activation of TLR 1/2 and TLR2/6 pathways. MyD88 and TRIF in TLR4 and endosomal TLR3 signaling play accessory roles by facilitating glycolysis/reactive oxygen species (ROS) production and necroptosis, respectively. TLR4 signaling involves both MyD88-dependant and independent pathways for the phosphorylation of IKK into IKKα, IKKβ, and IKKγ, and MAPKs into ERK, Jnk, and p38. This leads to the transcription of pro-inflammatory cytokines and type-1 IFN genes

Immune effector cell mobilization and cellular clearance

An efficient host response toward K. pneumoniae-infected cells always includes the recruitment of antigen-presenting or dendritic cells (DCs), macrophages, monocytes, and natural killer (NK) cells. An insufficiency of these immune cells drives poor neutrophil mobilization and successful Klebsiella infections. Neutrophil mobilization at the site of infection mediated by T helper type-17 cell signaling plays a crucial role in phagocytosis and cellular clearance of K. pneumoniae-infected cells. Neutrophils have the ability to form neutrophil extracellular traps (NETs) that deliver bactericidal effects and are an important host defense mechanism against carbapenemase-resistant hvKp (CR-hvKp).^107,110 Antimicrobial proteins (AMPs) such as cathelicidins, collectins, defensins, and lysozymes form the arsenal of chemical defenses. Collections comprise four lung surfactant proteins (SPA-D), which function as immunomodulators by mediating phagocytosis and inflammatory responses while also facilitating neutrophil recruitment and opsonization by binding surface carbohydrates in pathogens. A study conducted by¹¹¹ showed that diminished neutrophil count in mice lowered the production of CXCL2, IL-1β, IL-6, IL-17, IFN-γ, and TNF-α, which facilitates cellular clearance and phagocytosis. The expression of IL-33 and the chemokines CXCL5 and CXCL10 are known to drive neutrophil and macrophage mobilization.^112,113

The epithelial cells present in the respiratory tract produce a class of antimicrobial peptides known as human β-defensins (HBD1-3), which induce cps gene expression, shielding the bacterial cell from antimicrobial polypeptides. The CPS functions by inhibiting DC maturation, allowing K. pneumoniae to bypass innate host defenses. DCs produce pro-Th1 cytokines IL-12 and TNF-α, and IL-17/IL-23, the subsequent reduction of which leads to cell death of immature DCs. This consequently impairs T cell activation and impairs the movement of DC-mediated NK cells.¹⁰⁹ Biochemical markers seen in human pneumonia, such as chemokines (CCL3/CXCL1/CXCL5/CXCL8/CXCL15) and TNF receptor-1 (TNFR1), are produced by macrophages and are pivotal in mediating pro-inflammatory signaling by enhancing the bactericidal ability of neutrophils.¹⁰³ IL-17 and IL-22 are diagnostic markers that regulate the antimicrobial activity of the lung epithelium, produce CXCL5 and lipocalin-2, and help in the cellular clearance of K. pneumoniae.^108,114 The release of IFN-γ and IL-4 by CD4 tissue-resident memory T cells (CD4 T_RM) has been substantiated for aiding in the control of CR-hvKp infections.¹¹⁵

The nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing (NLRP1/NLRP3/NLRC4) NLRs form a multiprotein structure known as an inflammasome complex, which induces the activation of caspase-1 and releases IL-1β. This protease modulates the cytosolic processing of pro-IL-1β and pro-IL-18 into their mature forms, which coordinate immune responses. NLRP6 and NLRP12 are negative regulators of MAPK and NF-κB signaling pathways, which lower intestinal inflammation. Activated caspase-1 triggers an autonomous cellular defense mechanism known as pyroptosis, a form of apoptosis that is regulated by the apoptosis-associated speck-like protein (ASC)/NLRP3 pathway, and stimulates alveolar macrophages that engulf bacteria and eradicate them through efferocytosis. Caspases 4/5/11 recognize LPS and produce pro-inflammatory cytokines IL-1β and IL-18 by cleaving gasdermin-D (GSDMD). K. pneumoniae evades host defenses by inducing pathogenic transition and programmed cell death through necroptosis and lowering efferocytic uptake of infected neutrophils by activating the RIPK1/RIPK3/MLKL cascade.^116,117

Immune evasion mechanisms

The evolution of immune evasion mechanisms based on the recognition and eradication of the mediators of humoral and cell-mediated immunity by K. pneumoniae involves a complex interplay between various pathogenic virulence factors. Bacterial membrane components such as CPS and a thick capsule help in the evasion of innate immune mechanisms through resisting phagocytic uptake, preventing the adhesion of epithelial cells and phagocytes, inhibiting MAC-induced lysis, downregulating antimicrobial β-defensins in the lungs, and evading complement deposition and opsonization.^{68,118,119,120,121} The pathogen also subverts host defenses by interacting with soluble effectors of the immune system. The creation and function of MACs assembled by the activation of the classical complement pathway can easily be impeded by the cell wall components, such as CPS, LPS, and Omps. The outer membrane of K. pneumoniae contains OmpK36 and OmpK35 porins that confer resistance to antimicrobial peptides while also activating the classical complement system by binding with Cq1. Activation of the complement system in this manner inhibits the pathway, thus facilitating serum resistance. C3b deposition on the bacterial surface is limited by its neutralization by freely floating vesicles containing LPS^112,122 K. pneumoniae has also been shown to survive in mammalian macrophages inside an acidic vacuolar intracellular compartment known as the Klebsiella containing vacuole (KCV). They display programmed cell death within these macrophages and survive by controlling phagosome maturation by blocking the fusion between KCV and lysozyme through tampering with the PI3K-AKT-Rab14 axis.¹⁰²

High levels of the anti-inflammatory cytokine IL-10 often accompany Klebsiella-triggered pneumonia. Induction of this anti-inflammatory response is mediated by the activation of myeloid-derived suppressor cells. Induction of IL-10 reduces cellular clearance by inhibiting efferocytosis and pyroptosis. The host recognizes this maneuver and employs IFNγ to abrogate Klebsiella-induced IL-10-dependent evasive strategy. Another evasive mechanism to thwart innate immune responses involves the bacterial CPS inhibiting the activation of NF-κB by lowering bacterial internalization in epithelial cells, disrupting the TLR4/2-MyD88 signaling pathway, and the production of defensins.^103,123

K. pneumoniae competes with the host for the acquisition of iron needed for bacterial replication and the formation of iron-loaded proteins. The sequestration of iron by the host and the secretion of siderophores such as enterobactin by K. pneumoniae are done to deprive the competitors from harnessing iron and induce inflammation. The host responds by releasing lipocalin-2 to prevent the pathogen from binding with these siderophores. To counteract this response, K. pneumoniae secretes additional siderophores such as aerobactin, salmochelin, and yersiniabactin, which help in the evasion of lipocalin-2 for easier iron acquisition.^4,102 K. pneumoniae is known to evade these innate immune mechanisms through the modification of lipid-A present in the host microenvironment, altering the outer membrane and promoting antimicrobial peptide resistance. K. pneumoniae evades the complement system by preventing C3b deposition through the usage of surface polysaccharides. The elongation of the LPS O-polysaccharide and CPS also prevents the deposition of C3b on the bacterial surface and facilitates evasion from the complement system. K. pneumoniae also releases freely floating CPS that binds with cationic antimicrobial peptides, neutralizing any possible bactericidal effect.^104,107

Management of carbapenem resistance K. pneumoniae

Clinical subtypes: hvKp, CRKp, CR-hvKp, and multidrug-resistant (MDRKp)¹²⁴ require tailored approaches in management. The hvKp variant typically causes invasive community-acquired syndromes such as liver abscesses and metastatic infections and remains susceptible to third-generation cephalosporins, fluoroquinolones, or carbapenems. CRKp and MDRKP often demand newer β-lactam/β-lactamase inhibitor combinations (ceftazidime-avibactam, meropenem-vaborbactam, and imipenem-relebactam), polymyxins, tigecycline, or cefiderocol, guided by susceptibility testing. CR-hvKp combines hypervirulence with multidrug resistance, necessitating combination therapy and stringent infection control.^16,125,126 Subsections 6.1–6.10 (cellulitis-UTIs) detail infection-specific strategies, including cellulitis, osteomyelitis, ventilator-associated pneumonia, and sepsis integrating antimicrobial selection with supportive care (Figure 5).

Figure 5.

Sites of infection caused by multidrug-resistant Klebsiella pneumoniae and associated treatment options

The schematic outlines major clinical infection sites in the human body, including CNS, respiratory tract, endocardium, skin and soft tissues, intra-abdominal cavity, urinary tract, bloodstream, and bones. For each infection type, recommended antimicrobial agents, such as colistin, tigecycline, ceftazidime-avibactam, meropenem-vaborbactam, polymyxins, and other last-resort antibiotics are indicated based on current treatment guidelines

Cellulitis

Cellulitis is a skin infection characterized by redness, swelling, and pain and can lead to the development of abscesses, wound infections, and widespread non-suppurative infections affecting the dermis and subcutaneous tissue. Wound infections, in particular, are among the most common hospital-acquired infections, contributing significantly to patient morbidity and accounting for 70%–80% of such cases.¹²⁷ Treatment involves antibiotics, wound care, and managing any underlying conditions. Use antibiotics based on susceptibility, such as ceftazidime-avibactam, meropenem-vaborbactam, eravacycline, amikacin, polymyxins (colistin or polymyxin B), or tigecycline¹²⁸ (Figure 5). In hvKp-associated cellulitis, standard β-lactams or fluoroquinolones may suffice, while CRKp or CR-hvKp isolates demand agents such as ceftazidime-avibactam or polymyxins to prevent dissemination.¹²⁹

Osteomyelitis

Osteomyelitis is a bone infection leading to pain, swelling, fever and is diagnosed via imaging (CT, radiography) and bone biopsy, independent of long bone classification¹³⁰ CPE-PJI (carbapenemase producing Enterobacteriaceae prosthetic joint infection) required microbiological confirmation with at least two positive periprosthetic samples or joint fluid cultures.¹³¹ Treatment involves prolonged antibiotic therapy, and sometimes surgical drainage or debridement. Options may include ceftazidime-avibactam,¹³² meropenem-vaborbactam,¹³³ polymyxins (colistin), or tigecycline¹³⁴ based on susceptibility. Prolonged intravenous (IV) antibiotics, often for 6–8 weeks, are typically required (Figure 5). Surgical debridement to remove infected tissue or bone may be necessary, if required especially in cases with abscesses or extensive necrosis. Bone stabilization may require if the infection causes bone instability or fractures. Further, hvKp osteomyelitis often responds to susceptible cephalosporins, but CRKp and CR-hvKp may require prolonged combination regimens incorporating β-lactamase inhibitors or tigecycline; surgical debridement may remain essential^60,135

Ventilator-associated pneumonia

VAP is a lung infection in mechanically ventilated patients, and is the most serious ICU-acquired infection of the lung parenchyma that develops after 48 h of endotracheal intubation and mechanical ventilation.¹³⁶ Treatment involves broad-spectrum antibiotics, ventilator care, and supportive measures, tailored based on culture results. Management includes the use of broad-spectrum antibiotics, such as ceftazidime-avibactam plus aztreonam,¹³⁷ meropenem-vaborbactam, meropenem-ciproflaxin,¹³⁸ fosfomycin, polymyxins (colistin), or tigecycline. Optimizing ventilator settings, reducing sedation, performing regular oral care with chlorhexidine, and ensuring early extubation are necessary (Figure 5). In the case of hvKp strains in VAP typically retain some β-lactam susceptibility, whereas CRKp and MDRKp infections may necessitate dual therapy, commonly ceftazidime-avibactam with aztreonam or a polymyxin-based regimen.¹³⁹

Peritonitis

Peritonitis is an infection of the peritoneum, often caused by bacteria due to a perforation or spontaneous infection. The first case of nosocomial spontaneous bacterial peritonitis (SBP) caused by CRKp was reported in Italy in 2012. Since then, numerous global reports have highlighted its emergence. The combination of SBP, which has a mortality rate exceeding 80% when associated with septic shock, and CRKp infections, with over 50% mortality in bloodstream infections (CRKp-BSIs), presents a serious threat to patient survival.¹⁴⁰ Treatment includes broad-spectrum antibiotics, surgical intervention if needed, and supportive care. Initiating broad-spectrum antibiotics like ceftazidime-avibactam, meropenem-vaborbactam, cefiderocol, tetracyclines or polymyxins (e.g., colistin) is important based on susceptibility testing, and one should avoid aminoglycoside because of the concern of the toxicity, if secondary peritonitis is present, perform surgery to address the source of infection (e.g., bowel perforation)¹⁴¹ (Figure 5). In hvKp-related peritonitis generally exhibits better antibiotic response than CRKp or CR-hvKp, which often require combination therapy and early surgical management due to increased virulence and mortality.^142,143

Cholangitis

Cholangitis is a bile duct infection often caused by bacterial contamination due to biliary obstruction, presenting with fever, jaundice, and abdominal pain. Acute cholangitis remains a life-threatening biliary disease caused by bile duct obstruction from stones or tumors, often leading to multiple organ dysfunction.¹⁴⁴ Treatment involves antibiotics, biliary decompression (e.g., Endoscopic Retrograde Cholangiopancreatography [ERCP]), and supportive care. Start with broad-spectrum antibiotics like cefmetazole,¹⁴⁵ ceftazidime-avibactam, meropenem-vaborbactam, plazomycin, polymyxins (colistin) (Figure 5). Perform ERCP for stone removal or stent placement to relieve the obstruction, or consider percutaneous biliary drainage if ERCP is unavailable.¹⁴⁶ In case of hvKp, cholangitis can often be managed with third-generation cephalosporins or fluoroquinolones, while CRKp or CR-hvKp infections may demand broader agents like ceftazidime-avibactam or meropenem-vaborbactam in combination with biliary decompression^13,147

Liver abscess

A liver abscess is a localized collection of pus in the liver, often caused by bacterial, parasitic, or fungal infections. Next-generation sequencing (NGS), sputum culture, drug sensitivity testing, and other diagnostic methods can identify the specific pathogens in advance.¹⁴⁸ Treatment involves antibiotics, drainage (percutaneous or surgical), and addressing the underlying cause. Management of carbapenem-resistant Klebsiella (CR-Klebsiella) liver abscess starts with the broad-spectrum antibiotics such as ceftazidime-avibactam,¹⁴⁹ meropenem-vaborbactam, cefiderocol, polymyxins (colistin), or tigecycline¹⁵⁰ (Figure 5). Percutaneous drainage is preferred for abscesses that are large, symptomatic, or causing complications. Surgical drainage may be necessary for large, multiloculated, or complicated abscesses. Provide fluid resuscitation, pain management, and close monitoring for sepsis. hvKp is the predominant cause of community-acquired liver abscesses and usually remains susceptible to cephalosporins; however, CR-hvKp variants require intensive combination therapy (e.g., ceftazidime-avibactam ± aztreonam) and vigilant drainage due to higher metastatic potential.^151,152

Sepsis

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection, leading to widespread inflammation and potential organ failure. Management involves early identification, aggressive fluid resuscitation, broad-spectrum antibiotics, and supportive care. Start with broad-spectrum antibiotics, such as ceftazidime-avibactam, meropenem-vaborbactam, and polymyxins (colistin and polymyxin B) (Figure 5). Drainage of the abscess via percutaneous methods is often required for large or symptomatic abscesses.¹⁵³ hvKp frequently leads to septicemia with metastatic abscesses and responds to susceptible β-lactams, while CRKp and CR-hvKp sepsis mandate early, aggressive combination therapy and infection control measures^154,155

Meningitis

Meningitis is an inflammation of the protective membranes covering the brain and spinal cord, usually caused by infection. It can be viral, bacterial, or fungal and requires prompt medical treatment to prevent complications. Management of carbapenem-resistant Klebsiella meningitis typically involves a combination of effective antibiotics based on susceptibility testing, such as colistin, tigecycline, or ceftazidime-avibactam (Figure 5). In some cases, combination therapy may be necessary to enhance efficacy. If there are abscesses or increased intracranial pressure, surgical drainage or other interventions may be required. Prompt identification and initiation of targeted therapy are essential due to the high morbidity and mortality associated with CRKp.¹⁵⁶ hvKp meningitis may respond to third-generation cephalosporins if sensitive, but CR-hvKp strains may require agents with CNS penetration such as colistin, tigecycline, or ceftazidime-avibactam,¹⁵⁷ often in combination.¹⁵⁸

Infective endocarditis

Infective endocarditis is an infection of the heart valves or endocardium, usually caused by bacteria, leading to inflammation and potential valve damage. Symptoms often include fever, heart murmurs, and signs of septic emboli, requiring urgent antimicrobial treatment and sometimes surgery. Management of carbapenem-resistant Klebsiella infective endocarditis involves the use of appropriate antibiotics based on susceptibility testing, typically including agents like eravacycline, plazomycin, colistin, tigecycline, or ceftazidime-avibactam (Figure 5). Combination therapy is often recommended for enhanced effectiveness. Valve replacement or debridement may be necessary in cases of large vegetations, heart failure, or persistent infection despite appropriate antibiotics.¹⁵⁹ hvKp endocarditis is rare but often invasive; CRKp or CR-hvKp infections are associated with poor outcomes and require combination regimens, sometimes including eravacycline or cefiderocol, along with valve surgery when indicated.^158,160,161

Urinary tract infections

UTIs are bacterial infections affecting any part of the urinary system, commonly the bladder or urethra. Symptoms include painful urination, frequent urge to urinate, and cloudy urine, with treatment typically involving antibiotics. Management of carbapenem-resistant Klebsiella UTIs involves the Selection of effective antibiotics based on susceptibility testing. Options may include colistin, imipenem-relebactam, amikacin, cefiderocol tigecycline, fosfomycin, or ceftazidime-avibactam, with combination therapy¹⁶² (Figure 5). The hvKp isolates in UTIs generally remain susceptible to oral agents, whereas CRKp, CR-hvKp, and MDRKp necessitate parenteral treatment with agents such as ceftazidime-avibactam, fosfomycin, or cefiderocol, tailored to susceptibility testing^163,164

Emerging convergence of hypervirulence and carbapenem resistance in Klebsiella pneumoniae: Global surveillance insights and diagnostic and clinical challenges

At the beginning of 2024, the Global Antimicrobial Resistance and Surveillance System on Emerging Antimicrobial Resistance Reporting (GLASS-EAR) initiated a global assessment of hvKp ST23 isolates harboring carbapenemase genes, reflecting their rising international prevalence and potential for carbapenem resistance. Over a number of years, this lineage has been reported to be continuously transmitted, and in recent years, hvKp strains from different nations have been found to carry the genes linked to antibiotic resistance.^165,166

The hypervirulent phenotype of hvKp is caused by genetic components found on a large virulence plasmid and may be integrative conjugal elements. Increased synthesis of aerobactin and capsules is the known virulence factors specific to hvKp.¹³ New hvKp strains arise when extensively drug-resistant cKp strains acquire hvKp-specific virulence determinants, leading to nosocomial infection. Like cKp, hvKp strains are also growing more resistant to antimicrobials through the acquisition of mobile elements carrying resistance determinants. Although clinical laboratories currently cannot distinguish between cKp and hvKp, a number of biomarkers and quantitative siderophore production have recently been demonstrated to accurately predict hvKp strains. This could result in the development of a diagnostic test that clinical laboratories can use for epidemiologic surveillance and research studies, as well as for the best possible patient care.

Recently, the largest known cohort of K. pneumoniae ST11-K64 strains that co-produce NDM-1 and KPC-2 carbapenemases was found at a Wuhan tertiary hospital. More than half of the 109 isolates exhibited carbapenem resistance, and 45 of them were hvKp.¹⁶⁷ The results demonstrate how multidrug resistance and hypervirulence coexist, highlighting the critical need for improved infection control, stewardship of antibiotics, and continuous monitoring to stop the spread of these dangerous clones in medical settings.

Hypervirulence and carbapenem resistance can converge when virulence and resistance plasmids coexist in the same cell or when hybrid plasmids that possess both characteristics emerge. Furthermore, the transmission of plasmids carrying mobile genes, plasmid fusion, and recombination among bacteria, are the main causes of the high mutation and transmission rates of hvKp.¹⁶⁸ The prevalence of hvKp and the molecular evolutionary mechanisms of resistance and virulence-bearing plasmids can be better understood to track and manage these superbugs.

hvKp is disseminating worldwide, distinct from the cKp usually associated with healthcare environments.^{11,169,170,171} Unlike cKp, hvKp induces severe, multifocal infections in healthy persons within the community. The precise identification of hvKp is essential for appropriate clinical therapy and epidemiological surveillance. In one of the studies, researchers assessed the candidate biomarkers (Phenotypic and Genotypic) for their diagnostic efficacy in hvKp-rich (n = 85) and cKp-rich (n = 90) cohorts using isolates from North America and the UK.⁹¹ Five genes, peg-344, iroB, iucA, prmpA, and prmpA2, were shown to have more than 95% accuracy in distinguishing hvKp strains. Experimental validation in a mouse sepsis model validated their correlation with severe diseases.

An hvKp isolate that produced carbapenemase was found in a patient in the United States who was attending an outpatient clinic.¹⁷² In addition to having a carbapenemase-encoding plasmid harboring blaKPC-2, the strain was identified as K. pneumoniae serotype K1, ST23 and was discovered to possess hypervirulence-associated genes (rmpA, rmpA2, iroBCDN, peg-344, and iucABCD-iutA).¹⁷² The coexistence of resistance and virulence factors draws attention to the alarming convergence of pathogenicity and antibiotic resistance. The development of hvKp highlights the urgent need for increased clinical vigilance and continuous surveillance to track the pathotype’s effect and dissemination.

The most reliable genomic marker for identifying hvKp is the presence of five virulence-associated genes: iucA, iroB, peg-344, rmpA, and rmpA2¹⁷³. The clinical and genetic characteristics of hvKp were revealed by researchers in a long-term cohort study carried out in high-risk areas. They showed, using propensity score matching, that hvKp strains are linked to disease severity and higher mortality but are less likely to develop antibiotic resistance. The most common clinical manifestation of these strains was pneumonia, and they were primarily isolated from hospital environments. Among hvKp isolates, capsular serotype KL1 and ST23 were most commonly found. Infections acquired in the community and those linked to medical care have both been identified as separate risk factors.¹⁷³ The mechanisms by which the bacteria increase their ability to cause the disease are still not fully understood. It will take further investigation to provide diagnostic tools that can quickly identify diseases caused by hvKp strains and are feasible in nations with limited laboratory resources. New therapeutic options must be found in order to treat infections brought on by hypervirulent variations as well as multi-resistant diseases.

Further, with the increase in the drug resistance of K. pneumoniae, there is an urgent need for vaccines to manage infections caused by them. However, to date, there are no FDA-approved vaccines to manage Klebsiella infections. To overcome these circumstances, pharmaceutical giants are in a rush to develop vaccines against these bugs to reduce mortality rates among vulnerable populations such as immunocompromised individuals, neonates and young vulnerable populations. Vaccines that are under clinical trial are known to target surface sugars, common O-antigen subtypes, surface antigens and whole cells of K. pneumoniae. Table 1 highlights the currently known vaccine candidates that target K. pneumoniae.

Table 1.

Current vaccine candidates against hvKp

Vaccine candidate	Developer	Presentation	Phase	Population	Dose/route of administration	Outcome
Under clinical trials
Kleb4V/GSK4429016A	LimmaTech Biologicals AG/GSK	Adjuvant (a recombinant bioconjugates of O1, O2a, O2afg, and O3b with/without AS03)	Phase 1/2	2 subsets N = 166 18–40 years 50–70 years	IM, taking two dosages, two months apart.	Under investigation174

N, number; IM, Intramuscular.

Therefore, clinical recognition of CR-hvKp remains difficult owing to the lack of standardized diagnostic assays in most laboratories. Preliminary identification often depends on characteristic phenotypes such as hypermucoviscosity (positive string test), unusually severe community-acquired infections in healthy individuals, and rapid disease progression, followed by molecular confirmation using hvKp-associated biomarkers (peg-344, iroB, iucA, rmpA, and rmpA2). Therapeutic management poses additional challenges due to the convergence of hypervirulence and extensive drug resistance, frequently necessitating the use of novel β-lactam/β-lactamase inhibitor combinations (e.g., ceftazidime-avibactam and meropenem-vaborbactam) or last-resort agents such as cefiderocol and polymyxins. Rigorous antimicrobial stewardship, enhanced infection control, and rapid molecular surveillance are critical to contain these high-risk clones and reduce their clinical burden.

Conclusion

Although research and development have raised alarms on hvKp, there is a scarcity of evidence to inform hvKp infection prevention and control methods beyond routine precautions. The reservoirs and mechanisms of dissemination for hvKp strains have not been identified. Furthermore, it is uncertain which of these reservoirs, if any, is the primary source of transmission. Regardless of the method, it is obvious that hvKp strains can spread nosocomially. Currently, the relative roles of environmental and person-to-person acquisition remain unclear. As a result, both low- and high-income countries should strengthen their screening and diagnostic methods to detect CR-hvKp. Multiple virulence factors and phenotypes have been widely studied and have been used as key markers to identify CR-hvKp.

Until a clear preventive method is understood, counties should proceed with caution when using existing techniques to identify hvKp, such as biomarker detection, Kleborate virulence scores, or other accessible approaches. Further, by increasing the surveillance strategies, epidemic outbreaks of CR-hvKp could be stopped in future. This review, therefore, provides an overview of the present condition of CR-hvKp, prompting the scientific community to take early efforts to reduce CR-hvKp infections before it is too late to treat.

Acknowledgments

We acknowledge Duy Tan University, Danang, Vietnam for supporting this study.

Declaration of interests

The authors declare no competing interests.