Proteus spp.—taxonomy, clinical significance, and antibiotic resistance of a highly versatile pathogen

SUMMARY

Proteus spp. are members of the Enterobacterales order and are widely found in humans, animals, and the environment. Some species, particularly P. mirabilis, are highly pathogenic and are among the most frequent causes of urinary tract and bloodstream infections. Proteus spp. possess a variety of virulence factors, such as swarming and urease activity, leading to persistent and severe infections. An increasing resistance to antibiotics has been reported for isolates and poses a substantial threat to global health. In recent years, several new Proteus spp. have been described, but their potential as pathogens has not yet been determined. Here, we provide an update on the taxonomy, virulence factors, and antibiotic resistance of Proteus spp.

INTRODUCTION

Proteus spp. belong to the family Morganellaceae in the order of Enterobacterales (1, 2). They are gram-negative rods with a diameter of 0.4–0.8 µm. The cells can present as swimmer or swarmer cells, with swimmer cells having a length of 1–4 µm and swarmer cells up to 80 µm (3, 4). Proteus spp. are highly relevant pathogens that cause urinary tract infections, bloodstream infections, and other infections in humans and animals (5–11). Additionally, they are present in the gastrointestinal tract of vertebrates (12–14) and invertebrates (13, 15, 16) and in a wide diversity of habitats, such as wastewater or soil (17–19). In recent years, antimicrobial resistance (AMR) has been increasingly reported in Proteus spp. (20–22), which is a threat to global health (23, 24). Members of the Proteus genus have, however, also been found to be promising candidates for bioremediation (25–27).

This review aims to give an overview of the history of the discovery, identification, and taxonomy of Proteus spp., their clinical relevance, as well as unique characteristics such as swarming and biofilm formation. Virulence factors, AMR acquisition, and potential for bioremediation will be elucidated to put this extraordinary genus into perspective.

The changeful history of Proteus taxonomy

The first two Proteus spp. were described in 1885 by the German bacteriologist Gustav Hauser as Proteus mirabilis and Proteus vulgaris (see Fig. 1). Both were able to liquefy gelatin and exhibit swarming behavior (28). Hauser differentiated the two species by the rate of gelatin liquefaction: P. vulgaris was a more rapid liquefier compared to P. mirabilis. The name Proteus was inspired by the prophetic sea god in Greek mythology, “Proteus,” who was able to change his shape and thus was, similar to members of the Proteus spp., versatile in appearance. Hauser described Proteus zenkeri at the same time but rescinded this finding in 1892 (29).

Fig 1.

Timeline of Proteus phylogeny. Proteus appendicitidis, as the newest addition to the Proteus genus, has not yet been validly published.

In 1966, Cosenza and Podgwaite (30) obtained new isolates from gypsy moth larvae, which were named as the new species Proteus myxofaciens. This species, however, was found to represent a new genus, Cosenzaea, in 2011 by Giammanco et al. (31), thus naming it Cosenzaea myxofaciens comb. nov. In 2021, Behrendt et al. (32) reevaluated the taxonomy and nomenclature and proposed to preferably use the name Proteus myxofaciens. To date, both species names are validly published and hence constitute synonyms (2).

The species originally named P. vulgaris, representing the type species of the genus Proteus, was found to be heterogeneous and consists of three biogroups based on their ability to metabolize indole, salicine, and esculin, as well as their chloramphenicol sensitivity (33, 34). While members of biogroup 1 cannot metabolize any of these compounds and are chloramphenicol resistant, members of biogroup 2 are able to metabolize all three and are chloramphenicol sensitive. Members of biogroup 3 are able to metabolize indole, but not salicine and esculin. While a subgroup corresponding to the later-defined biogroup 1 was first defined by Brenner in 1978 (33, 34), Hickman in 1982 defined the other two biogroups and proposed naming biogroup 1 Proteus penneri, in honor of John Penner, a Canadian microbiologist (33, 34) (with the type strain CDC 1808-73 = ATCC 33519). In 1995, Brenner et al. determined P. vulgaris biogroup 3 to consist of four genomospecies (genomospecies 3–6) using DNA relatedness studies (35). Notably, genomospecies 3 consisted of only two strains: one strain at that time serving as the neotype culture of the type strain, originating in the Pribram Collection in Vienna (ATCC 13315 = NCTC 4175) (35), and another strain (CDC 1732-80 = ATCC 700826). Since both strains did not represent the genomospecies that was most frequently isolated from human urine and did not adequately represent most of the P. vulgaris strains described in the literature at that point in time, Brenner et al. (35) proposed replacing the original type strain of P. vulgaris with the type strain CDC PR1 = ATCC 29905 of biogroup 2. This proposal was granted in 1999 (36). Genomospecies 3, the group containing the original P. vulgaris type strain ATCC 13315, was proposed to be reclassified as Proteus hauseri in honor of Hauser by O’Hara et al. in 2000 (37), but with CDC 1732-80 = ATCC 700826 as the type strain. At the time, genomospecies 4, 5, and 6 remained unassigned to a Proteus species. Since 2015, several species have been added to the genus: Proteus terrae and Proteus cibarius in 2015 and 2016, respectively (18, 38). These two species have later been found to be heterotypic synonyms of genomospecies 5 by Dai et al. (39), who used multilocus sequence analysis and confirmed by Behrendt et al. (32) using genome-based phylogeny. These species, including genomospecies 5, were all incorporated into P. terrae, and two subspecies instead of two species were created, P. terrae subsp. terrae and P. terrae subsp. cibarius (32, 39). In parallel, Dai et al. (39) first characterized the species Proteus alimentorum, Proteus columbae, Proteus faecis, and Proteus cibi in 2018 and 2019 (40–42). P. appendicitidis, as the newest addition to the genus, was described by He et al. in 2024 (43) (see Fig. 2) but has not been validly published in the List of Prokaryotic names with Standing in Nomenclature so far.

Fig 2.

Phylogenetic tree of all Proteus type strains described to date, based on whole genome sequences, analysis performed by Tygs web services (44), and imaged using iTOL (45).

In addition to the taxonomy of Proteus at species level, the diverse order “Enterobacterales” was reclassified in 2016 by Adelou et al. (46). They divided it into seven distinct families, placing the genus Proteus into the Morganellaceae family, together with Morganella spp., Providencia spp., and others.

Significance of Proteus spp. and their habitats

The field of Proteus research has grown significantly in the last few years. In just 10 years, five new species within the genus have been described. Hence, the significance of a single species has yet to be determined. As of 2024, P. mirabilis is the most intensively investigated species of the Proteus genus and appears to be the most clinically prevalent species, judging from the amount of accessible genome sequences at NCBI (47). Around 93% of genomes openly accessible have been assigned to P. mirabilis, followed by approximately 2% unidentified Proteus sp., 2% P. terrae, and 1% P. vulgaris. This signifies the research focus, which has been primarily on P. mirabilis as a human pathogen.

This notion is enforced by the origin of isolates accessible via NCBI BioSamples: the majority (approximately 95%) is of human origin, with 2% originating from swine and chicken, respectively. The majority of specimens have been obtained in the clinical context, e.g., urine, blood, sputum, or fecal swabs, underlining the clinical relevance of Proteus spp.

It can be assumed, however, that not all of the reported strains have been characterized correctly, since the differentiation of closely related Proteus spp. is challenging. For many isolates, species-level identification was performed when not all species we know today had already been described. Moreover, these findings might be biased toward P. mirabilis as the most clinically relevant species.

CLINICAL RELEVANCE OF PROTEUS SPP.

Proteus is considered a constituent (<0.05%) of the healthy human gut microbiome (48), but it can also cause serious infections in humans due to the presence of urease as a characteristic of the genus and other virulence factors. The majority of Proteus infections are caused by P. mirabilis (49, 50), but infections by other Proteus spp., such as P. vulgaris, P. hauseri, P. penneri (51, 52), as well as P. alimentorum (53), have been reported. P. mirabilis is a common cause of both community-acquired urinary tract infections (UTIs) (2% of these UTIs were caused by P. mirabilis) (54), hospital-acquired UTIs (9.4%) (55), and complicated UTIs (2.4%–35%), secondary to catheterization (56, 57). P. mirabilis has been recognized as causing cystitis (58), pyelonephritis (59, 60), prostatitis (61, 62), wound infections (51), and burn infections (63). Secondary to UTI and catheter-associated UTIs (CAUTIs), it is a common cause for bacteremia (64–66). P. mirabilis has also been implicated in the emergence of rheumatoid arthritis (67, 68). P. vulgaris and P. penneri have, similarly to P. mirabilis, been isolated from UTIs, wound and burn infections, as well as bloodstream infections and respiratory tract infections (69–77).

Proteus in UTIs

Proteus spp. have been reported as a cause for UTI from as early as 1892, where they were reported to be highly pathogenic to rabbits and causing “intense nephritis with abscesses in the kidneys” (78). P. mirabilis is the species most commonly reported from UTI, and relatively little data are available on other Proteus spp. UTIs are frequent in the human population, with approximately 50%–60% of women suffering a UTI in their lifetime (79). In both uncomplicated and complicated UTIs, P. mirabilis is found as a causative agent in 2%–4% of cases (54, 80–82), representing the third most frequent gram-negative species causing UTI (after Escherichia coli and Klebsiella pneumoniae). P. mirabilis is, however, more prevalent in CAUTIs (5, 11, 56, 58, 83). CAUTIs are UTIs resulting from the often long-term use of a catheter, as frequently seen in nursing homes and hospitals (84). UTIs are the most common nosocomial infection, with approximately 80% of these being CAUTIs (85, 86). In CAUTIs, P. mirabilis has been shown to be present in 26.4% of the cases of mono-bacterial infections and in 35% of poly-microbial infections, making it one of the most frequent species causing CAUTIs (56).

The route of infection in CAUTIs is often explained by the presence of fecal contaminants or skin residents in the periurethral area (87–91). Bacteria can enter the urinary tract either directly via catheter insertion, through the catheter lumen, or along the catheter-urethral surface (92). It is notable that P. mirabilis has been shown to have the highest capability of adhering to catheters compared to a group of several other gram-negative bacteria (93). Within the bladder, the use of a catheter often leads to the retention of urine (10–100 mL), resulting in a reservoir for bacteria to prosper (94). P. mirabilis is an avid former of biofilm and can adhere to the surface of catheters, as well as swarm (95–99). These capabilities make Proteus spp. one of the biggest threats in CAUTIs, as they can both swarm through catheters, invading the urinary tract, and also use the presence of the catheter to form protective biofilm (100, 101). Once in the bladder, P. mirabilis has been shown to be effective in invading host cells (102). While the invasion success is highly dependent on the particular strain, as well as the environmental conditions, P. mirabilis can display invasion rates higher than other invading pathogens, such as Salmonella enterica Serovar Typhimurium, E. coli, and Klebsiella pneumoniae (102–104). In contrast to E. coli, P. mirabilis does not establish larger intracellular populations in the host cells but rather appears to use the host cells as a place for initial replication and survival, before re-entering the bladder lumen and forming multicellular aggregates (105–108). These aggregates have been demonstrated to protect the incorporated bacteria from invasion by the host’s neutrophils and thus prove an effective immune system evasion technique (106). Aggregation and biofilm formation have also been linked with phenotypic resistance to antibiotics and hindered treatment, despite a genetic susceptibility to antibiotics, once again underlining the clinical significance of Proteus spp. infections (109–113).

During infection in the bladder, P. mirabilis causes inflammation and submucosal edema, leading to cystitis, which is, in general, more severe in CAUTIs compared to UTIs (114, 115). P. mirabilis UTIs are strongly correlated with the formation of bladder and kidney stones (106, 116–118) since P. mirabilis produces higher amounts of urease compared to other species commonly found in UTIs such as E. coli, Klebsiella oxytoca, Staphylococcus saprophyticus, and Enterococcus faecalis (119). A comparison of several P. mirabilis strains with representative strains of Providencia stuartii, Providencia rettgeri, and Morganella morganii revealed an appreciably higher urease activity of P. mirabilis strains despite substantially lower substrate affinities (120). The urease activity is, in some cases, even increased in polymicrobial infection (114). Urease catalyzes the hydrolysis of urea into carbon dioxide and ammonia, increasing the pH in the environment (121). The pH increase can lead to the precipitation of calcium phosphate and magnesium-ammonium phosphate and the subsequent formation of carbonate apatite and struvite crystals (122–124). These crystals can be integrated into crystalline biofilm or obstruct the urinary flow through the catheter, leading to the retention of urine, bacteriuria, and facilitating ascension of bacteria into the kidneys (125–127). In long-term catheterized patients, approximately 50% experience catheter blockage due to crystal formation (5, 95, 128). Proteus spp. have been isolated in 70% of bacteria-induced stone formation (129).

When ascending into the kidney, P. mirabilis frequently causes kidney stones and can lead to more severe kidney damage than other pathogens (114, 115). It proliferates within the tubular epithelium of the kidneys and can result in both necrosis and nephrosis, as well as inflammatory lesions (114, 130). If the infection cannot be treated in a timely manner, permanent renal damage can be caused. Moreover, UTIs can progress to bacteremia and sepsis, which, interestingly, is more often observed in polymicrobial infections (114, 128). The mortality of patients with UTI caused by P. mirabilis has been studied in-depth by Chen et al. (9). The overall mortality rate of 191 patients with P. mirabilis bacteremic and non-bacteremic UTIs was found to be 8.9%, while bacteremic patients’ mortality rate was 16.4%. In comparison, the mortality rate of urosepsis caused by E. coli has been reported to be 12.8% (131). In a study investigating community-acquired mono-microbial bacteremia, E. coli was detected in 72.3% of cases, Klebsiella in 24.9%, and Proteus spp. in 2.7%. Mortality rates reported were similar to the rates reported by Chen et al., with 7.6%, 18.3%, and 16.1% for E. coli, Klebsiella, and Proteus, respectively, once again underlining the severity of Proteus infections (132).

Proteus spp. as pathogens in the gastrointestinal tract

Proteus spp. have been linked to gastrointestinal diseases in a variety of cases, ranging from diarrhea and food poisoning to Crohn’s disease (133–135). Notably, research into Proteus as a gastrointestinal pathogen is still relatively young and might yield more insights on Proteus and gastrointestinal health (136). Proteus spp. are, as mentioned earlier, known human gut commensals. Studies have shown that the alteration of relative gut microbe abundances is often linked to an altered state of health in patients (137). Such alterations in relative abundances have also been described for Proteus spp. in a variety of cases.

P. mirabilis, for example, has been isolated in patients with diarrhea more often (10.8% of samples) than in healthy patients (2.7% of cases) (133), while the presence of P. vulgaris is generally associated with gastrointestinal symptoms (138). In Crohn’s disease, more pathogenic bacteria have been found in the small bowel serosa in affected patients than in healthy patients (27% and 15%, respectively), 33% of which included Proteus spp. (139). A microarray-based study by Mondot et al. (140) highlighted the overrepresentation of Proteus spp., especially P. vulgaris, in Crohn’s patients. In stool samples and colonic mucosa, Proteus spp. have been found to be enriched, with 18.5% of samples from Crohn’s disease patients containing Proteus, compared to 5.6% in the control group (141). Moreover, the presence of Proteus spp. favors the postoperative recurrence of Crohn’s disease (140, 142).

Proteus spp. have furthermore been linked to foodborne gastroenteritis, cirrhosis, and pancreatic infections (134, 135, 143–145). Proteus biofilm was found in 10 of 81 stents removed from patients (age range 20–86 years) with bile duct stones (n = 46, 56.8%), benign strictures (n = 29, 35.8%), and malignant strictures (n = 6, 7.4%) (146). In all of these 10 stents, Proteus formed a polymicrobial biofilm with, e.g., Klebsiella spp. or Pseudomonas spp. Unfortunately, in this specific study, there was no further investigation into the composition of biofilm formers and their influence on disease progression or the influence of the underlying cause for stent placement on the biofilm composition. Together with K. pneumoniae, P. mirabilis has been demonstrated to elicit colitis in murine models. Additionally, it has been shown that a high P. mirabilis burden correlates with the presence of colitis (147). Proteus spp. can, partly due to their ability to form biofilms, colonize medical devices in the gastrointestinal tract, such as shunts, stents, and nasogastric tubes (148–152), and can contaminate gastroscopes and colonoscopes (153). Gastrointestinal carriage of Proteus spp. can also lead to Proteus infections in other parts of the body, such as the urinary tract (154).

Proteus and rheumatoid arthritis

Proteus has been linked to rheumatoid arthritis (RA), a chronic inflammatory disorder affecting 0.5–1% of humans worldwide (155, 156). In 1971, Chandler et al. (157) reported increased titers of antibodies against Proteus in RA patients. This finding has been replicated in several studies worldwide. In 1992 and 1995, Ebringer et al. (158) and Wilson et al. (159), respectively, found homologies in HLA-DR4 antigen associated with RA and P. mirabilis hemolysin, as well as in type XI collagen and P. mirabilis urease (158, 159). The sequences in question are EQRRAA/ESRRAL and LRREI/IRRET. In 1999, cross-reactivities between synthetic Proteus hemolysin peptides with HLA-DR1/DR4 antigens and synthetic urease peptides and collagen antigens were reported (160), a finding strengthened by Wilson et al. (161) in 2003, demonstrating the lysis of red blood cells coated with HLA-DR4/1-containing EQRRAA by anti ESRRAL-Proteus antibodies. The notion that Proteus UTIs and RA are linked was further substantiated by studies reporting P. mirabilis to be the most common microbe found in the urine of RA patients and studies confirming an increased abundance of anti-Proteus antibodies in RA patients worldwide (161–163). The mechanism behind this link between bacteria and RA was proposed by Ebringer and Rashid (67) as follows: Proteus infects the host and leads to a UTI, triggering the generation of anti-Proteus antibodies. These antibodies cross-react with the aforementioned motifs in collagen and HLA-DR4/1, leading to local reactions and the release of auto-antigenic materials, which in turn lead to the production of auto-antibodies.

In a recent study, the comparison of urine microbiota of RA patients and healthy individuals has shown that RA patients exhibit a significantly higher microbial richness and that, among others, Proteus spp. are enriched (164). Moreover, a new putative cross-reaction pair has been found recently: antibodies against P. mirabilis O3 lipopolysaccharide appear to react with collagen type I (165).

VIRULENCE AND THERAPEUTIC APPROACHES

Swarming, adhesion, and biofilm formation

Proteus spp. are well-known for their ability to swarm very efficiently (see Fig. 3). While other bacteria, such as E. coli and P. aeruginosa, are also able to swarm (166), Proteus spp. can do so under a wide variety of conditions and at much higher agar concentrations compared to other species (167, 168). Swarming behavior on 1.5% agar has been reported for all Proteus type strains to date, with the exception of the P. penneri type strain ATCC 33519^T (41, 43). Proteus cells show a dimorphism as elongated, multi-nucleated and multi-flagellated “swarming” cells and as short, mononucleated “swimming” cells with 6–10 peritrichious flagella (169, 170). Swarming depends strongly on environmental conditions, such as the availability of carbon and nitrogen sources or NaCl (168). Several patterns of swarming have been described (171), but an in-depth analysis of the impact species and genotype have on swarming patterns remains to be done. Often, their swarming on the plate consists of swarming phases and consolidation phases, resulting in the characteristic bull’s-eye pattern (172) (Fig. 3B). In the context of infection, there is evidence that P. mirabilis swarming cells are better able to invade uroepithelial cells, with a 15-fold higher invasion rate than vegetative cells, possibly due to an increased cytolytic activity related to higher levels of hemolysin production (108). In contrast, non-swarming mutants have been found to be less infective in murine models (173). Moreover, P. mirabilis has been found to be capable of swarming rapidly over urinary catheters at a rate of 10 cm in 24 h, accentuating this genus’ serious role in host colonization in catheterized patients (96).

Fig 3.

Swarming phenotypes of different Proteus isolates on blood agar. Shown is the phenotype of colonies after 24 h. (A) Non-swarming isolate of P. hauseri sampled from dog feces, (B) bull’s-eye pattern of swarming P. terrae sampled from water, (C and E) root-like structures seen in swarming P. terrae sampled from water, (D) P. terrae sampled from dog feces swarming irregularly, and (F) unilateral, thin swarming layer produced by P. terrae isolate from dog feces.

Adhesion of Proteus spp. cells to both host cells and other surfaces plays an important role in both infection and biofilm formation (174, 175). Adhesion occurs through both fimbriae and non-fimbrial adhesins. In P. mirabilis strain HI4320, 17 gene clusters for the expression of fimbrial proteins have been found (176). Uropathogenic E. coli CFT073 harbors 12 such gene clusters in its genome (177–179). Fimbriae investigated in P. mirabilis include the mannose-resistant Proteus-like fimbriae (MR/P), uroepithelial cell adhesin, also called nonagglutinating fimbriae, P. mirabilis fimbriae, and several others, which have been implicated in biofilm formation, colonization of the urinary tract, and CAUTIs (175, 180–189) (see Table 1 for an overview of important virulence genes in P. mirabilis). Next to P. mirabilis, adhesion to human uroepithelial cells has been demonstrated for P. vulgaris isolates (190). P. vulgaris has also been described to adhere to other surfaces, such as catheters and stainless steel (191, 192). For P. penneri, the expression of fimbriae associated with adhesion and biofilm formation has been shown (193).

TABLE 1.

Virulence genes of P. mirabilis with their corresponding protein function

Gene	Protein function	Significance in virulence	Reference
aipA	Hemagglutinin	Host cell invasion	(194)
hpmA	Hemolysin	Hemolytic and cytotoxic activity	(102, 115, 195)
mrpA	Major MR/P fimbria protein	Fimbria involved in swarming, biofilm formation, and adhesion to host cells	(106, 183, 184, 196, 197)
nrpR	Siderophore synthase	Siderophore synthase, iron acquisition	(198)
pmfA	Major fimbrial subunit	Fimbria involved in swarming and adhesion to host cells	(188, 189, 196)
Pta	Autotransporter	Surface-associated cytotoxin	(115)
taaP	Adhesin	Promotes auto-agglutination	(194)
ureC	Urease subunit	Nitrogen and carbon acquisition, pH increase leading to optimal pH in urine and stone formation	(59, 106, 199)
zapA	Serralysin	Degrades host immunoglobulins, but also cell matrix and cytoskeletal proteins, and antimicrobial peptides	(200)

After initial adhesion, often to catheters in the case of P. mirabilis, biofilm formation can be initiated. Biofilms are characterized by the presence of bacterial cells in extrapolymeric substances and have been implicated as providing protection against both physical forces and biological factors, such as the treatment with antibiotic compounds (201–205), making biofilm infections especially challenging to treat (206).

Therapeutic approaches to swarming and biofilm formation

A specific challenge in preventing and treating Proteus infections is posed by the ability of Proteus spp. to swarm and ascend on catheters into the urinary tract, as well as adhere to them and form biofilms. Consequently, there have been numerous efforts to design catheters that are inhibitory to biofilm formation. Each of these efforts comes with its own chances and risks, and as of now, none of the solutions has led to the development of a commonly applicable solution for the medical field (207). Possible ideas are catheter coatings that could contain either antimicrobial substances, biofilm inhibitors, bacteriophages, or iron scavengers (58, 207–212). With catheter coatings, the release of substances plays a vital role in the viability of applications in clinical routine, as often the substances are depleted far earlier than the catheter would routinely be changed (213). A solution for this would be contact killing or rather non-release-based approaches, where the antimicrobial compound is covalently anchored to the catheter surface (214). Moreover, alternatives to the common Foley catheter are explored (215–218), as well as sensors that can be integrated into the catheter and detect the formation of crystalline biofilm by measuring pH (219, 220). Recently, a polymer resistant to bacterial biofilm, swarming, and encrustation was discovered and tested with P. mirabilis. Further tests with this polymer will be of special interest in the development of a novel catheter designed to prevent swarming and biofilm formation (221).

The Centers for Disease Control and Prevention issued guidelines for the prevention of CAUTIs in 2009, which were updated in 2017 (222). Here, the focus is on appropriate catheter use, proper insertion techniques, and proper maintenance. If the implementation of these guidelines does not lead to a decrease in CAUTIs, the use of a different catheter material is encouraged. Moreover, surveillance of CAUTI prevalence is recommended.

A problem in connection with biofilm and aggregate formation is the decreased susceptibility to antibiotics of cells within these structures (223). A study by Irwin et al. (224) determined the minimum inhibitory concentration (MIC) for planktonic bacteria and the minimum biofilm eradication concentration (MBEC) for the same bacteria in biofilm. Depending on the antibiotic, MIC and MBEC for P. mirabilis differed drastically, e.g., the MIC for ciprofloxacin was 0.08 mg/L at pH 7, whereas the MBEC was >125 µg/mL. It is important to note that the ability of bacterial cells in biofilms to tolerate higher concentrations of antimicrobial agents does not represent resistance (225) but is attributed to multiple factors, such as limited penetration of the antibiotics (226, 227), restricted growth of cells in the inner layers of the biofilm (226, 228, 229), the presence of persister cells (230–232), and more. Treatment options include topical administration of antimicrobials, the combination of antimicrobial therapies, and the use of adjuvants to increase the antimicrobial effect.

Topical administration of antibiotics in high concentration results in a high local concentration of the administered agent, while concentrations in other parts of the body are decreased (206, 233). This results in both a chance to treat biofilm-incorporated cells and a decreased overall chance of developing antimicrobial resistance (234, 235). Combining antimicrobial agents for treatments, either simultaneously or sequentially, can be used to delay the development of antimicrobial resistance and can help combat different subpopulations in the biofilm more efficiently (236–241). An example of a combination therapy tested in vitro on P. mirabilis planktonic and biofilm cells is the combination of a lytic bacteriophage and ampicillin (242). In the study performed by Yazdi et al. (242) in 2018, a bacteriophage with high lytic activity against some P. mirabilis strains (and no activity to the tested K. pneumoniae and E. coli strains) was used. The combination of ampicillin and bacteriophage treatment showed a higher impact on the population density of planktonic P. mirabilis strains than both treatments alone. More importantly, after combination treatment of P. mirabilis biofilm, 93% of biofilm was removed compared to the control treatment, followed by 70% biofilm removal by the phage alone and 44% by ampicillin (at a concentration of 246 µg/mL) alone.

Proteus urease

The production of urease, an enzyme catalyzing the hydrolysis of urea into carbon dioxide and ammonia, has a significant impact on the pathogenesis of Proteus infections (243). The reaction leads to both an increase in utilizable nitrogen supply with the increase of ammonia and an increase in pH in the environment. While Proteus spp. can withstand pH ranges from pH 5–10, their optimal pH ranges from 7 to 8, a condition usually not met in the slightly acidic urine with a median pH of around 6 (41, 244). Thus, by producing urease, Proteus spp. get access to ammonia as a nitrogen source, as well as optimize their growth conditions by adjusting pH. The increase in pH, as discussed above, not only promotes the growth of Proteus spp. but also the formation of stones, as it initially leads to the precipitation of magnesium and calcium minerals. The resulting urinary stones can obstruct the urinary flow and lead to tissue damage (117). Moreover, catheters can be obstructed, and the formation of crystalline biofilm sheltering the bacteria is facilitated (Fig. 4). Urease-negative P. mirabilis strains have been demonstrated to lack the ability to induce stone formation (243). Urease activity has been identified in P. mirabilis, P. penneri, P. vulgaris, and P. columbae (72, 245, 246). While it is highly likely that urease is also active in other Proteus reference strains, no published research has yet confirmed this.

Fig 4.

Sections of an encrusted pigtail catheter colonized by P. mirabilis and E. coli. (A) Overview picture of a catheter’s side hole with encrustations, magnified in (B). (C) Catheter in cross-section. The lumen diameter is significantly decreased, leading to obstruction of urinary flow.

How to manage urease production?

Since urease production and the subsequent pH increase enhance Proteus spp. and other urease-producing bacteria’s ability to thrive in the urinary tract, as well as ultimately induce stone formation, the application of urease inhibitors appears to be an important component in fighting Proteus UTIs (247). One kind of urease inhibitor is quinolones. The two fluoroquinolone antibiotics levofloxacin and ciprofloxacin have been shown to exhibit antibacterial activity against P. mirabilis and to inhibit urease activity (248). Moxifloxacin also inhibits urease activity, with inhibition activity being even larger when combined with silver in an Ag-Mox nanoparticle (249).

Natural products such as cranberry juice have been used to prevent and treat UTIs—or support other therapeutic agents—for a long time. The presence of flavonoids acting as urease inhibitors is considered to be the reason for this treatment (250). While there is an ongoing debate on the effectiveness of cranberry products on the prevention and treatment of UTIs (251), further in vitro data on the activity of other flavonoids are promising. In a study using Helicobacter pylori urease, quercetin, a flavonoid present in capers, coriander, and kale (252), was a strong urease inhibitor (253). There are many more components exhibiting urease-inhibiting activity (254), including natural products such as boswellic acid and palmatine (255, 256), heterocyclic compounds (257), and covalently binding inhibitors such as disulfiram (258).

Proteus-specific urease inhibitors have been described by Milo et al. (259), who report on 2-mercaptoacetamide, a small molecular enzyme inhibitor able to inhibit P. mirabilis urease, as well as preventing catheter encrustation in an in vitro bladder model. Grela et al. (260) have tested 21 compounds for P. mirabilis urease-inhibiting activity. Of these 21 compounds, 18 have shown inhibitory qualities, making the implementation of urease inhibitors a potentially new route in combating Proteus UTIs. To date, two urease inhibitors have been approved by the FDA, namely acetohydroxamic acid (Lithostat) and hydroxycarbamide/hydroxyurea (261, 262), which both have significant side effects (263), making routine application not feasible.

Immune evasion

Proteus spp. have several ways to evade the host immune response, ranging from the invasion of host cells to the production of antimicrobial-degrading peptides and LPS modification. The innate immune response to UTIs with P. mirabilis is a proinflammatory response. Studies in mice have shown elevated levels of CXCL1 and interleukin-10 (114). CXCL1 is a known neutrophil chemoattractant, resulting in the recruitment of neutrophils to the region of infection. Formation of P. mirabilis clusters in the bladder lumen further elicits a recruitment of neutrophils (106). Neutrophils can clear bacteria by mediating phagocytosis or NETosis, a process where neutrophil extracellular traps, consisting of condensed chromatin and antimicrobial proteins, are released (264, 265). However, it is generally assumed that the formation of big clusters, as well as the embedment in urinary stones, can protect Proteus cells (106, 117).

P. mirabilis cells are able to both invade and lyse host cells. Contrary to E. coli, however, they do not form large intracellular niches but appear to use invasion as a way to evade the host’s immune response and initially establish a small intercellular population, then return to the bladder lumen. Factors responsible for invasion and lysis are the AipA autotransporter for bladder cell invasion, as well as the Proteus toxic agglutinin (Pta) and hemolysin (HpmA) for lysis (115, 194). P. mirabilis has been found to be resistant to killing by polymyxins, polycationic antimicrobial peptides, as well as other antimicrobial peptides (266). This resistance is mediated by modifications of the LPS, the initial targets of polymyxins, altering the surface charges, and the production of Zap protease, a toxin able to degrade antimicrobial peptides (267), as well as secretory IgA and IgG (200, 268). Moreover, P. mirabilis periplasmic proteinaceous inhibitor PliC is able to inhibit c-type lysozymes (269), another part of the human innate immune system (270).

There have been several efforts to aid the immune system in combating Proteus infections by vaccination (271–274). Choubini et al. (275) developed a vaccine based on MrpA, UcaA, and Pta antigens, targeting the major MR/P fimbria protein involved in biofilm formation, the adhesion pilin UcaA, and an autotransporter. Treatment of mice with the resulting vaccine significantly decreased the infection success of P. mirabilis in a murine experimental UTI challenge. In another study performed by Habibi et al. (276), a vaccine was designed based on P. mirabilis MrpA and Flagellin (FliC). Vaccinated mice were challenged with P. mirabilis HI3240, and the colony count in bladders and kidneys was determined. The fusion vaccine MrpA.FliC was most effective in preventing infection, reducing the median colony count in the bladder 14,950-fold and in the kidney 6,429-fold compared to the control. While efforts to develop an applicable vaccine for clinical use are ongoing, no such vaccine is available to date.

Toxins

The main toxins in P. mirabilis associated with virulence are hemolysins, agglutinins, and metalloproteases. Hemolysins are secreted pore-forming cytotoxins. The predominant hemolysin in the Proteus genus is HpmA, which, together with HpmB, constitutes a two-partner (or Type V_b), Sec-dependent secretion system, encoded by the genes hpmA and hpmB (277). HpmB forms a β-barrel transmembrane channel for extracellular secretion of HpmA from the periplasm after N-terminal proteolytic processing (278). HpmA is capable of lysing several types of cells from several host species and has been reported in P. mirabilis, P. vulgaris, and P. penneri (105, 195, 279–281). To date, however, deletion of hpmA has not been found to significantly affect the virulence of P. mirabilis, most likely because it is masked by other factors (102, 115, 187, 195, 282–284). Pta, the Proteus toxic agglutinin, is a protein found in the outer membrane that is described as both an agglutinin and a cytotoxin (285). Pta has been shown to lyse bladder cells in vitro and contribute to the colonization of bladder and kidney in a murine co-challenge experiment (285). ZapA is a metalloprotease with a relatively broad specificity, capable of cleaving IgA, IgG, antimicrobial peptides, actin, collagen, and more (268, 286–289). Due to its broad specificity, its presence contributes strongly to virulence (200).

Virulence of species other than P. mirabilis

Since P. mirabilis is the most common Proteus spp. encountered in the healthcare setting, and most research has been performed on this species, little is known about the virulence of other Proteus spp. While P. vulgaris, P. penneri, P. hauseri, and P. appendicitidis have been associated with human infections, there is very little evidence for the other Proteus spp. Virulence can be assessed in numerous ways, including animal models (290, 291), comparative genomics (292, 293), transcriptomics (294, 295), and (genome-wide) mutagenesis (296–298). Recent advances in microfluidics even permit experiments in bladder models outside of test animals (299). For this review, we assessed if known virulence genes of P. mirabilis were also present in the genomes of other Proteus spp. BLAST (300) was used to search for the amino acid sequence of proteins described in highly virulent P. mirabilis HI4320 in the translated whole-genome assemblies of the type strains for P. penneri (ASM314453v1, GCF_003144535.1), P. vulgaris (PVA, GCF_000754995.1), P. terrae subsp. terrae (ASM314432v1, GCF_003144325.1), and P. terrae subsp. cibarius (ASM314449v1, GCF_003144495.1) (Fig. 5). This analysis can only give a brief insight into the presence of genes coding for certain virulence proteins in the Proteus type strains, and a more in-depth analysis is necessary to confirm conservation of these proteins between species. It appeared that a variant of all eight assessed virulence genes was present in the respective Proteus spp.

Fig 5.

Heatmap of genes associated with virulence in P. mirabilis uropathogenic strain HI4320 compared to a selection of type strains of other Proteus species.

Urease, the protein responsible for hydrolyzing urea and thus ultimately nitrogen acquisition, is present in all tested Proteus spp. with high protein similarity (Fig. 5). This is most likely due to the fact that urea is present abundantly in both humans and animals, thus presenting an attractive source of nitrogen and carbon. In the clinical context, the presence of urease in all tested Proteus spp. could be an indication of their ability to colonize the human host, specifically the bladder, once introduced to the system. MrpA, the major MR/P fimbriae protein, is involved in swarming, biofilm formation, and adhesion to host cells and thus crucial to virulence. Notably, P. terrae subsp. terrae possesses the protein with the highest protein similarity to P. mirabilis uropathogenic strain HI4320, indicating it to be a strong candidate for human colonization and infection. PmfA, the major fimbrial subunit involved in swarming and adhesion to host cells, is conserved in all tested Proteus spp. The same does not hold true for serralysin ZapA and the autotransporter Pta. ZapA plays an important role in the immune evasion of Proteus infections, as it is able to degrade—among others—antimicrobial peptides. NrpR, involved in siderophore synthase, and TaaP, an adhesin involved in auto-agglutination, are only weakly conserved in Proteus spp. compared to Proteus mirabilis HI4320. This might be explained by the preferred niche of the species. While P. mirabilis is commonly found in the bladder, which is an iron-limited environment, little is known about the natural environments of other Proteus species. It is possible that in these environments, highly effective iron-scavenging enzymes are not necessary for survival. HpmA, a hemolysin, is also present in all tested Proteus spp. and moderately conserved, again hinting at the possibility of human infection through representatives of other Proteus spp.

Prediction of pathogenicity of Proteus type strains using PathogenFinder (301) yields a positive result for all Proteus spp. type strains known to date. Interestingly, the species P. terrae subsp. cibarius is predicted to be a human pathogen with a higher probability (P = 0.871) than the documented human pathogen P. mirabilis (P = 0.784). A first clinically relevant P. terrae subsp. cibarius isolate has been described in 2018, originating from a human wound sample in Nigeria (302).

Antimicrobial resistance

A prevalent problem when dealing with Proteus spp. is their ability to acquire antibiotic resistance genes (ARGs). While P. mirabilis wild type is naturally susceptible to almost all beta-lactams (Table 2), more and more strains with acquired beta-lactamases have been reported. In a recent study from Egypt (303), 51.7% of P. mirabilis clinical isolates produced extended-spectrum beta-lactamases (ESBLs), while 10.3% were carbapenemase producers. Of 58 strains, 17 (29.3%) were considered multidrug-resistant (MDR), making them complicated to treat. In another study from Italy (8) investigating bloodstream infections caused by P. mirabilis, 44% of the cases were caused by ESBL-producing P. mirabilis. A large-scale study (304) investigating a total of 1,406 P. mirabilis isolates collected from multiple sites around the world, including Latin America, North America, and Europe, reported the highest prevalence of ESBL-producing P. mirabilis among all isolates in Latin America (22.4%), followed by Europe (11.1%). Percentages were much lower in Canada, the United States, and the Western Pacific (3.1%, 4.9%, and 1.8%, respectively).

TABLE 2.

Expected phenotypes of different Proteus spp. according to EUCAST^b

	P. mirabilis	P. penneri	P. vulgaris
Ampicillin/amoxicillin	S	R	R
Amoxicillin-clavulanate	S	S	S
Ampicillin-sulbactam	S	S	S
Ticarcillin	S	S	S
Cefazolin, cefalexin	S	R	R
Cefoxitin	S	S	S
Cefuroxime	Ia	R	R
Tetracyclines	R	R	R
Polymyxin B, colistin	R	R	R
Nitrofurantoin	R	R	R

^a According to CLSI guidelines, P. mirabilis is not considered “I” but “S” for cefuroxime (305).

^b R, resistance expected; S, susceptibility expected; and I, susceptibility at increased exposure expected (306).

Conjugation plays a key role in the acquisition of ARGs in Proteus spp. (307) and is the most effective mechanism of horizontal gene transfer (307, 308). It allows rapid spread of ARGs and can confer MDR in a single conjugation event (309, 310). Conjugative plasmids can contain multiple ARGs (311) and either stay as plasmids in the recipient cell for further conjugation events or integrate into the host’s chromosome if they contain transposons (312) or integrons (313). These genetic elements encode for integrase or transposase enzymes, which can incorporate the ARGs into the chromosome. The spread of MDR is often facilitated by gene cassettes containing multiple ARGs, which can be incorporated into the chromosome in a single step (314). Conjugation has been extensively studied in Proteus spp. for many years. Multiple studies have demonstrated that most resistance genes were found on conjugative plasmids and integrons (307, 308, 315).

Proteus spp. can exchange ARGs via conjugation intraspecies and interspecies. Among the Enterobacteriaceae family, conjugation events are common (316, 317). For example, E. coli has been identified as both a donor and receiver of ARGs (318–320), as well as K. pneumoniae (321), and Salmonella spp. (322). They have been shown to exchange plasmids containing ESBLs (323, 324), fluoroquinolone resistance genes (325, 326), aminoglycoside-modifying enzymes (327), as well as carbapenemase genes (328). Conjugative plasmids encoding genes that confer resistance to diverse beta-lactams (329–331), tetracyclines (332), sulfonamides (333), and aminoglycosides (331) were also identified in P. mirabilis.

Carbapenemases in Proteus spp.

Of increasing global importance is the rapid spread of carbapenemase genes as carbapenems are used as “last resort” antibiotics in many countries for the treatment of ESBL-producing Enterobacterales (334). Carbapenem resistance in P. mirabilis poses a particularly dangerous global health issue as it frequently results in pandrug-resistant isolates (335). This is caused by the intrinsic resistance of P. mirabilis against polymyxins (336), which are other “last resort” antibiotics that previously showed an effect against many carbapenemase producers (337). Three classes of carbapenemases have been identified in Proteus spp.: Ambler classes A, B, and D beta-lactamases (338–340) (Table 3). Of class A carbapenemases, KPC-2 is the most common one among Enterobacterales worldwide (341). It was initially identified in K. pneumoniae (321). KPC enzymes confer resistance to different classes of beta-lactams, including carbapenems and cephalosporins (342, 343). They are usually encoded on a conjugative plasmid with a transposon (341, 344, 345).

TABLE 3.

First discoveries of carbapenemases in Proteus spp.

Carbapenemase	First discovered in Proteus spp.	Reference
OXA-23	1996–1999, France	(346)
VIM-1	2006, Greece	(347)
KPC-2	2008, USA	(338)
IMP-27	2009, USA	(348)
NDM-1	2009-2010, Switzerland	(349)
OXA-48	2012, Palestine	(340)
OXA-58	2017, Belgium	(350)
OXA-24/40	2019, Algeria	(351)
OXA-162	2023, Germany	(352)
OXA-181	2023, Germany	(352)

In contrast to Ambler class A, C, and D enzymes, which are serine-beta-lactamases, class B enzymes are metallo-beta-lactamases with zinc ions in their active sites. This makes them susceptible to inhibition by cation chelators like EDTA, but not by regularly used beta-lactamase inhibitors (353). A widespread member of this class, IMP-1, which was originally identified in P. aeruginosa, is able to hydrolyze imipenem, penicillins, and extended-spectrum cephalosporins but not aztreonam (354). Cefiderocol is a novel siderophore cephalosporin that was shown to have high stability against clinically relevant carbapenemases like IMP-1, KPC-3, and NDM-1 and is efficiently taken up by bacterial cells (355).

Class D beta-lactamases include oxacillin-hydrolyzing enzymes called OXAs (356, 357). The class is further divided into multiple subfamilies with some possessing carbapenem-hydrolyzing activities (358). Their abundances differ between continents and the species of Enterobacterales. In contrast to other members of the order, the most prevalent OXA-type carbapenemases in Proteus spp. seem to be OXA-23, OXA-48, and OXA-58 (359–361). A recent study investigating isolates from France and Belgium found that among 61 isolates with an OXA-type phenotype, 21 isolates produced OXA-23 (359). While OXA-58 is commonly found in Acinetobacter spp., a study in Germany found that only 4 out of 37 strains of P. mirabilis isolated from different patients produced OXA-58 (361). The same study stated that the presence of OXA-58 may easily be missed in P. mirabilis as its detection with MIC and CarbaNP tests is difficult. The detection of OXA-48 in P. mirabilis poses a similar problem. It was previously rarely identified in this organism (362). A recent study in Germany found that, similar to OXA-58, OXA-48-producing P. mirabilis isolates exhibited low MIC values for carbapenems. Additionally, in contrast to other carbapenemase-producing Enterobacterales, MIC values for piperacillin-tazobactam were also low (363). Even though MIC testing implied that OXA-48 only confers resistance against low concentrations of carbapenems in P. mirabilis, another study showed that carbapenems are, in fact, not an effective treatment against OXA-48-producing Enterobacterales (364).

The effective treatment of resistant Proteus infections depends on the prompt detection of the particular strain’s resistance mechanisms and the resulting appropriate antibiotic therapy. However, for P. mirabilis, it has been noted that standard testing procedures such as determination of the MIC or disk-diffusion tests with current breakpoints lack sensitivity to reliably detect carbapenemase production in P. mirabilis isolates producing OXA-23, OXA-48-like, and OXA-58 (352). Additionally, the detection of carbapenemases by other methods is also more challenging compared to other Enterobacterales. Immunochromatographic tests are now often employed for the detection of carbapenemases in clinical routine diagnostics but have been shown to fail more often in Proteus spp. compared to other species (365–368), potentially related to lower expression of the carbapenemase enzyme. Similarly, detection of carbapenemase genes may fail since many commercial assays target only the major genes found in Enterobacteriaceae (bla_KPC, bla_NDM, bla_VIM, bla_OXA-48-like, and bla_IMP) but do not include bla_OXA-23 and bla_OXA-58 (352, 369).

Thus, special care is recommended when treating P. mirabilis infections to prevent inappropriate and ineffective therapy. Recently, a simple algorithm for the phenotypic detection of carbapenemase production in P. mirabilis has been proposed. By combining disk diffusion tests using temocillin and ticarcillin-clavulanate with mzCIM, 100% specificity and sensitivity were obtained both in a retrospective and a prospective analysis of 81 and 91 strains, respectively (352).

POTENTIAL OF PROTEUS SPP. IN MEDICINE AND BIOTECHNOLOGY

Despite Proteus spp. prevalence and persistence in infections, Proteus spp. also offer several chances for applications in the medical fields and biotechnology for the future. Proteus spp. have been found in a number of extreme environments, having adapted to these environments by the use of surprising metabolic features. A number of strains have been reported to withstand higher than usual concentrations of toxic compounds, such as copper and other heavy metals (370–372), high salinity (373), and high acidity (374).

Of special interest to medicine and biotechnology are species that are not only able to withstand extreme conditions but also able to utilize compounds that are either toxic or non-degradable. One of these compounds is oil, which is often released into the environment on accident, with remediation proving very difficult. Kim et al. (375) published the finding of a P. vulgaris strain expressing an extracellular alkaline lipase as early as 1995. Whangsuk et al. (376) found another Proteus sp. strain able to convert palm oil into biodiesel, whereas Ibrahim et al. identified a P. vulgaris and a P. mirabilis strain as two of the most active crude oil degraders in a tested set of strains (377). P. mirabilis has been successfully used to degrade hydrocarbons in oil sludge, and there are plans for in situ application of said strain, possibly bioremediating oil spills or other polluted sites (378).

Certain strains of Proteus spp. have also been found to be capable of phenol utilization (379), DDT reduction (380), pesticide degradation (381–383), and azo dye neutralization (384, 385). Taken together, Proteus spp. appear to be particularly well suited for adapting to extreme lifestyles and might thus be of great importance in developing strategies to neutralize toxic compounds in contaminated soil and wastewater. Moreover, with their high resistance to heavy metals, Proteus spp. have been implicated in aiding plant growth in contaminated soil regions. One P. mirabilis strain was reported to effectively block zinc from being taken up by maize roots, reducing the plant’s oxidative stress and thus promoting its growth (386). In addition to this, some Proteus strains reportedly solubilize phosphate, which again promotes plant growth (387).

CONCLUSION

The Proteus genus contains pathogenic species with a high diversity that has only been uncovered recently. While most of the clinical research has been focused on P. mirabilis and its ability to swarm and produce urease, leading to complications in infections such as UTIs, little is known about the other members of the Proteus genus. Here, we show that infection with all Proteus spp. appears to be possible, as many virulence genes are shared. Experimental confirmation of this is needed to assess the risk and severity of infections.

Proteus spp. are highly relevant in the context of AMR dissemination, as they are able to acquire AMR genes, and the presence of some of these genes is not always detected by standard assays. This can result in prolonged infections and thus the opportunity for AMR genes to be transferred to other pathogens. Further investigation into accurate testing for the presence of AMR genes in Proteus spp. is necessary to effectively prevent further spread of AMR.

However, the high diversity and adaptability of Proteus spp. also appear to be beneficial, as there have been several strains of Proteus spp. reported to degrade or utilize toxic compounds. Using Proteus spp. in directed evolution approaches could lead to novel ways to efficiently degrade toxic compounds both in the lab and in remediation projects.

Biographies

Linea Katharina Muhsal, PhD, is advancing the fields of medical microbiology and microbial ecology as a postdoctoral researcher at Germany's Osnabrück University and guest scientist at Carl von Ossietzky University of Oldenburg. Her academic journey began with a Bachelor of Science in Biology from the University of Bielefeld, Germany, followed by a Master of Science in Biotechnology from the Wageningen University and Research Center in the Netherlands. At the University of Osnabrück, she earned her doctorate studying bacterial ecology and evolution, where she combined laboratory experimentation with modeling techniques to investigate the evolution of cooperative behavior in bacteria. In 2024, Dr. Muhsal broadened her research to include medical research with a PostDoc at Carl von Ossietzky University of Oldenburg, focusing on P. mirabilis and carbapenemase detection. Her current work bridges microbiology and technology, exploring machine learning applications for antimicrobial resistance detection while maintaining her core focus on microbial ecology and evolution — specifically how environmental factors influence species and community behavior in both natural and medical contexts

Lisa Theis, B.Sc., is currently completing her Master´s thesis at the Institute for Medical Microbiology and Virology at the Carl von Ossietzky University of Oldenburg, Germany. In May 2024, she joined Professor Hamprecht’s research group, where she investigates natural competence in Gram-negative bacteria. She earned her Bachelor´s degree in Biology with specialization on Microbiology at the Philipps University of Marburg in Germany in cooperation with the Max-Planck-Institute for Terrestrial Microbiology. She has been pursuing her Master´s in Microbiology since 2022 in an international Master´s program coordinated by the Institute for the Chemistry and Biology of the Oceans (ICBM).

Jörg Overmann, PhD, serves as Scientific Director of the Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures since 2010 and is Professor of Microbiology at the Technical University of Braunschweig. He studied Biology with a focus on Microbiology at the Universities of Bochum and Freiburg, completing his doctorate at the University of Konstanz in 1991. Following postdoctoral research in Vancouver, Canada, and his habilitation at Carl von Ossietzky University Oldenburg, he became Professor of Microbiology at Ludwig-Maximilians-University Munich before moving to Braunschweig. His research focuses on bacterial genome evolution, the diversity and functions of bacterial communities in natural environments, and on molecular mechanisms of bacterial interactions, including microbiomes. Dr. Overmann developed the bacterial metadata platform BacDive and has conducted extensive field research across Africa and South America. He served and serves on several prestigious committees, including the DFG Senate Commission on Biological Diversity and the Council of Scientists of the Human Frontier Science Program Organization. He is member of the Executive Committee of the Leibniz Association.

Axel Hamprecht is a professor of medical microbiology and virology of the Carl von Ossietzky University Oldenburg with a long-standing focus on antimicrobial resistance. He serves as the director of the Institute of Medical Microbiology and Virology at Oldenburg Hospital. Previously he worked as professor for multidrug-resistant pathogens at the University Hospital Cologne. Axel studied medicine at the Universities of Heidelberg, Bordeaux, Geneva, Nottingham and Berlin. He has always been fascinated by Gram-negative bacteria and their great arsenal of resistance mechanisms, a topic he has been working on for more than 10 years. His reseach focus is on carbapenemases in Enterobacterales and horizontal gene transfer. Additionally, he is interested in medically relevant fungi and improvement of diagnostic methods in microbiology.