Staphylococcus aureus Evasion of Host Immunity in the Setting of Prosthetic Joint Infection: Biofilm and Beyond

Abstract

Purpose of Review

The incidence of complications from prosthetic joint infection (PJI) is increasing, and treatment failure remains high. We review the current literature with a focus on Staphylococcus aureus pathogenesis and biofilm, as well as treatment challenges, and novel therapeutic strategies.

Recent Findings

S. aureus biofilm creates a favorable environment that increases antibiotic resistance, impairs host immunity, and increases tolerance to nutritional deprivation. Secreted proteins from bacterial cells within the biofilm and the quorum-sensing agr system contribute to immune evasion. Additional immunoevasive properties of S. aureus include the formation of staphylococcal abscess communities (SACs) and canalicular invasion. Novel approaches to target biofilm and increase resistance to implant colonization include novel antibiotic therapy, immunotherapy, and local implant treatments.

Summary

Challenges remain given the diverse mechanisms developed by S. aureus to alter the host immune responses. Further understanding of these processes should provide novel therapeutic mechanisms to enhance eradication after PJI.

Introduction Structure and Formation of Biofilm

Total joint replacement is one of the most common elective surgeries performed in the USA, and the incidence of complications from prosthetic joint infection (PJI) is increasing. Unfortunately, treatment failure of PJI remains high, ranging from 30 to 50% after surgical debridement and implant retention to 10–20% after implant removal with two-stage surgical treatment [1–3]. Staphylococcus aureus is the most common pathogen isolated from PJI, where up to 50% of cases are caused by hard-to-treat methicillin resistant S. aureus (MRSA) strains [4, 5]. A major cause of treatment failure is bacterial resistance to antimicrobials; therefore, it is critical to understand the mechanisms responsible. While genetic alterations can arise to engender bacterial resistance to specific antibiotics (i.e., MRSA), the primary and much broader mechanism of resistance to drugs and host defenses involves biofilms, which protects pathogens that would otherwise be eradicated in their planktonic, or free floating form. As defeating biofilm is central to preventing and treating PJI, here, we review the current literature on S. aureus pathogenesis and biofilm, as well as challenges, and novel therapeutic strategies.

The formation of biofilms evolved over time as an adaptive response to hostile environments, allowing single-celled organisms to survive in a multi-cellular community [6]. In this process, biofilm formation proceeds in four generalized steps, including (1) bacterial cell attachment to a surface, (2) cell proliferation, (3) biofilm maturation, and (4) detachment and dispersal. Many of these processes contain species-specific characteristics. For example, S. aureus has a number of mechanisms to adhere to host tissue and foreign material surfaces. Once adherence has occurred, formation of a microcolony or small group of initial cells occurs, followed by cell proliferation and biofilm formation via production of extracellular matrix. Most of the biofilm mass consists of self-produced matrix of extracellular polymeric substances (EPS) composed of polysaccharides, proteins, and extracellular nucleic acids [7]. The EPS encases the bacterial cells contained within the biofilm to provide mechanical stability, protect against antimicrobial serum factors (i.e., antibodies and complement) and immune cell invasion, and retain essential nutrients and enzymes [7]. Observations of bacterial biofilms show their complex and heterogeneous organization, composed of void spaces and water channels to facilitate oxygen and nutrient transport through the bulk volume [8, 9••]. Cell-to-cell communication and exchange of genetic material can occur, showing the dynamic nature of biofilm. Furthermore, a proportion of bacteria have decreased metabolic activity creating less opportunity for effective antibiotic eradication [10]. It is within this specialized microenvironment that S. aureus is unaffected by antibiotic exposures up to 1000 times greater than the minimal inhibitory concentration (MIC) of its planktonic phenotype [11].

Initially, it was hypothesized that diffusion of drug molecules into the biofilm was the limiting factor that allowed survival of bacteria within this glycocalyx structure [12]. However, many static and dynamic culture systems of 3D biofilms have demonstrated that despite thorough drug penetration, many bacterial cells still survive treatment [13–15]. Thus, this drug-resistant phenotype is a product of a variety of cellular and environmental factors including slowed growth rate, mutability, pH alteration, and altered nutrient requirements [7]. Additionally S. aureus biofilms are capable of containing slow growing persister cells, as well as small colony variant (SCV) cells, which have an auxotrophic defect, both of which enable extreme resistance to conventional antimicrobial treatments [16–20].

While there have been many studies on biofilm formation and structure utilizing in vitro models, no methods have emerged for “gold standard” biofilm growth, or biofilms that faithfully represent the glycocalyx of in vivo biofilms [21–26]. However, the use of dynamic biofilm culture systems has improved their clinical relevance [22–26]. Although these methods may produce a biofilm-like growth, with bacterial cells encased in a matrix of EPS, we cannot assume that they are clinically relevant models for biofilm growth and in vivo biofilm formation is less well characterized. Contrasts between in vitro and in vivo biofilm growth across many species has been documented and scientists have been cautioned to consider the clinical relevance of their biofilm models [27•]. To address this, several in vivo models have been developed using central venous catheters, or subcutaneous foreign body infection, to evaluate biofilm eradication by potential drug therapies [28, 29]. Overall, these methods can greatly improve the clinical relevance of a preclinical study.

Staphylococcus Biofilm Evasion of Host Immunity

S. aureus biofilm creates a favorable environment that increases antibiotic resistance, impairs host immunity, and increases tolerance to nutritional deprivation. In this section, we summarize recent research focused on better understanding the effect of S. aureus biofilm infections on host immune response.

Cell Wall-Anchored Proteins

S. aureus expresses a broad range of proteins on its surface (up to 24 proteins have been identified), and are collectively known as cell wall-anchored (CWA) proteins due to their covalent linkage to pathogen’s peptidoglycan layer [30]. Based on the presence of certain unique structural motifs, the CWA proteins are classified into four groups: (1) the microbial surface component recognizing adhesive matrix molecule (MSCRAMM) family, (2) the near-iron transporter (NEAT) motif family, (3) the three-helical bundle family, and (4) the G5-E repeat family [30, 31]. Examples of each family of CWA and their functions are summarized in Table 1.

Table 1.

Summary of selected cell wall-anchored proteins relevant to staphylococcal biofilm

Cell wall-anchored protein motif	Examples	Ligand binding	Function
MSCRAMM family proteins	Clumping factors A and B (ClfA and ClfB)	Bind fibrinogen (A,B), keratin (B), loricrin (B), complement factor I (A), desquamated epithelial cells (B)	Bind host extracellular matrix components particularly fibrinogen. Degradation of complement (C3b)
	SdrC, SdrD, SdrE, bone sialoprotein-binding protein	Beta-neurexin binding (SdrC), binds to desquamated epithelial cells, complement factor H (SdrE)	Binding to epithelium
	Fibronectin binding proteins A and B (FnBpA, FnBpB)	Bind fibrinogen, fibrin, elastin	Binds extracellular matrix
	Collagen adhesion (CNA)	Binds collagen, complement protein C1q	Binds extracellular matrix, inhibits complement activation
NEAT motif family	IsdA-E, IsdG, IsdH, IsdI	Bind/transport heme or hemoglobin, integrins, and extracellular matrix components	Iron/heme uptake and transport, resistance to neutrophil killing
Three-helical bundle	Protein A	Binds Fc region of IgG to inhibit opsonophagocytosis, activates platelet aggregation via von Willebrand factor binding, B cell superantigenic activity by crosslinking Fab region of VH3 bearing IgM, activates TNFR1	Immune evasion Infection pathogenesis
	Staphylococcal complement inhibitor (SCIN)	C3 convertases on bacterial surface	Inhibits complement activation
G5-E repeat family	Surface protein G (SasG)	Unknown	Cell-to-cell adhesion during biofilm formation

MSCRAMM microbial surface component recognizing adhesive matrix molecules, NEAT near-iron transporter (NEAT) domains

The MSCRAMM family of CWA proteins plays a key role in S. aureus bone/joint infections by initiating staphylococcal attachment to host plasma proteins [30–37]. The Clf-Sdr class of MSCRAMMs that include clumping factor proteins (ClfA and ClfB), and surface-anchored proteins (SdrC, SdrD, SdrE) mediate biofilm formation by promoting S. aureus attachment to both indwelling prosthetic devices and plasma-coated biological surfaces [30, 34, 35, 38]. Wang and colleagues showed that ClfA exacerbates orthopedic implant-related hematogenous S. aureus infection in a rodent model and that neutralizing antibodies against ClfA in combination with α-hemolysin (Hla) inhibited S. aureus biofilm formation [39••].

The near-iron transporter (NEAT) motif family of CWA proteins (IsdA-E, IsdG, IsdH, IsdI) is summarized in Table 1 [40]. These proteins induce biofilm formation under iron starving conditions and are essential for S. aureus’ survival against host immune defenses [41–43].

An important three-helical bundle CWA protein, expressed in all S. aureus strains, is the hypervariable staphylococcal protein A (SpA) (Table 1). This multifunctional protein, often used for S. aureus strain genotyping, is essential for nasal carriage of S. aureus in humans and is critical for the pathogenesis of staphylococcal infections [44–50]. SpA promotes bacterial aggregation and significantly contributes to the development of biofilm-associated infections and S. aureus abscess community formation in vivo [51]. Studies have reported an association between SpA expression and bone infection severity including osteoclast activation, osteoclast differentiation, and cortical bone destruction via TNFR1 and EGFR-mediated signaling pathways [52–55].

Immunoevasive Proteins

S. aureus biofilms harbor an extensive exoproteome of proteins with functions related to immunoevasion and pathogenesis including hemolysins, nucleases, lipases, proteases, and collagenases [56–61]. The most important of these proteins related to biofilm pathogenesis are summarized in Table 2.

Table 2.

Summary of selected immunoevasive proteins relevant to staphylococcal biofilm

Class of protein	Examples	Function
Hemolysins	α-Hemolysin (α-toxin, H1a), β-hemolysin, γ-hemolysin	Pore-forming toxin (α- and γ-hemolysin), sphingomyelinase (β-hemolysin). Lyse red blood cells, other leukocytes, epithelial/endothelial cells; alters immune cell signaling pathways involved in cell proliferation, immune response, and cytokine expression
Leukocidins	LukAB, LukDE, PVL	Pore-forming toxins. Lyse neutrophils, monocytes, and macrophages
Enzymes	Autolysin (AtlA)  - Aminidase (Amd)  - Glucosaminidase (Gmd)	Peptidoglycan hydrolases. Cell separation, generates extracellular DNA in biofilm matrix
	Aureolysin	Protease. Inactivates PSMs and can activate other proteases
	Staphylokinase	Activates plasminogen to plasmin. Cleaves complement factor C3b
	Nuclease	Inactivate neutrophil extracellular traps (NETs)
Phenol-soluble modulins	δ-hemolysin, PSMα1–α4	Small amphipathic peptides. Lyse neutrophils and break down biofilm matrix, critical for biofilm disassembly
Superantigenic exotoxins	Toxic shock syndrome toxin	Stimulate T cells nonspecifically without typical antigenic recognition. Can cause toxic shock syndrome. Activates bone resorption, and inhibits host immunity in osteomyelitis
	Staphylococcal enterotoxins (A–E, G)	Activate cytokine release and involved in gastroenteritis, sepsis, kidney injury
Wall teichoic acids	–	Roles in cell division, complement activation, beta-lactam resistance in methicillin-resistant S. aureus, antibiotic susceptibility, and resistance to neutrophil intracellular killing
Chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS)	–	Inhibits neutrophil migration and activation and prevents complement activation

Evasion of Innate Immunity

Neutrophils and macrophages present the first line of host innate defense against S. aureus infections. Polymorphonuclear neutrophils (PMN) can efficiently kill planktonic S. aureus using several antimicrobial strategies such as phagocytosis, production of bactericidal peptides, oxidative bursts, and secretion of proinflammatory cytokines and chemokines [62]. S. aureus has developed many strategies to resist neutrophil-mediated killing (Tables 1 and 2). Secreted peptides such as PSMs and pore-forming toxins disrupt the integrity of the plasma membrane of neutrophils and cause cell lysis [63••]. Scherr et al. showed that S. aureus biofilms increase expression of the accessory gene regulator (agr) locus (which encodes a quorum-sensing system) in the presence of neutrophilic challenge, and this may play a role in resistance to PMN killing and phagocytosis [64]. Membrane-bound pigment such as staphyloxanthin and production of enzymes such as superoxide dismutase scavenge reactive oxygen species, protecting S. aureus from neutrophil-mediated killing [65]. Another direct innate host defense mechanism represents the formation of neutrophil extracellular traps (NETs), in which bacteria are trapped for clearance by neutrophils and macrophages [66•, 67]. By producing nucleases, S. aureus can trigger the degradation of NETs to deoxyadenosine, which induces macrophage cytotoxicity in abscess environments [68••]. Many strains of S. aureus also produce factors that inhibit complement activation, providing another mechanism against host defenses [69•]. Using these various defense mechanisms and virulence factors, S. aureus can survive neutrophil-directed killing and phagocytosis, persist within neutrophils, and use these cells as vehicles to circulate through host tissue [69•].

Classically activated macrophages (M1) elicit proinflammatory responses such as inducible nitric oxide synthase (iNOS) production, TNFα, IL-1β, and IFN-γ expression against intracellular S. aureus and efficiently promote bacterial clearance. Staphylococcal biofilms actively evade recognition by toll-like receptors (TLR2 and TLR9) and skew macrophage responses towards an anti-inflammatory state (alternatively activated, M2) [70, 71•, 72]. M2-macrophages were inefficient in phagocytosing S. aureus cells within the biofilm and promoted a pro-fibrotic environment that enriched biofilm formation [64, 72]. S. aureus biofilms also actively secrete virulence factors such as LukAB and α-hemolysin that can impair macrophage phagocytosis and function [73].

A heterogeneous subset of immature monocytes and granulocytes called myeloid-derived suppressor cells (MDSCs) has recently been shown to play an important in promoting S. aureus orthopedic biofilm infection [74•]. In a murine model of prosthetic joint infection (PJI), Heim and colleagues demonstrated that MDSCs actively suppressed T cell recruitment and proinflammatory cytokine production at the site of infection, thereby facilitating S. aureus biofilm persistence and proliferation [74•]. The authors showed that MDSCs suppressed monocyte/macrophage inflammation at the local biofilm milieu by augmenting anti-inflammatory mediators like arginase-1 (Arg-1) and IL-10 [74•, 75, 76]. Peng and colleagues used a rat PJI model to show that S. aureus biofilms polarize a subset of MDSCs into M2-macrophages, and this preferential expansion can further promote bacterial persistence at the site of infection [77].

Evasion of Adaptive Immunity

Adaptive immunity against staphylococcal biofilms consists of cell-mediated responses dominated by T cells and humoral antibody responses mediated by B cells. CD4+ T helper 1 (Th1) cells facilitate cellular immune responses against intracellular bacteria, while Th2 cells mediate humoral immunity against extracellular pathogen [76, 78]. Th17 cells, though traditionally thought to complement Th1 cells in mediating intracellular bacterial clearance, are now known to promote biofilm-associated chronic inflammation [79]. In a porcine osteomyelitis infection model, Jensen et al. showed that the antibody responses against extracellular S. aureus in biofilms are skewed to a predominantly Th1- and Th17-biased immune response, which is ineffective in clearing extracellular pathogens [80]. To further illustrate the Th1/Th17 bias, the group showed that Th1/Th17-biased C57BL/6 mice were unable to clear chronic S. aureus biofilm bone infections, while most of the Th2-skewed BALB/c mice, which produced anti-inflammatory Th2 cytokines (IL-4, IL-10), and regulatory T cells (Treg), cleared the infection [81].

Numerous studies have described anti-S. aureus humoral immune responses following colonization or infection [82–85]. Our laboratory determined that IsdA, IsdB, aminidase, and glucosaminidase are immunodominant antigens during S. aureus infection in both mice and patients with deep musculoskeletal infections [86•]. IgG titers against many different S. aureus-associated proteins are common in healthy humans; however, due to previous infection, colonization, and subclinical infection, it is challenging to use them for diagnostic purposes [86•]. Additionally, the presence of these antibodies only confers limited protection against future S. aureus infections [82–85].

Agr and Emigration from Biofilm

Biofilm disassembly converts S. aureus to its planktonic state, allowing it to reestablish local infection and travel to distant sites resulting in systemic infection. One of the most important regulators of biofilm development and disassembly in S. aureus is the accessory gene regulator (agr) system. It is a well-characterized two-component peptide quorum-sensing system present in all staphylococci [87]. It is controlled by a circuit initially triggered by autoinducing peptide (AIP), which is synthesized and secreted into the extracellular environment [88]. At a certain threshold concentration, it activates the histidine kinase AgrC, which then leads to the production of proteases, toxins, and PSMs, which are surfactant-like peptides that can act against host defenses and help break down the biofilm matrix contributing to disassembly [88, 89]. During the initiation phase of biofilm development, agr expression is low, upregulating the expression of bacterial adhesion factors, while simultaneously downregulating toxin production. Isogenic agr mutants exhibited pronounced biofilm formation and decreased ability to disseminate into other tissues [90–93]. In a mature biofilm, increased expression of agr leads to protease, toxin, and PSM production leading to dispersion of S. aureus from the biofilm to planktonic state and secretion of extracellular factors that disrupt the host immune response. A significant portion of the agr mutant phenotype with regard to biofilm structure and inability to disassemble is due to a lack of PSM expression, illustrating their importance to the agr system [9••]. Other genes besides PSMs expressed through the agr system also can play a role in biofilm resistance, but these are less well characterized. For instance, in a mouse PJI model, S. aureus biofilms thwarted macrophage phagocytosis via agr-mediated production of Hla and LukAB toxins [73].

Beyond Biofilm: Canalicular Invasion and Colonization

A high rate of treatment failure occurs in PJI despite extensive soft tissue and bone debridement, implant exchange, and extensive antibiotic therapies, suggesting that other mechanisms of bacterial persistence exist [94–96]. Recent findings from our group have discovered that S. aureus colonizes long submicron cracks and canaliculi adjacent to the medullary canal in chronic osteomyelitis, and this serves as a major bacterial reservoir [97••, 98]. Within the canalicular network of live bone, the bacterial cells have an inexhaustible source of nutrients while remaining protected from the threat of immune cell attack [99].

These observations do not conform to established dogma that historically defined S. aureus as non-motile cocci ~ 1 μm in diameter. Based on the absence of any observable motility structures such as flagella, cilia, or pseudopods, it is theorized that S. aureus is capable of invading the canaliculi of live bone with a novel motility mechanism [97••]. By performing immunoelectron microscopy following BrdU metabolic labelling, it was demonstrated that live bone colonization is an active process that involves bacterial replication, rather than dormant persistence [97••, 98]. It appears that S. aureus utilizes haptotaxis to identify the 3D structure of the canalicular openings and durotaxis to identify the rigidity of the 3D structure and invade canaliculi via extrusion of daughter cells through asymmetric binary fission [97••, 98]. The bacteria are able to deform from spherical cocci to rod-shaped bacterium and migrate towards the osteocytic lacunae via proliferation at the leading edge [97••, 98].

Others have also challenged the classical understanding of S. aureus by reporting evidence of active cell motility via spreading dendrites, as well as reporting altered cell growth phenotype by identifying SCVs [19, 100, 101]. S. aureus passive spreading across solid or soft media, aided by PSM production, is well established [102, 103]. However, evidence that these bacteria are capable of active propulsion has only recently emerged.

Collectively, these findings suggest a novel mechanism of bacterial persistence in chronic osteomyelitis and may provide insight as to why S. aureus infection in the setting of chronic PJI is so challenging to treat.

Beyond Biofilm: Soft Tissue Abscess Communities

Another hallmark of S. aureus infection is the formation of abscess lesions or staphylococcal abscess communities (SACs) [69•]. SACs, similar to biofilms, serve as another strategic survival mechanism where the organism thrives in a multi-cellular community while impairing host immunity. It has been shown that abscess formation is not solely a host driven response but rather a pathogen driven process where S. aureus synchronously triggers the stages of SAC formation by discrete regulation of protein production [69•, 104]. After invasion of host soft tissue, S. aureus produces factors that actively recruit host immune cells; however, this recruitment is not effective in killing the organism [104]. Instead, a central, replicating group of bacteria develop and are surrounded by a protective pseudocapsule of fibrin [104]. This SAC is then surrounded by a peripheral ring of necrotic neutrophils, which add to soft tissue injury and further impair immune function, followed by an outer layer of surviving immune cells, which are ineffective in removing the pathogen [104]. Proteins such as staphylococcal nuclease and adenosine synthase A convert neutrophilic NETs into products that induce macrophage cytotoxicity, which may be a significant mechanism in protecting bacteria within the SAC [68••]. Eventually, these SACs can mature and rupture, releasing the bacterium into the surrounding tissue [69•]. The formation of SACs rely on many genes that are important in biofilm formation including ECM binding factors such as ClfA, iron transport proteins, protein A, and genes encoding capsular polysaccharides [69•, 104].

Emerging Treatments: Targeting Biofilm

In recent years, there has been a renewed focus on emerging therapeutic treatments directed at biofilm formation including modifications of systemic antibiotic protocols, novel systemic therapies, and local treatments of the infected periprosthetic environment.

The clinical emergence of S. aureus resistance to conventional systemic antibiotics has led to modifications in antibiotic regimens to target bacteria within biofilm. Rifampicin inhibits bacterial RNA polymerase and exhibits bactericidal activity against biofilm-forming microorganisms; however, rapid resistance develops when used as a monotherapy. In vivo studies suggest that addition of rifampicin to current standard of care systemic antibiotics reduces colony-forming units in infected periprosthetic tissues and may reduce biofilm formation [105, 106]. These data have been supported by clinical studies, which show improved infection eradication with the addition of rifampicin to other antibiotic regimens including vancomycin [2, 107–109]. The efficacy of current systemic antibiotic regimens in targeting bacteria within an established biofilm remains controversial, however, and minimum biofilm eradication concentrations (MBEC) for a given antibiotic can be many fold higher than minimum inhibitory concentration (MIC) [110–112]. Additionally, the MBEC can be greater than achievable concentrations from systemic treatment, suggesting that alternative treatments targeting biofilm itself are necessary [110].

Immunotherapy is another major area of interest, which may provide complementary treatment for antibiotic therapies against S. aureus infection. Vaccine strategies for S. aureus infection have been attempted, but none have been successful beyond phase I clinical trials [113, 114•, 115]. Some of these vaccine strategies have failed because they attempted to target components of the cell wall that are not universally expressed across strains including poly-N-acetyl glucosamine and LTA acid [113]. One S. aureus vaccine that showed promise in early clinical studies targeted capsular polysaccharides (types 5 and 8) conjugated to a recombinant Pseudomonas aeruginosa exotoxin A. However, in a larger trial, this vaccine failed to reduce infection in hemodialysis patients [113, 116]. Another vaccine from Merck (V710) showed preclinical promise by targeting IsdB [114•]. Unfortunately, V710 failed to reduce infection rates or mortality in a phase 2b/3 trial, which attempted to prevent S. aureus infection after cardiothoracic surgery. Moreover, patients who did get infected were more likely to die in the vaccine group, suggesting that this vaccine may actually impair host immunity against sepsis [114•]. Thus, developing animal models that more faithfully replicate human surgical site infections may be critical for elucidating host humoral and cell-based immune responses to S. aureus and are important to future development of vaccine-based strategies.

Passive immunization strategies to components of biofilm or bacterial cells may show more promise than vaccine-based approaches. One approach targeting the biofilm itself used a monoclonal antibody (mAb) to DNA binding proteins from the DNABII family, which has conserved homologs across many bacterial species including S. aureus [117]. Using this mAb in combination with daptomycin systemic therapy, reductions in both planktonic and adherent bacteria were found in a murine implant-associated infection model relative to daptomycin monotherapy [117]. Wang et al. used monoclonal antibodies to α-toxin and ClfA to directly target biofilm formation. Combination of the two mAbs resulted in decreased colony-forming units from bone/joint tissue, reduced propensity for infection, and less biofilm aggregates in a murine model of hematogenous MRSA infection [39••]. Other work has focused on developing immunotherapy targeting the glucosaminidase (Gmd) subunit of autolysin (Atl), which is an immunodominant S. aureus antigen [118, 119•]. In a murine model of implant-associated MRSA infection, synergistic activity between anti-Gmd therapy and systemic vancomycin reduced peri-implant bacterial burden, osteolysis volume from established osteomyelitis, and resulted in sterilization of staphylococcal abscess communities (SACs) relative to monotherapies alone [119•]. Intracellular reservoirs of S. aureus within host immune cells may be another mechanism of bacterial persistence and reemergence after treatment [120••]. In order to target these cells, Lehar et al. created an antibody-antibiotic conjugate (AAC) that consists of a monoclonal antibody that recognizes the alpha-O-linked N-acetylglucosamine sugars on wall teichoic acids (WTAs) bound to rifamycin class derivative antibiotic (similar in properties to rifampicin) [120••]. This AAC binds to the surface of S. aureus, and then upon opsonization, the proteolytic environment of the phagolysosome of the host cell causes release of the active antibiotic form [120••]. This AAC demonstrated improved results versus systemic vancomycin alone in a murine MRSA hematogenous infection model.

Dispersal of S. aureus biofilms is another major focus for antimicrobial therapeutics. By converting the biofilm bacteria to planktonic form, these cells will be more susceptible to common antibiotics that were previously unable to infiltrate the biofilm. Enzymatic treatments like proteinase K, trypsin, dispersin B, lysostaphin, DNases, and fibrinolytics have shown promise in their ability to disperse staphylococci from biofilm by acting on key structural components of the biofilm matrix [88, 121, 122]. Dispersion B for example is an enzyme discovered in Aggregatibacter actinomycetemcomitans and acts on in methicillin-sensitive strains by hydrolyzing the polysaccharide intercellular adhesin (PIA), which is a key factor in biofilm formation for S. aureus [121]. Other agents like lysotaphin lyse the interbridges on the bacterial cell walls but are not strain dependent [121]. Fibrinolytics like streptokinase or nattokinase break down the fibrin matrix within biofilm and decrease the minimum biofilm eliminating concentration (MBEC) of available systemic antibiotics [110, 122]. Targeting the quorum-sensing system is another strategy to trigger biofilm dispersal, and autoinducing peptide type I (AIP-1) treatment in vitro was able to trigger dispersal of MRSA on titanium discs, increasing its susceptibility to antibiotic therapy [123]. One of the major concerns surrounding dispersal agents is that bacteria cells disassembled from their biofilms are more capable of systemic infection in other locations in the body. Thus, agents like the ones previously stated likely must be used in combination with systemic therapies such as antibiotics [121, 122].

Biomaterials-based approaches have also shown promise in the treatment of PJI. Improving biofilm resistance on the implant surface would reduce rates of treatment failure after PJI. Silver-based implant coatings have shown clinical promise in megaprostheses after segmental bone resection and post-fracture [124, 125•, 126]. A silver-coated megaprosthesis (Alguna) reduced rates of postoperative infection (11.8 versus 22.4%) and had improved success after debridement and implant retention relative to titanium implants in a small case control study [125•]. It has also been used on urinary catheters, vascular grafts, and endotracheal tubes with varying degrees of success [127, 128]. Concerns about local and systemic toxicity have led to alternative approaches to using silver as an antimicrobial in PJI. Use of additive manufacturing and nanoparticle-based silver may improve upon its efficacy while reducing the potential off-target effects [129].

Novel antibiotic or small molecule implant coatings may also be a successful strategy to reduce biofilm formation on orthopedic implants; however, most of these studies have not progressed beyond the preclinical stages. One group of strategies focus on increasing the local concentration of antimicrobial therapies, while promoting host cell binding to the implant surface to the exclusion of bacterial colonization. Covalent bonding of an antibiotic to titanium may reduce biofilm formation on the implant without impairing osseointegration and host cell attachment [130]. Surface modifications that enhance osseointegration of titanium in combination with antibiotic coatings may be another strategy to increase resistance to biofilm formation while promoting host cell adhesion [131]. Use of antibiotic carriers such as hydrogels or phosphatidylcholine-based materials allow point of care temporary implant coatings that elute antibiotics from the implant surface and provide biofilm resistance [132–134]. Treatment of the infected implant may provide another strategy to reduce the morbidity of PJI. Ehrensberger et al. used cathodic voltage-controlled electrical stimulation to titanium with an established biofilm, and this reduced both planktonic and biofilm-associated MRSA in vivo [135]. Other local treatments available in the operating room include heat (autoclave) and topical scrubs. Leary et al. showed that the combination of autoclave and scrubbing with 4% chlorhexidine was able to remove over 99% of biofilm on a cobalt chromium disc with established S. aureus and Staphylococcus epidermidis [136]. Treatment of the implant surface remains a critical component to infection eradication in PJI; however, further advancement in the clinical development of these treatments is necessary.

Conclusion

Improvements in our knowledge of S. aureus pathogenesis, and host immunity against this common pathogen, should provide novel therapeutic mechanisms to enhance eradication after PJI. Challenges remain given the diverse mechanisms developed by S. aureus to alter the host immune responses. The addition of local treatments to the implant to increase its resistance to bacterial colonization and therapies that target biofilm would increase the modalities at our disposal for treating this challenging infection.

Conflict of Interest

Edward M Schwarz reports grants from NIH and AOTrauma during the conduct of the study and personal fees and other from Telephus, LLC, outside the submitted work. In addition, Dr. Schwarz has a patent on passive immunization and diagnostics for S. aureus licensed to Telephus, LLC.

The other authors declare that they have no conflicts of interest.

EMS has patents related to this work. EMS has received financial compensation and stock from Telephus Medical LLC.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors.

Grant Sponsors

National Institutes of Health (EMS), Grant Numbers P30 AR069655 and P50 AR07200 (EMS), and AOTrauma Clinical Priority Program (EMS).