Impact of Staphylococcus aureus on Pathogenesis in Polymicrobial Infections

Abstract

Polymicrobial infections involving Staphylococcus aureus exhibit enhanced disease severity and morbidity. We reviewed the nature of polymicrobial interactions between S. aureus and other bacterial, fungal, and viral cocolonizers. Microbes that were frequently recovered from the infection site with S. aureus are Haemophilus influenzae, Enterococcus faecalis, Pseudomonas aeruginosa, Streptococcus pneumoniae, Corynebacterium sp., Lactobacillus sp., Candida albicans, and influenza virus. Detailed analyses of several in vitro and in vivo observations demonstrate that S. aureus exhibits cooperative relations with C. albicans, E. faecalis, H. influenzae, and influenza virus and competitive relations with P. aeruginosa, Streptococcus pneumoniae, Lactobacillus sp., and Corynebacterium sp. Interactions of both types influence changes in S. aureus that alter its characteristics in terms of colony formation, protein expression, pathogenicity, and antibiotic susceptibility.

INTRODUCTION

Staphylococcus aureus is an opportunistic and resilient human pathogen that colonizes the mucosal surfaces. It is the causative agent of many serious acute and chronic infections. The anterior nares are the primary reservoirs of S. aureus. Asymptomatic colonization occurs in approximately 20% of the normal population, and 60% are transiently colonized, while 20% appear to be rarely or never colonized (1). Extranasal colonization of S. aureus also takes place in several locations, including the skin, rectum, axillae, vagina, pharynx, and gastrointestinal tract (2).

S. aureus causes numerous infections, including skin infections (boils, furuncles, styes, impetigo), surgical and trauma wounds, urinary tract infections, gastrointestinal tract infections, pneumonia, osteomyelitis, endocarditis, thrombophlebitis, mastitis, meningitis, infections on indwelling medical devices, toxic shock syndrome (TSS), and septicemia (3, 4). The factors contributing to the rise of this organism as a formidable pathogen involve multiple mechanisms of virulence. These include the evolution of strategies to resist antibiotics and evade host defenses, as well as the production of an arsenal of virulence factors such as capsule, coagulase, lipase, hyaluronidase, protein A, fibrinogen binding proteins, fibronectin binding proteins, and secreted toxins such as secreted enterotoxins (SEs), toxic shock syndrome toxin-1 (TSST-1), Panton-Valentine leucocidin (PVL), hemolysins, and phenol-soluble modulins (PSM) (5–9).

Several studies have confirmed S. aureus as one of the coinfecting microbes in many patients with polymicrobial infections (10). The interactions between S. aureus and the coexisting microbes are either cooperative, as with Candida albicans (11–14), Enterococcus faecalis (15, 16), Haemophilus influenzae (17–19), and influenza virus (20, 21), or competitive, as with Pseudomonas aeruginosa, Streptococcus pneumoniae (18, 19), Lactobacillus sp. (22–27), and Corynebacterium sp. (17, 28–30). Irrespective of whether the interactions are cooperative (Fig. 1) or competitive (Fig. 2), S. aureus within a community behaves differently with respect to its monomicrobial growth. This article focuses on reviewing the significance of interactions between S. aureus and other microorganisms and its effect on disease progression and outcome.

FIG 1.

Cooperative interactions between S. aureus and other microbes. S. aureus can cocolonize with H. influenzae, E. faecalis, C. albicans, and influenza virus. S. aureus-induced lysis of red blood cells (RBC) leads to the release of hemin and NAD, which act as nutrients and support the growth of H. influenzae. S. aureus secretes proteases that cleave the host sialic acid receptor and increase the infectivity of influenza virus by releasing the virus from the host cell surface. S. aureus gained vancomycin resistance from E. faecalis due to horizontal gene transfer and became more resistant to antibiotics during coinfection with C. albicans. Symbols: SA, S. aureus; VRSA, vancomycin-resistant S. aureus; VREF, vancomycin-resistant E. faecalis.

FIG 2.

Competitive interactions between S. aureus and other microbes. S. aureus exhibits antagonism toward P. aeruginosa, Streptococcus sp., and Lactobacillus sp. P. aeruginosa produces phenazine (PZ), hydrogen cyanide (HCN), quinolone oxidase (QO), and pyocyanin (PY), resulting in the respiratory blockage of S. aureus, which in turn leads to the formation of small-colony variants (SCVs). SCVs are more persistent and are resistant to antibiotics. Lactobacillus sp. and Streptococcus sp. inhibit the growth of S. aureus by producing hydrogen peroxide (H₂O₂). S. aureus produces staphyloxanthin and catalase, which neutralize the toxic effects of H₂O₂. Additionally, Lactobacillus spp. produce organic acids and bacteriocins that limit the growth of S. aureus. Certain S. aureus strains also produce bacteriocins such as staphylococcin Au 26, which in turn inhibit the growth of lactobacilli. Blocked arrows indicate antagonism, and arrows indicate survival strategies of S. aureus.

Interactions with Candida.

Both Candida species and S. aureus usually exist as commensals and colonize human mucosal surfaces. Furthermore, they are opportunistic pathogens and cause a wide range of infections such as sepsis, pneumonia, denture stomatitis, and neonatal sepsis. Despite causing a number of infections independently, C. albicans and S. aureus can also be coisolated from several diseases such as cystic fibrosis, superinfection of burn wounds, urinary tract infections, and diabetic foot wounds and from the surfaces of various biomaterials, including dentures, voice prostheses, implants, endotracheal tubes, feeding tubes, and catheters (31–34).

Biofilm-embedded microbes are extremely resistant to both host clearance mechanisms and antimicrobial agents. S. aureus and C. albicans are often isolated concurrently from mixed bacterial-fungal biofilms on implanted medical devices (35). During biofilm-associated coinfections, C. albicans forms the base of the biofilm and facilitates the biofilm formation of S. aureus. C. albicans hyphal protein agglutinin-like sequence 3 (Als3p) mediates the binding of S. aureus with C. albicans hyphae (14, 36, 37). Within the polymicrobial biofilm, S. aureus exhibits enhanced resistance to vancomycin (13).

Independent studies demonstrated that the interactions between S. aureus and C. albicans enhance disease severity in several ways (33, 38). Candidal infections cause physical damage to organ walls, allowing S. aureus to penetrate the internal organs more easily. S. aureus, on the other hand, secretes different proteases that help C. albicans to enhance its adhesion to the mucosal layer (12). During systemic infections, each organism helps the other to evade phagocytic killing mediated by polymorphonuclear leukocytes (PMNs). C. albicans secretes a proteinase that degrades the Fc portion of immunoglobulin G (IgG) and greatly reduces the opsonizing activity of human PMNs against S. aureus (39). On the other hand, S. aureus secretes coagulase and extracellular fibrinogen binding proteins (Efb) that protect Candida sp. from PMN-mediated phagocytosis. Coagulase activates prothrombin, which mediates the conversion of fibrinogen to fibrin. Formation of fibrin clots surrounding the candidal cells helps Candida spp. to evade phagocytic killing by granulocytes (40). Additionally, Efb binds to C3 complement and interferes with complement activation and C3-mediated opsonization (41). The cooperative infection of C. albicans and S. aureus represents a significant therapeutic challenge, and their coisolation from blood is an indication of a dire prognosis (42).

Competitive or antagonistic relationships between C. albicans and S. aureus have also been reported where the farnesol quorum-sensing molecule secreted by C. albicans inhibits the biofilm formation of S. aureus. Farnesol disrupts the S. aureus cell membrane integrity and thereby its viability. Additionally, in vitro results demonstrated that farnesol-treated S. aureus showed enhanced susceptibility to a variety of clinically important antibiotics (43). However, it is as yet unclear how much farnesol C. albicans secretes under in vivo conditions and whether the secreted concentrations are sufficient to inhibit the growth of S. aureus in vivo. Nevertheless, all available in vivo data suggest that S. aureus and C. albicans exist in synergy. Apart from Candida albicans, S. aureus was also isolated together with Candida tropicalis, Candida parapsiolosis, and Trichosporon asahii (44, 45).

Interactions with influenza virus.

The mechanisms of interaction of S. aureus with influenza virus are much more complex than the interactions between S. aureus and C. albicans. Superinfection of influenza virus and S. aureus is one of the major causes of severe influenza pneumonia, prolonged inflammation, and higher mortality rates. This represents the best-known model of bacterial-viral coinfection (20).

Influenza virus A infection promotes and enhances the nasopharyngeal adherence of S. aureus (46). On the other hand, S. aureus promotes the infectivity and spread of the influenza virus particles. Hemagglutinin (HA), a trimeric glycoprotein, present in multiple copies in the membrane envelope of influenza virus, is responsible for the attachment of the virus particle to sialic acid-containing receptors of the host ciliated columnar epithelial cells. Proteolytic cleavage of the hemagglutinin is an important prerequisite for the infectivity of the influenza virus and for the spread of the virus in the host organism and associated pathogenicity. Several strains of S. aureus have been found to secrete serine proteases that activate infectivity of influenza virus by proteolytic cleavage of the hemagglutinin (21).

Coinfections of S. aureus and influenza virus may lead to severe disease outcome, as influenza virus infection enhances the deleterious effects of staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin 1 (TSST-1) (47, 48). SEB and TSST-1 are superantigens that activate T cells in an uncontrolled manner and cause massive systemic release of cytokines. Concurrent S. aureus and influenza virus infection induces enterotoxin-mediated massive release of tumor necrosis factor alpha (TNF-α) and gamma interferon (IFN-γ). This results in fever, rash, hypotension, tissue injury, and shock. It has been hypothesized that the lethal synergism between concurrent influenza infection and S. aureus superantigen exposure may contribute to sudden and unexpected death from influenza virus infection (49).

Interactions with other bacteria.

The majority of the interactions between S. aureus and other bacterial species are competitive in nature, and only a few interactions are cooperative. Cooperative interactions involving S. aureus exist with H. influenzae and E. faecalis. Competitive interactions are observed between S. aureus and other bacteria, viz., Pseudomonas aeruginosa, Streptococcus pneumoniae, lactic acid bacteria (LAB), Corynebacterium sp., or S. epidermidis. That the interactions are competitive does not mean that these organisms completely inhibit the colonization of S. aureus; rather, S. aureus employs numerous defense strategies for its survival, counterattacking the competing bacteria and surviving in the same ecological niche. Cooperative or competitive interactions lead to the development of more-persistent S. aureus strains with altered colony morphology, antibiotic resistance, and increased virulence. The interactions of S. aureus with other bacterial species are listed below.

(i) Interactions with Haemophilus influenzae.

S. aureus and H. influenzae both colonize the nasopharynx and, in some instances, the conjunctivae and genital tract. H. influenzae reaches higher colony densities when the resident colonizer is S. aureus. The higher H. influenzae colony densities have been attributed to the availability of nutrients that S. aureus provides to facilitate its growth (19). S. aureus produces three major hemolysins (α, β, and γ) which lyse erythrocytes by compromising their membrane integrity (50). The hemolysis of erythrocytes by S. aureus-secreted hemolysins releases nutrients such as hemin and NAD, which are vital for the growth of H. influenzae (51–53). Margolis et al. demonstrated synergistic interactions of S. aureus and H. influenzae in the rat nasopharynx (19). However, Pettigrew et al. and van den Bergh et al. studied the compositions of nasal microflora among children and have reported antagonism or negative association between S. aureus and H. influenzae (54, 55). Both of those studies were designed to determine the microflora composition among children in the age group between 6 and 36 months.

(ii) Interactions with Pseudomonas aeruginosa.

The relation between S. aureus and P. aeruginosa is competitive in nature, although the two organisms are frequently found together in clinical settings. They have common niches within the host, for example, the lungs of cystic fibrosis (CF) patients, peritoneum of dialysis patients, catheters, diabetic foot wounds, and other type of wounds caused by skin injury or skin burn (44, 56). S. aureus is often reported as the primary pathogen infecting the lungs of the CF patients, followed by P. aeruginosa. Although coinfections of these pathogens are very common under in vivo conditions, several independent in vitro studies demonstrated that, when cocultured together, P. aeruginosa thrives better than S. aureus (57–59). The better survival of P. aeruginosa is attributed to its ability to produce respiratory toxins such as pyocyanin, hydrogen cyanide, and alkyl-hydroxyquinoline N-oxides that can block the electron transport pathway, thereby inhibiting the growth of S. aureus and other pathogenic staphylococci (57, 58).

Despite its sensitivity to respiratory inhibitors, S. aureus does not get completely cleared away by P. aeruginosa. To counter the effect of the respiratory toxins produced by P. aeruginosa, S. aureus forms electron transport-deficient small-colony variants (SCVs) that grow as tiny, nonpigmented colonies (57). Purified 4-hydroxy-2-heptylquinoline-N-oxide (HQNO) or pyocyanin produced by P. aeruginosa is sufficient to induce SCV selection in S. aureus (57, 59). These SCVs are auxotrophic to hemin or menadione and are resistant to antibiotics, especially aminoglycosides, trimethoprim-sulfamethoxazol (60), and the host antimicrobial peptide lactoferricin B (8). The resistance of SCVs is due in part to their severely decreased membrane potential as well as their reduced growth rate and metabolic processes. These SCVs also persist better than their normal counterparts.

P. aeruginosa also produces a 20-kDa endopeptidase, LasA, which selectively cleaves S. aureus peptidoglycan. LasA cleaves the glycyl-glycine and glycyl-alanine bonds of the pentaglycine interpeptide bridge in the S. aureus peptidoglycan and induces lysis (61, 62). Using the rat model of infection, Mashburn et al. showed that P. aeruginosa can lyse S. aureus cells and that the iron-containing proteins released from the lysed S. aureus cells serve as the source of iron, thereby increasing the pathogenic potential of P. aeruginosa (63, 64). However, this result is yet to be validated in clinical settings. P. aeruginosa exhibits a similar kind of antagonistic relationship with S. epidermidis, as well as with species representatives of S. haemolyticus, S. saprophyticus, S. hyicus, S. muscae, and S. lugdunensis (58).

(iii) Interactions with Streptococcus pneumoniae.

The relation between S. pneumoniae and S. aureus is antagonistic. S. pneumoniae and S. aureus colonize the upper respiratory tract of children and compete with each other for the same niche (59, 65, 66). Various studies have shown that colonization of the upper airway by S. pneumoniae is negatively correlated with S. aureus colonization and that children who are vaccinated with pneumococcal conjugate vaccines are at major risk of S. aureus infections (18). This inverse relation suggests that one organism interferes with the colonization of the other. In vitro data demonstrate that hydrogen peroxide (H₂O₂), a byproduct of aerobic metabolism produced by S. pneumoniae, is responsible for the antagonistic relationship between these two pathogens (67). H₂O₂ production leads to the production of DNA-damaging hyperoxides through the Fenton reaction that induces the SOS response. The SOS response induces the resident prophages, resulting in the lysis of lysogenic staphylococci. Because the vast majority of S. aureus strains are lysogenic, the production of H₂O₂ is a very effective antistaphylococcal strategy of S. pneumoniae. H₂O₂, at concentrations typically produced by pneumococci, kills lysogenic but not nonlysogenic staphylococci (68). Pneumococci, however, are not SOS induced upon exposure to H₂O₂, as they are resistant to the DNA-damaging effects of the Fenton reaction (69).

It is interesting that S. aureus, which produces so many antioxidants and free radical scavengers, including catalase, alkyl hydroperoxide reductase, superoxide dismutase (SodA and SodM), and staphyloxanthin (16, 70), is susceptible to H₂O₂ produced by S. pneumoniae. A possible explanation could be that the amounts of free radical scavengers that S. aureus produces are not sufficient to neutralize all the H₂O₂ produced by S. pneumoniae. Regev-Yochay et al. demonstrated that staphylococcal species that secrete higher concentrations of catalase are resistant to S. pneumoniae (67).

However, other studies have offered hypotheses suggesting that the production of hydrogen peroxide may not be the main reason for the antagonistic relationship between these pathogens in vivo (71). Although both pathogens colonize the upper respiratory tract, their microniches are different. Therefore, direct antagonism mediated by H₂O₂ is an unlikely reason for their antagonism. Rather, the antibody response generated during S. pneumoniae infection, although ineffective in restricting this pathogen itself, is effective in providing cross-protection against S. aureus (71, 72).

(iv) Interactions with LAB.

The lactic acid bacteria (LAB) consist of a group of heterogeneous bacterial species comprising nonsporulating, Gram-positive cocci and bacilli that are able to ferment sugars predominantly into lactic acid. This leads to acidification of the environment down to a pH of 3.5. LAB colonize the gut and urogenital tract and contribute to defense against S. aureus-mediated food poisoning and genital infections. The antistaphylococcal activity of LAB strains is attributed to the production of H₂O₂, organic acids, antimicrobial proteins, biosurfactants, surface proteins, and quorum-sensing inhibitors. The most commonly studied members of intestinal and vaginal LAB include Lactobacillus acidophilus, L. casei, L. fermentum, L. salivarius, L. rhamnosus, L. gasseri, L. vaginalis, L. johnsonii, and L. delbrueckii (25, 73–75).

In similarity to the results seen with S. pneumoniae, LAB-produced hydrogen peroxide (H₂O₂) inhibits the growth of S. aureus (76, 77). Additionally, LAB secrete organic acids (lactic, acetic, formic, caproic, propionic, butyric, and valeric acids) that inhibit the growth of S. aureus (78). LAB-produced bacteriocins interfere with cell wall structure and biosynthesis and form pores in the S. aureus membrane (79). Among the bacteriocins produced by LAB, the most important are nisin, produced by Lactococcus lactis; pediocin, produced by Pediococcus acidilactici; and lacticin 3147, produced by Lactococcus lactis DPC 3147 (79, 80).

Apart from inhibiting the growth of S. aureus by the use of H₂O₂, organic acids, and bacteriocins, LAB compete with S. aureus for the host cell adhesion sites. Biosurfactants and surface proteins of LAB strains are involved in this competitive exclusion process. L. fermentum, L. acidophilus, L. crispatus CRL 1266, L. paracasei subsp. paracasei CRL 1289, L. salivarius CRL 1328, L. rhamnosus GG, Lactococcus lactis subsp. lactis, and Propionibacterium freudenreichii subsp. shermani were shown to disrupt the adherence of S. aureus to the intestinal and urogenital tract by competing for the same adhesion sites. Some LAB strains were also shown to displace previously adhered S. aureus from the vaginal epithelial cells (27). In a recent study, it was also shown that the small signaling molecule cyclic dipeptides cyclo(l-Tyr-LPro) and cyclo(l-Phe-l-Pro), produced by the human vaginal isolate L. reuteri RC-14, are able to interfere with the staphylococcal quorum-sensing system agr, a key regulator of virulence genes, and repress the expression of staphylococcal exotoxin TSST-1 (81).

To counter the detrimental effects of LAB species, S. aureus produces bacteriocins that have antibacterial activity against LAB. For example, S. aureus secretes bacteriocins such as staphylococcin Au-26, Bac1829, BacR1, aureocin A70, and aureocin A53 that inhibit the growth of lactobacilli (80).

(v) Interactions with Corynebacterium sp.

S. aureus and Corynebacterium sp. are two of the most important species infecting the skin and nasopharynx. Both organisms are associated with catheter-related infections. A lower incidence of S. aureus colonization has been observed in individuals heavily colonized by Corynebacterium sp. (C. accolens, C. pseudodiptheriticum, and C. tuberculostearicum). Corynebacterium spp. utilize competitive exclusion strategies similar to those of LAB in competing with S. aureus for the same adhesion site with host mucosal epithelial cells (30). No bacteriocin-like activity of Corynebacterium sp. against S. aureus has been reported. However, a number of bacteriocins secreted by S. aureus are active against Corynebacterium sp. These bacteriocins include Bac1829 (17), aureocin A70 (29), aureocin A53 (82) and staphylococcin 188 (28).

(vi) Interactions with S. epidermidis.

Besides these interactions, S. aureus is also known to interact with members of the same genus. Several reports indicate antagonistic relationships between S. aureus and S. epidermidis. Both S. aureus and S. epidermidis are opportunistic and nosocomial pathogens. Unlike S. aureus, which causes severe acute infections, S. epidermidis frequently causes chronic infections and has an exceptional capacity to attach to the indwelling medical devices during surgery and form biofilms. The presence of S. epidermidis in the nasal cavities has been reported to correlate with the absence of S. aureus (83). Similar to S. pneumoniae, this pathogen uses multiple strategies to inhibit S. aureus colonization. These include production of autoinducing peptide (AIP), phenol-soluble modulins (PSM), and bacteriocins. The production of virulence factors and other extracellular proteins in staphylococci is globally regulated by the accessory gene regulatory system (agr). agr encodes a two-component signaling pathway whose activating ligand is AIP, which is also encoded by agr (84). The AIPs can activate the agr response in the other members of the same group but show mutually inhibitory effects between members of different groups. Based on the agr loci present, S. aureus strains have been divided into 4 major groups, agr-1_Sa to agr-4_Sa, and S. epidermidis into 3 major groups, agr-1_Se to agr-3_Se (85). S. epidermidis AIP has been proven to inhibit the activity of agr-1_Sa to agr-3_Sa and thereby suppress the expression of virulence factors such as the alpha-toxin, β-toxin, δ-toxin, serine protease, DNase, fibrinolysin, enterotoxin B, and toxic shock syndrome toxin 1 in S. aureus. Among S. aureus AIPs, only agr-4_Sa weakly inhibits the activity of agr-1_Se (30, 86).

Additionally, S. epidermidis secretes an extracellular serine protease (Esp) that, alone or in combination with host β-defensin 2, eliminates S. aureus biofilms. Esp cleaves S. aureus major autolysin (Atl) protein and interferes with its function (87). Activity of Atl is necessary for DNA release and biofilm formation of S. aureus (88). Phenol-soluble modulins (PSMγ and PSMδ) and bacteriocins (Pep5, epidermin, epilancin K7, and epicidin 280) produced by S. epidermidis inhibit the growth of S. aureus. S. epidermidis-secreted PSM peptides cooperate with each other and with the host antimicrobial peptide, LL-37, to exert selective antimicrobial action against S. aureus (9, 89).

(vii) Interactions with Enterococcus faecalis.

The anterior nares are generally considered to be the primary site of colonization of S. aureus; however, low concentrations (≤10⁵ CFU/g of feces) of this organism cocolonize the intestinal tracts together with E. faecalis in healthy humans. Both S. aureus and E. faecalis normally exist as commensals, but they can turn into opportunistic pathogens causing urinary tract infections, bacteremia, and infective endocarditis (15). Apart from the intestinal tract, E. faecalis and S. aureus are frequently isolated from the respiratory tract, urinary tract, and chronic foot ulcers and from diabetic foot wounds (44). The interaction between E. faecalis and S. aureus is neither truly synergistic nor antagonistic.

Many studies have focused on the mechanisms by which S. aureus acquired the vancomycin resistance gene from E. faecalis. Vancomycin-resistant S. aureus (VRSA) strains emerged due to horizontal transfer of a Tn1546 transposon containing the vanA gene from vancomycin-resistant E. faecalis (90–92). The transposon Tn1546 harboring the vanA gene present on the pAM830 plasmid is related to the Inc18 family of broad-host-range conjugative plasmids and is responsive to the cAM373 pheromone secreted by the plasmid-free (recipient) strains of E. faecalis. cAM373 triggers the process of conjugation, leading to the transfer of the vanA gene from the vancomycin-resistant E. faecalis (donor) strains to the vancomycin-susceptible E. faecalis (recipient) strains (93). S. aureus is also known to secrete a peptide, staph-cAM373 (amino acid sequence AIFILAA), with activity similar to that of E. faecalis cAM373 (amino acid sequence AIFILAS) that triggers the process of conjugation between vancomycin-resistant E. faecalis (donor) and S. aureus (recipient) (94). This conjugation results in the transfer of the vanA gene from E. faecalis to S. aureus. Genetic analysis of several vancomycin-resistant S. aureus (VRSA) strains showed that transposon Tn1546 harboring the vanA gene either jumped into a staphylococcal plasmid or integrated into the S. aureus chromosome (16, 91, 95). The acquisition of vanA by S. aureus resulted in incorporation of d-alanyl-d-lactate (d-Ala-d-Lac) precursors into the peptidoglycan instead of d-alanine-d-alanine (d-Ala-d-Ala). The E. faecalis and S. aureus cell wall harboring the d-Ala-d-Lac precursors has 1,000-fold less affinity for vancomycin, a drug that is considered the last-resort antibiotic to treat methicillin-resistant S. aureus (MRSA) infections (96). Interactions between these two bacteria have led to an increase in the numbers of multidrug-resistant staphylococci.

CONCLUSION

Most infections are polymicrobial in nature and can be seen in almost every niche in the human body, particularly in mucosal surfaces, where different species of microorganisms such as bacteria, fungi, and viruses coexist as communities. S. aureus is one of the most common pathogens found in polymicrobial infections. In polymicrobial infections, S. aureus differentially modulates host immune responses and disease severity and acquires altered growth and antibiotic susceptibility patterns. The altered immune response during polymicrobial infections could be beneficial or detrimental for S. aureus. For example, influenza virus infection inhibits Th₁₇-mediated adaptive immune responses (97). Activated Th17 cells are necessary for protection against S. aureus infection, because this subset of T cells enhances neutrophil recruitment to sites of infection and kills S. aureus (98, 99). Therefore, Th₁₇ cell-mediated immune activation is necessary to limit S. aureus infections. By inhibiting the Th₁₇ cell-mediated immune response and subsequent neutrophil infiltration, influenza virus helps S. aureus to colonize and to cause severe secondary bacterial pneumonia (97, 100). In contrast to the immune suppression mediated by influenza virus that aids S. aureus, S. pneumoniae-mediated immune activation is detrimental to S. aureus. The antibody response generated during S. pneumoniae infection against its glyceraldehyde-3-phosphate dehydrogenase, although ineffective in inducing opsonophagocytic killing of S. pneumoniae, can cross-react with staphylococcal protein 1-pyrroline-5-carboxylate dehydrogenase and induce opsonophagocytic killing of S. aureus (71, 72). S. pneumoniae itself is protected from opsonophagocytic killing due to its antiopsonic polysaccharide capsule.

Additionally, S. aureus in polymicrobial infections displays enhanced persistence and antibiotic tolerance. S. aureus acquired vancomycin resistance genes from E. faecalis and became resistant to vancomycin (16, 91, 95). S. aureus, during coinfection with C. albicans, showed increased vancomycin resistance (13, 101). This bacterium forms electron transport-deficient small-colony variants during coinfection with P. aeruginosa (57, 58). These SCVs persist better than their normal counterparts and are resistant to aminoglycosides and trimethoprim-sulfamethoxazols (102).

A 23-valent polysaccharide vaccine against S. pneumoniae which was recently introduced into the market indeed prevented S. pneumoniae nasopahryangeal colonization, but the vaccinated individuals were subject to an increased risk of S. aureus nasal colonization (72). Therefore, prevention of one pathogenic infection provides opportunities to the competing pathogens to cause disease. These findings highlight the potential complications that could arise from conventional treatment and disease prevention strategies that target a single organism, thereby necessitating the need to introduce modified therapeutic approaches that take into account the coinfecting organisms. Several strategies could be used to address the difficulties in treatment of polymicrobial infections of S. aureus. One could be the use of combined vaccines against two or more coinfecting microbes; however, such vaccines are still in the experimental stages. The next approach could be the judicious use of antimicrobial drugs. A coinfection of S. aureus and influenza virus should be treated with antiviral and appropriate antibacterial drugs. A third approach is the use of LAB strains to prevent not all but some of the S. aureus infections. Probiotic LAB can prevent intestinal and urogenital tract coinfections. Studies have shown that regular intake of probiotic LAB and fermented milk can even reduce S. aureus colonization in the upper respiratory tract. Similarly, probiotic LAB species also confer protection against influenza virus by modulating innate immunity. Thus, probiotic bacteria can be used to prevent coinfections of S. aureus and influenza virus.

In summary, S. aureus in polymicrobial infections represents a clinical challenge greater than that of S. aureus in monomicrobial infections. The coexisting microbes significantly influence the outcome of the infection by altering invasion ability, growth, gene expression, and drug sensitivity patterns. Further investigations are required to design appropriate treatment strategies to tackle polymicrobial infections mediated by S. aureus.