ABSTRACTCandida albicansandStaphylococcus aureusare among the most prevalent nosocomial pathogens that are responsible for severe morbidity and mortality, even with appropriate treatment. Using a murine model of polymicrobial intra-abdominal infection (IAI), we have previously shown that coinfection with these pathogens results in synergistic lethality that is partially dependent on exacerbated prostaglandin signaling, while monomicrobial infection is nonlethal. Therefore, the objective of this study was to identify staphylococcal virulence determinants that drive lethal synergism during polymicrobial IAI. Using the toxigenicS. aureusstrain JE2, we observed that coinfection withC. albicansled to a striking 80 to 100% mortality rate within 20 h postinoculation (p.i.) while monomicrobial infections were nonlethal. Use of a green fluorescent protein (GFP)-P3 promoterS. aureusreporter strain revealed enhanced activation of the staphylococcalagrquorum sensing system duringin vitropolymicrobial versus monomicrobial growth. Analyses by quantitative real-time PCR (qPCR), Western blot, and toxin functional assays confirmed enhancedagr-associated gene transcription and increases in secreted alpha- and delta-toxins.C. albicans-mediated elevated toxin production and hemolytic activity were determined to beagrAdependent, and genetic knockout and complementation ofhlaidentified alpha-toxin as the key staphylococcal virulence factor driving lethal synergism. Analysis of mono- and polymicrobial infections 8 h p.i. demonstrated equivalent bacterial burdens in the peritoneal cavity but significantly elevated levels of alpha-toxin (3-fold) and the eicosanoid prostaglandin E2(PGE2) (4-fold) during coinfection. Importantly, prophylactic passive immunization using the monoclonal anti-alpha-toxin antibody MEDI4893* led to significantly improved survival rates compared to those following treatment with isotype control antibody. Collectively, these results define alpha-toxin as an essential virulence determinant duringC. albicans-S. aureusIAI and describe a novel mechanism by which a human-pathogenic fungus can augment the virulence of a highly pathogenic bacteriumin vivo.IMPORTANCERelatively little is known about the complex interactions and signaling events that occur between microbes and even less so about how microbial “cross talk” shapes human health and disease.Candida albicans(a fungus) andStaphylococcus aureus(a bacterium) are formidable human nosocomial pathogens, causing severe morbidity and mortality. Moreover, they are frequently coisolated from central venous catheters and deep-seated infections, including intra-abdominal sepsis. In this work, we have shown that coinfection withC. albicansandS. aureusis highly lethal, leading to >80% mortality by day 1 postinfection, whereas monoinfection withC. albicansorS. aureusdoes not cause mortality. This infectious synergism is dependent on the expression of staphylococcal alpha-toxin, and secretion of this potent virulence factor is actually augmented byC. albicansvia anagr-dependent mechanism. Moreover, prophylactic neutralization of alpha-toxin with a monoclonal antibody is sufficient to elicit protection during coinfection. Therefore, we have demonstrated that a pathogenic fungus can enhance virulence determinants of a bacteriumin vivowith devastating consequences to the host. These results have important implications in the surveillance and treatment of polymicrobial disease and highlight the dynamic intersection of environment, pathogens, and host.
Influence of membrane fluidity on the assembly of Staphylococcus aureus alpha-toxin, a channel-forming protein, in liposome membrane.
