Staphylococcus aureus adherence to Candida albicans hyphae is mediated by the hyphal adhesin Als3p

Abstract

The bacterium Staphylococcus (St.) aureus and the opportunistic fungus Candida albicans are currently among the leading nosocomial pathogens, often co-infecting critically ill patients, with high morbidity and mortality. Previous investigations have demonstrated preferential adherence of St. aureus to C. albicans hyphae during mixed biofilm growth. In this study, we aimed to characterize the mechanism behind this observed interaction. C. albicans adhesin-deficient mutant strains were screened by microscopy to identify the specific receptor on C. albicans hyphae recognized by St. aureus. Furthermore, an immunoassay was developed to validate and quantify staphylococcal binding to fungal biofilms. The findings from these experiments implicated the C. albicans adhesin agglutinin-like sequence 3 (Als3p) in playing a major role in the adherence process. This association was quantitatively established using atomic force microscopy, in which the adhesion force between single cells of the two species was significantly reduced for a C. albicans mutant strain lacking als3. Confocal microscopy further confirmed these observations, as St. aureus overlaid with a purified recombinant Als3 N-terminal domain fragment (rAls3p) exhibited robust binding. Importantly, a strain of Saccharomyces cerevisiae heterologously expressing Als3p was utilized to further confirm this adhesin as a receptor for St. aureus. Although the parental strain does not bind bacteria, expression of Als3p on the cell surface conferred upon the yeast the ability to strongly bind St. aureus. To elucidate the implications of these in vitro findings in a clinically relevant setting, an ex vivo murine model of co-infection was designed using murine tongue explants. Fluorescent microscopic images revealed extensive hyphal penetration of the epithelium typical of C. albicans mucosal infection. Interestingly, St. aureus bacterial cells were only seen within the Abbreviations: AFM, atomic force microscopy; CLSM, confocal laser scanning microscopy; ConA-Texas red, concanavalin A conjugated to Texas red; H&E, haematoxylin and eosin; IFA, indirect immunofluorescence; PNA-FISH, peptide nucleic acid probes for fluorescent in situ hybridization; SEM, scanning electron microscopy. A supplementary figure is available with the online version of this paper. Microbiology

Clinical Implications of Oral Candidiasis: Host Tissue Damage and Disseminated Bacterial Disease

“…Although C. albicans is commonly encountered as a commensal in healthy individuals, this opportunistic pathogen is capable of invading virtually any site of the human host, from superficial sites to deep tissues and organs. C. albicans and S. aureus are frequent colonizers of human mucosal surfaces; using confocal fluorescent and atomic force microscopy, our previous in vitro studies revealed a unique mixed biofilm architecture wherein S. aureus associated closely with the hyphae (34,52). However, in order to investigate the clinical implications of this interaction during cocolonization of host mucosal tissue, a suitable animal model is needed.…”

“…Our in vitro studies have demonstrated that S. aureus exhibits high affinity to the C. albicans hyphae as these species coexist in biofilm and identified the C. albicans hyphal specific adhesin Als3p to be involved in the coadherence process (34,35). Further, our previous in vivo studies have indicated that the interaction of these species may carry important clinical implications (36,37).…”

Fungi at the Scene of the Crime: Innocent Bystanders or Accomplices in Oral Infections?

“…Genetic manipulation of this adhesin has been shown to reduce bacterial binding to hyphal filaments of C. albicans, reducing the invasive capacity of this relationship [55]. The role of this phenotype has been shown in several bacterial pathogens including Staphylococcus aureus, Streptococcus gordonii, and Streptococcus agalactiae [56][57][58]. Interestingly, when the Δals3 strain was grown with S. mutans, no reduction was observed in mixed species biofilm formation, suggestive of a different mechanism of interaction [59].…”

Candida albicans Mucin Msb2 Is a Broad-Range Protectant against Antimicrobial Peptides

“…Approximately one-fourth of all C. albicans bloodstream infections appear to be polymicrobial (33), and coinfection with C. albicans and the important bacterial pathogen S. aureus in mice is synergistic to increase mortality (36). C. albicans and S. aureus were shown to interact directly by forming a mixed biofilm (34) that depended to a considerable extent on the C. albicans Als3 cell wall protein (37). Importantly, mixed biofilm formation with C. albicans enhanced the resistance of S. aureus to vancomycin (35), which, like the more recent antibiotic daptomycin, is being used as a reserve-type antibiotic to combat multiresistant Gram-positive bacterial pathogens, including Staphylococcus, Enterococcus, and Corynebacterium species.…”