Candida albicans and Staphylococcus aureus Pathogenicity and Polymicrobial Interactions: Lessons beyond Koch's Postulates

Abstract

While Koch's Postulates have established rules for microbial pathogenesis that have been extremely beneficial for monomicrobial infections, new studies regarding polymicrobial pathogenesis defy these standards. The explosion of phylogenetic sequence data has revolutionized concepts of microbial interactions on and within the host. However, there remains a paucity of functional follow-up studies to delineate mechanisms driven by such interactions and how they shape health or disease. That said, one particular microbial pairing, the fungal opportunist Candida albicans and the bacterial pathogen Staphylococcus aureus, has received much attention over the last decade. Therefore, the objective of this review is to discuss the multi-faceted mechanisms employed by these two ubiquitous human pathogens during polymicrobial growth, including how they: establish and persist in inter-Kingdom biofilms, tolerate antimicrobial therapy, co-invade host tissue, exacerbate quorum sensing and staphylococcal toxin production, and elicit infectious synergism. Commentary regarding new challenges and remaining questions related to future discovery of this fascinating fungal-bacterial interaction is also provided.

Keywords: Candida albicans; Staphylococcus aureus; biofilm; co-infection; fungal-bacterial; polymicrobial; quorum sensing; synergism; toxin.

Figure 1

Schematic of the agr quorum sensing system in Staphylococcus aureus. AgrA is the response regulator in a two-component system that acts as a transcription factor, modulating toxin production. (1) AgrA activates transcription from the P2 promoter, driving expression of RNAII, an operon consisting of four agr genes. AgrB is a membrane-bound permease that processes AgrD, a pre-signal peptide, and releases it as AIP-2 (auto-inducing peptide 2). AIP-2 is sensed by AgrC, a membrane-bound histidine kinase that is part of the two-component signaling system. AgrC phosphorylates AgrA, activating it, leading to a positive feedback loop. (2) Activated AgrA also drives transcription from the P3 promoter, driving expression of RNA III, the effector of the QS system. RNA III directly encodes for delta toxin (hld) and also binds to repressor of toxin (rot) transcript, allowing for toxin production by inhibiting rot translation. RNAIII also binds to a number of adhesin-related genes to similarly block their translation.

Figure 2

In vitro polymicrobial biofilm formation by C. albicans and S. aureus. The images demonstrates S. aureus (green/merged, white arrows) attached throughout the biofilm and along the hyphal filaments of C. albicans (blue). The extracellular matrix (red) largely encases a majority of the staphylococci. Methods: A polymicrobial biofilm was formed in vitro in RPMI-1640 medium with C. albicans (strain SC5314) and S. aureus (strain M2) using 1 × 10⁶ CFU of each microbe to inoculate a Permanox chamber slide for 24 h at 37 °C. Biofilms were washed with sterile saline to remove non-adherent cells, fixed in 4% formalin, and stained with a cocktail containing calcofluor white (50 µg/mL), Concanavalin A-Texas Red (50 µg/mL), and Syto9 (1.67 µM). Images were captured using 405 nm, 488 nm, and 565 nm lasers and DAPI, FITC, and Texas Red filter sets with a Zeis 510 confocal scanning laser microscope. Corresponding Z-stacks were constructed using packaged Zeis software depicting a side view. Scale bar represents 20 µm.

Figure 3

Schematic of C. albicans–S. aureus interactions. (1) S. aureus preferentially attaches to the hyphal filaments of C. albicans via binding of the candidal adhesin Als3p. (2) Encasement of S. aureus in fungal biofilm matrix components (including β-1,3-glucan) impairs penetration of antibiotics (purple triangle) by sequestration of drug. The C. albicans QS molecule farnesol also upregulates drug efflux pumps in S. aureus to enhance tolerance to antibacterials. (3) S. aureus is able to gains access into subepithelial spaces by “hitchhiking” onto the invasive C. albicans hyphae. S. aureus may also then disseminate to distant sites (including the kidneys) following co-invasion via the bloodstream. (4) C. albicans enhances the staphylococcal agr QS system, ultimately leading to elevated levels of ⍺-toxin (green bolts), cell damage, and synergistic lethality during polymicrobial IAI.