Vancomycin tolerance and host responses in Staphylococcus aureus-Candida albicans dual-species biofilm infections

Abstract

The polymorphic fungus Candida albicans and gram-positive bacterium Staphylococcus aureus are biofilm-forming organisms commonly found in immunocompromised patients, with each organism ranked by the CDC in the top causes of mortality for US hospitals. Murine models have demonstrated that C. albicans and S. aureus can form a polymicrobial biofilm on epithelial tissue and facilitate systemic infection of both organisms from the oral cavity. While many studies have examined host responses to each organism in mono-species infections, few have sought to determine the impact of dual-species interaction, despite most infections being polymicrobial in nature. In addition to collaborative pathogenesis, previous research has shown S. aureus gains tolerance to the glycopeptide antibiotic vancomycin when co-cultured with C. albicans. However, the mechanism behind this phenomenon has not been well elucidated. We hypothesized that interaction of C. albicans and S. aureus induces specific changes within both organisms that allow for increased antimicrobial tolerance, survival and virulence in dual-species biofilms. To examine this interaction further, we conducted two global transcriptomics studies (in vitro and in vivo) and one lipidomic study to analyze changes in genetic and lipid profiles in pathogen and host. In vitro transcriptomics revealed that genes in C. albicans remained mostly unchanged but numerous genes in S. aureus were differentially expressed. These results paralleled our in vivo transcriptomic analysis in our murine model of oral co-infection. From these results, we tested multiple S. aureus mutants to determine their vancomycin tolerance and found that mutation of a single stress response gene, clpP (proteolytic subunit), abolished acquisition of staphylococcal antimicrobial resistance. This induction of clpP-dependent antimicrobial tolerance was induced by farnesol produced by C. albicans at physiologically relevant concentrations. Finally, we used our murine co-infection model determine if cells of the innate immune system may be involved in systemic S. aureus infection. Organ culture and flow cytometry analysis revealed intracellular S. aureus in labeled macrophages and neutrophils within lymph nodes of only dual-species infected mice. These findings demonstrate the importance of the dual-species biofilm phenotype, its impact on antibiotic treatment, and its modulation of the host immune response to promote infection.