Enterococcus faecalis Inhibits Hyphal Morphogenesis and Virulence of Candida albicans

Abstract

The Gram-positive bacterium Enterococcus faecalis and the fungus Candida albicans are both found as commensals in many of the same niches of the human body, such as the oral cavity and gastrointestinal (GI) tract. However, both are opportunistic pathogens and have frequently been found to be coconstituents of polymicrobial infections. Despite these features in common, there has been little investigation into whether these microbes affect one another in a biologically significant manner. Using a Caenorhabditis elegans model of polymicrobial infection, we discovered that E. faecalis and C. albicans negatively impact each other's virulence. Much of the negative effect of E. faecalis on C. albicans was due to the inhibition of C. albicans hyphal morphogenesis, a developmental program crucial to C. albicans pathogenicity. We discovered that the inhibition was partially dependent on the Fsr quorum-sensing system, a major regulator of virulence in E. faecalis. Specifically, two proteases regulated by Fsr, GelE and SerE, were partially required. Further characterization of the inhibitory signal revealed that it is secreted into the supernatant, is heat resistant, and is between 3 and 10 kDa. The substance was also shown to inhibit C. albicans filamentation in the context of an in vitro biofilm. Finally, a screen of an E. faecalis transposon mutant library identified other genes required for suppression of C. albicans hyphal formation. Overall, we demonstrate a biologically relevant interaction between two clinically important microbes that could affect treatment strategies as well as impact our understanding of interkingdom signaling and sensing in the human-associated microbiome.

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

“…The protein responsible was identified as the bacteriocin EntV; this secreted protein demonstrated antihyphal and antivirulence properties without antagonising fungal growth [107•]. Co-infection of these organisms has been shown to attenuate C. albicans virulence in vivo, particularly with an initial administration of E. faecalis [108,109]. Interestingly, this effect was also mimicked in a murine oral candidiasis model, whereby initial administration of heatkilled bacteria was shown to have a protective antifungal effect [110].…”

“…Based on existing knowledge of polymicrobial biofilm communities, these findings could complicate treatment regimens and provide a possible explanation for recurrent root canal infections. Mechanistically, E. faecalis integrates into Candida biofilms and reduces the overall biomass [105], and also been shown to negatively affect C. albicans hyphal formation [106]. The protein responsible was identified as the bacteriocin EntV; this secreted protein demonstrated antihyphal and antivirulence properties without antagonising fungal growth [107•].…”

Streptococcus gordonii comCDE (competence) operon modulates biofilm formation with Candida albicans

“…The major signalling molecules produced by Gram-positive cocci are peptides, including CSPs and bacteriocins. There is evidence that peptides produced by S. mutans (Jarosz et al, 2009) and Enterococcus faecalis (Cruz et al, 2013) are inhibitory to filamentation by C. albicans. The ComCDE systems in S. gordonii, S. pneumoniae and S. mutans function as QS autoregulatory control systems, modulating gene expression patterns in different ways for the three streptococcal species (Vickerman et al, 2007;Mashburn-Warren et al, 2010;Son et al, 2012;Merritt & Qi, 2012).…”

Candida albicans morphology: still in focus

“…In the case of Fusobacterium nucleatum, however, hyphal inhibition was contact dependent [13]. Interestingly, both probiotic and facultative pathogenic bacteria were shown to negatively affect hyphal formation [9,10,[12][13][14][15]. It, therefore, appears possible that the interaction with bacteria, rather than distinct environmental conditions or transcriptional programs, is responsible for the predominance of C. albicans yeast morphology in the gastrointestinal tract.…”

“…In most studies, bacteria were found to inhibit fungal filamentation, often by secreted compounds that are linked to bacterial quorum sensing, virulence factors, or metabolism [9][10][11][12]. In the case of Fusobacterium nucleatum, however, hyphal inhibition was contact dependent [13].…”