Interspecies Interactions between and .

The facultative anaerobic polymorphic fungus and the strictly anaerobic Gram-positive bacterium are two opportunistic pathogens residing in the human gut. While a few studies have focused on the prevalence of in -infected patients, the nature of the interactions between these two microbes has not been studied thus far. In the current study, both chemical and physical interactions between and were investigated. In the presence of , was able to grow under aerobic, normally toxic, conditions. This phenomenon was neither linked to adherence of bacteria to hyphae nor to biofilm formation by . Conditioned medium of inhibited hyphal growth of , which is an important virulence factor of the fungus. In addition, it induced hypha-to-yeast conversion. -Cresol, a fermentation product of tyrosine produced by , also induced morphological effects and was identified as an active component of the conditioned medium. This study shows that in the presence of , can persist and grow under aerobic conditions. Furthermore, -cresol, produced by , is involved in inhibiting hypha formation of , directly affecting the biofilm formation and virulence of . This study is the first detailed characterization of the interactions between these two gut pathogens. and are two opportunistic pathogens that reside in the human gut. A few studies have focused on the prevalence of in -infected patients, but none have shown the interaction(s) that these two organisms may or may not have with each other. In this study, we used a wide range of different techniques to better understand this interaction at a macroscopic and microscopic level. We found that in the presence of , can survive under ambient aerobic conditions, which would otherwise be toxic. We also found that affects the hypha formation of , most likely through the excretion of -cresol. This ultimately leads to an inability of to form a biofilm. Our study provides new insights into interactions between and and bears relevance to both fungal and bacterial disease.