Bacteria and fungi have acquired the ability to interact and survive in many hostile environments both found in nature, as well as, the human body. Candida albicans, an opportunistic fungal pathogen, causes a variety of infections in immunocompromised or immunosuppressed individuals, but also asymptomatically colonizes 80% of the population within the intestinal tract, oral cavity, as well as, the female genitourinary system. A unique capability of C. albicans is the ability to change its morphology from benign circular yeast form, to oval pseudohyphal form, to cylindrical tissue-penetrating hyphal form. Our laboratory has previously identified a bacterium, Alcaligenes faecalis, which displays inhibitory characteristics towards C. albicans. A review of the literature shows that some bacteria have the ability to inhibit C. albicans, but only when in the hyphal form. We therefore wanted to explore if the morphological state of C. albicans dictated the degree of inhibition A. faecalis is able to exude, or simply “does morphology matter?” To determine this effect, a series of both solid and liquid media experiments were performed using a wild-type (able to convert between morphologies) strain of C. albicans, a mutant strain of C. albicans locked into the yeast morphological state, and a mutant strain of C. ablicans locked into the hyphal morphological state. For solid media experiments, the different strains of C. albicans were made into a lawn on agar plates, A. faecalis was spotted onto the lawns, and, after 24 hours, observed for signs of inhibition. For liquid media experiments, C. albicans strains were inoculated alone or co-cultured with A. faecalis for 24 hours and plated to enumerate colony forming units. Our results indicate that: (1) the morphological state of C. albicans is not a determining factor, which is a unique finding compared to other published reports; (2) Both A. faecalis and the closely related A. viscolactis both inhibit C. albicans showing that this is a shared ability among the Alcaligenes genus. (3) that the ability to inhibit C. albicans is thru some form of contact dependent mechanism, as the cell free supernatant of A. faecalis has no inhibitory action. Currently, the exact mechanism for this interaction is unknown, but could be one of the secretion systems bacteria use for interactions with other microbes. As there are very limited treatments for fungal infections and severe side-effects associated with current antifungals, exploiting these mechanisms are medically relevant to human health as they could potentially lead to novel treatments for problematic human fungal pathogens.

Quorum sensing molecules have become a recent focus of study to learn if and how they can be used, both on their own and in conjecture with current antimicrobial methods, as a means of bacterial control. One such quorum sensing molecule is the sesquiterpene alcohol, Farnesol, which is synthesized and released by the fungus, Candida albicans. In most in-vivo cases, our laboratory has shown that Alcaligenes faecalis overtakes C. albicans, preventing its growth. However, as a way to counteract this inhibitory effect, Farnesol may be one way that Candida has found to fight back. In this study, we focused on the inhibitory properties of Farnesol for growth and motility of A. faecalis, as well as, the molecule’s ability to prevent Alcaligenes from creating biofilms and/or degrading them once they have already been established. Our experiments show evidence that Farnesol is able to inhibit both the growth and motility of A. faecalis, and determination of the specific concentrations of Farnesol needed to see the largest effects on A. faecalis biofilms. Our hope is that in future studies, we will be able to add varying concentrations of the Farnesol to known and widely used antibiotics in order to increase the effectiveness of antibiotics against bacterial strains, both in the Alcaligenes genus and in other genus, that have previously been considered “antibiotic resistant”.