UC Merced

Biofilms are structured and densely packed communities of microbial cells attached to surfaces. They are considered the natural growth state for a vast majority of microorganisms. The ability to form biofilms is an important virulence factor for most pathogens, including the opportunistic human fungal pathogen Candida albicans. C. albicans is one of the most prevalent fungal species of the human microbiota. However, C. albicans can also cause severe and life-threatening infections when host conditions permit. Conventional C. albicans biofilms are often resistant to antifungal agents and the host immune response and can act as reservoirs of infectious cells, maintaining persistent infections and seeding new infections in a host. The majority of C. albicans clinical isolates are heterozygous (a/α) at the mating type-like (MTL) locus, which defines Candida mating types, and can form robust biofilms when cultured in vitro. These “conventional” biofilms, formed by MTL-heterozygous (a/α) cells, have been the primary focus of C. albicans biofilm research to date. Recent work in the field has uncovered novel mechanisms through which biofilms are generated by C. albicans cells that are homozygous or hemizygous (a/a, a/Δ, α/α, or α/Δ) at the MTL-locus. In these studies, the addition of mating pheromone of a particular mating type can induce cells of the other mating type to form specialized “sexual” biofilms. Although sexual biofilms are generally less robust than conventional biofilms, they could serve as a protective niche to promote the parasexual life cycle of mating-competent cells, and thus could be an adaptation which may increase population diversity in dynamic environments. In this work, we investigate the molecular and genetic mechanisms that enable sexual biofilm formation in C. albicans to gain insight into this unique biological process and establish a basis for comparison between sexual and conventional biofilms. Chapter 1 of this dissertation provides a detailed review of sexual biofilm formation in C. albicans. Chapter 2 of this dissertation provides detailed analyses and transcriptomics of several clinically isolated strains growing under sexual biofilm inducing conditions as well as co-cultures of white and opaque cells of various mating types. This led to the identification of several downstream genes that are involved in sexual biofilm formation. Chapter 3 of this dissertation identifies several TFs that are involved in regulating sexual biofilms. We find that many regulators of conventional biofilm formation are similarly involved in sexual biofilm formation. Interestingly, some transcription factors appear to switch from positive regulators of one system to negative regulators of the other. In addition, differential gene expression and gene ontology analyses reveal the unique role of these transcription factors in regulating sexual biofilm formation. Finally, Chapter 4 provides a detailed description of the conclusions and future directions for where other researchers can take this project, particularly to build clinical relevance for the sexual biofilm system and to uncover the transcription network of sexual biofilms and elucidate the evolutionary history of sexual and conventional biofilms.