The role of Enterococcus faecalis biofilm formation in the regulation of conjugation.

Abstract

Enterococcus faecalis has recently emerged as an important nosocomial pathogen. Pathogenicity of these organisms depends greatly on a few important aspect of enterococcal physiology. The ability of enterococci to form biofilms greatly enhances their virulence. Their innate resistance to many antibiotics and their ability to transfer these resistance genes through conjugation heightens their threat to human health. The work described in this thesis attempts to explain the roles of biofilm growth, conjugation, and cell communication in E. faecalis. To examine the role of biofilm growth on the E. faecalis transcriptome, RNAseq analysis was undertaken. We found that over 100 genes were measurably upregulated during biofilm growth while approximately 26 genes were downregulated. These data gives us important insights into the biology of enterococcal biofilms. In clinical settings, biofilms are likely locations for antibiotic resistance transfer events involving nosocomial pathogens such as E. faecalis. Conjugation is an important mode of horizontal gene transfer in bacteria, enhancing the spread of antibiotic resistance. In this work, I demonstrate that growth in biofilms alters the induction of conjugation by a sex pheromone in E. faecalis. Mathematical modeling suggested that a higher plasmid copy number in biofilm cells would enhance a switch-like behavior in the pheromone response of donor cells with a delayed, but increased response to the mating signal. Alterations in plasmid copy number and a bimodal response to induction of conjugation in populations of plasmid-containing donor cells were both observed in biofilms, consistent with the predictions of the model. The pheromone system may have evolved such that donor cells in biofilms are only induced to transfer when they are in extremely close proximity to potential recipients in the biofilm community. These results have important implications for development of chemotherapeutic agents to block resistance transfer and treat biofilm-related clinical infections.