How to Be a Bad Bug: Virulence Determinants of Enterococcus faecium

Abstract

Enterococcus faecium is a ubiquitous bacterium that only recently emerged as a nosocomial pathogen. Infections mostly affect immunocompromised patients and multi-resistance of E. faecium often hampers treatment. This thesis focuses on determinants, which give pathogenic potential to E. faecium. In the first paper, two TirE proteins are described as novel virulence factors of E. faecium. The tirE encoding genes are exclusive to nosocomial E. faecium strains and localized on a putative mobile genetic element of phage origin. Comparison of the wild-type and its isogenic mutant which lacks the tirE locus showed that the tirE locus promotes bacterial proliferation in human blood. Both TirE proteins were detected in bacterial supernatant and one of them was associated with membrane vesicles. Membrane vesicles of E. faecium are described for the first time in the second paper. An isolation protocol using ultracentrifugation and purification of vesicles over a density gradient was established. Using a proteomic approach the proteinaceous content of vesicles of nosocomial strains under varying culture conditions was described. In addition to virulence factors, the vesicles contain vaccine candidate proteins and antimicrobial resistance related proteins. In the third paper, a megaplasmid from a blood culture isolate encoding high-level gentamicin resistance was investigated for its putative virulence determinants. The megaplasmid was transferred to a commensal strain, where it enhanced proliferation in blood. Plasmid-encoded blood proliferation-enhancing factors are predicted to be involved in primary metabolic processes and membrane transport. Genes encoding these factors are enriched among clinical strains. Megaplasmids are therefore likely to serve as vessels for various genes, including those needed for niche adaption in infection. Overall, the findings of this thesis advanced the field of E. faecium’s virulence in different aspects and will hopefully inspire novel therapy development.