In the last two decades Enterococcus faecium has emerged as a nosocomial pathogen. Successful treatment is increasingly hampered because of antibiotic resistance. To prevent infections with and spread of these multi-resistant nosocomial pathogens, further knowledge of the pathogenesis of E. faecium is needed. E. faecium isolates responsible for the majority of nosocomial infections and hospital outbreaks are genetically distinct from indigenous intestinal isolates. These hospital-acquired E. faecium strains acquired antibiotic resistance and virulence genes conferring an infectious phenotype to E. faecium resulting in increased fitness of E. faecium in hospitalized patients. So far, different putative virulence genes are identified that possibly contribute to the pathogenesis of E. faecium. However, our knowledge about the functions of these genes is still limited. The enterococcal surface protein gene esp is one of the identified putative virulence genes and has been implicated in initial adherence and biofilm formation. We hypothesized that Esp, because of its linkage to hospital-acquired E. faecium isolates and potential involvement in adherence and biofilm formation, is an important virulence factor for E. faecium. Therefore, the studies of this thesis mainly focused on elucidating the role of Esp in the pathogenesis of different E. faecium infections. First, by constructing an E. faecium esp mutant we demonstrated that Esp is involved in biofilm formation. Inactivation of esp resulted in abolished cell surface Esp expression, significant lower initial adherence to polystyrene, and reduced biofilm formation compared to the Esp-expressing parent strain. Subsequently, the Esp-deficient mutant and its Esp-expressing parent strain were tested in different animal models mimicking intestinal colonization, urinary tract infection (UTI), peritonitis, bacteremia, and endocarditis. In the UTI, bacteremia and endocarditis model, the Esp-deficient mutant was significantly attenuated compared to the wild-type strain indicating that Esp contributes to the pathogenesis of these infections. No differences were observed in the peritonitis and intestinal colonization model. The importance of E. faecium Esp in UTI and endocarditis probably results from the fact that these models represent biofilm-associated infections indicating a niche specific role of Esp in the pathogenesis of E. faecium infections. Esp is one of the few virulence determinants identified in E. faecium. In an attempt to identify novel putative virulence genes, the deltarho-Web model was used, a computer program that can recognize genes acquired through horizontal gene transfer by their anomalous GC-content and dinucleotide frequency. By using this model, a genomic island was identified, which appeared highly specific for hospital-acquired E. faecium isolates and putatively encodes a metabolic island involved in carbohydrate transport and metabolism providing hospital-acquired E. faecium alternative metabolic pathways of energy production. In conclusion, the work described in this thesis helped to increase our understanding in the function of Esp and identified a novel genomic island possibly implicated in E. faecium fitness. With the limited therapeutic options available, E. faecium will increasingly become a clinical challenge. Improved understanding of the virulence properties of this species will contribute to the development of novel intervention strategies, which may help us to prevent spread and infections of E. faecium in hospitals
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