Physiology of Enterococcus faecalis and faecium
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Enterococcus faecium and Enterococcus faecalis are commensal bacteria that colonize the gastrointestinal tract. Although these bacteria are in principal non-pathogenic, E. faecium and E. faecalis have emerged as important nosocomial pathogens with high-level resistance to antibiotics causing clinical infections including urinary tract infections, bacteremia and bacterial endocarditis in elderly and immunocompromised patients. Since the prevalence of enterococcal infections in hospitalized patients is still increasing every year, much research is focussed on the pathogenicity of these specific strains. Thus far, no clear evidence is has been found that explains the ecological success of these infectious causing strains. In order to understand what characteristics of E. faecium and E. faecalis really favour its ability to colonize and infect its host, this review focuses on the physiology of these bacteria rather then its pathology. The potential to degrade mucin glycoproteins via the production of specific glycosidases might contribute to an enhanced fitness of the organism because this substrate can be used as an additional nutrient source. Furthermore, E. faecium and E. faecalis have been shown to be resistant to bile acids. Besides bile salt hydrolases (BSH) that can detoxify conjugated bile acids (CBA), also other proteins, like gls24, are associated with CBA resistance. In addition to these characteristics, E. faecalis and to a lesser extend also E. faecium, are the primary source of Reactive Oxygen Species (ROS). In healthy individuals, the mucosa protects the underlying epithelial cell layer against harmful luminal contents like ROS. However, since E. faecium and E. faecalis produce glycosidases that reduce thickness of the mucosa, less protection is provided against ROS. ROS can cause genomic alterations, which is correlated to colorectal cancer. In conclusion, better understanding of the physiology of E. faecium and E. faecalis is essential to gain more insight into the pathogenicity of these bacteria. This review will discuss some essential adaptations of E. faecium and E. faecalis that might contribute to its increased fitness and prevalence.