Enterococci are a diverse and species-rich group of lactic acid bacteria isolated from a variety of environments, including the digestive systems of humans, animals, and insects, as well as from natural biomes such as water, sewage, soil, and arable land[1, 2]. Enterococcus faecium (E. faecium) is a common lactic acid bacterium in the intestine, which is an oval-shaped, facultative anaerobic gram-positive bacterium and has been widely used in animal husbandry[3, 4]. E. faecium is one of the important pathogens of nosocomial infections, which can cause sepsis, urinary tract infections, infective endocarditis, meningitis, and wound infections, and ranks second among infections caused by gram-positive cocci[5].
Although E. faecium is the cause of most infections, the hospital-adapted genotype of E. faecium is more prone to developing multidrug resistance[6]. E. faecium possesses a thick cell wall and exhibits resistant to many antimicrobials, including β-lactams, fluoroquinolones, and aminoglycosides, thereby increasing the mortality rate of infection[7, 8]. Vancomycin-resistant enterococci (VRE) was first isolated in Europe in the late 1980s[9]. Since then, VRE has gradually become one of the most important bacterial causes of healthcare-associated infections in the world. Vancomycin-resistant E. faecium (VREfm) is the majority of cases of VRE[10]. This rapid increase in prevalence may be attributed to its high recombination rate and extensive horizontal gene transfer, allowing bacteria to easily acquire drug-resistant phenotypes[11]. The global phylogeny of E. faecium is characterized by the dominance of two distinct phylogenetic clades, A and B. Clade A can be further divided into two subclades: A1 consisting primarily of clinical strains, and A2 consisting of strains mainly found in animals but also some non-hospitalized individuals. Clade B encompasses community isolates[12] and was recently reclassified as Enterococcus lactis.
Although E. faecium is not considered highly virulent, species possess virulence factors (VFs) associated with colonization, host invasion and/or tissue damage[6], or otherwise bypassing the host immune system. In E. faecium most of the VFs are involved in interactions with the extracellular matrix proteins vital in biofilm formation and colonization[13].
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