Enterococci have until recently been accepted as low- virulentmicroorganisms. Although theyarenormalhuman commensals, they can act as opportunistic pathogens and cause serious and life threatening infections in especially immunocompromised individuals. Because of the fact that they have intrinsic and acquired resistance to many antimicrobial agents which is one of the most frequent isolated microorganisms from hospital-acquired infections in recently. There is increasing evidence that enterococci frequently have several specific important characteristics that allow them to survive for long periods on fomites in the hospital environments and colonize inpatients. All of these traits are attributed to the easy acquisition of multidrug resistance and virulence factors of the organisms. Amongst the enterococci, Enterococcus faecalis and E.faecium are medically the most important enterococci, with E.faecalis being the causative isolate in 80-90% of the cases. There

are still many unknown aspects about virulence factors of clinical Enterococcus isolates. Increasing multidrug resistance in nosocomial Enterococcus strains from all over the world recently enhances the need for further investigation of enterococci, especially the virulence factors. In this review, it was discussed exogenous acquisition of bacteria, transfer of genetic elements between enterococci, acquired host’s immun response against enterococcal infections, expressions of the virulence genes, escape mechanisms from the host’s immune system of enterococci and important virulence factors in enterococcal immunopathogenesis. A better understanding of the virulence factors of Enterococcus and host’s immune response may cause to the progress of new therapeutic and prophylactic approaches. Supporting the existence of enterococci as commensal bacteria in human microbiota and their non-infectious behavior can be seen as a new strategy for effective prevention and treatment of enterococcal infections.

Enterococcus Enterococcal infections Virulence factors Immunopathogenesis