Enterococcus faecalis

Enterococcus faecalis – formerly classified as part of the Group D Streptococcus system – is a Gram-positive, commensal bacterium inhabiting the gastrointestinal tracts of humans and other mammals. It is among the main constituents of some probiotic food supplements. Like other species in the genus Enterococcus, E. faecalis can cause life-threatening infections in humans, especially in the nosocomial (hospital) environment, where the naturally high levels of antibiotic resistance found in E. faecalis contribute to its pathogenicity. E. faecalis has been frequently found in root canal-treated teeth in prevalence values ranging from 30% to 90% of the cases. Root canal-treated teeth are about nine times more likely to harbor E. faecalis than cases of primary infections.

Physiology
E. faecalis is a nonmotile, facultatively anaerobic microbe; it ferments glucose without gas production, and does not produce a catalase reaction with hydrogen peroxide. It can produce a pseudocatalase reaction if grown on blood agar. The reaction is usually weak. It produces a reduction of litmus milk, but does not liquefy gelatin. Growth on nutrient broth is consistent with being facultatively anaerobic.

Pathogenesis
E. faecalis can cause endocarditis and bacteremia, urinary tract infections (UTI), meningitis, and other infections in humans. In the United States, E. faecalis is associated with nosocomial infections including catheter-associated UTI, central line-associated bloodstream infection, and surgical site infections. Several virulence factors are thought to contribute to E. faecalis infections. A plasmid-encoded hemolysin, called the cytolysin, is important for pathogenesis in animal models of infection, and the cytolysin in combination with high-level gentamicin resistance is associated with a five-fold increase in risk of death in human bacteremia patients. A plasmid-encoded factor called "aggregation substance" is also important for virulence in animal models of infection.

Antibacterial resistance
E. faecalis is resistant to many commonly used antimicrobial agents (aminoglycosides, aztreonam, cephalosporins, clindamycin, the semisynthetic penicillins nafcillin and oxacillin, and trimethoprim-sulfamethoxazole). Resistance to vancomycin in E. faecalis is becoming more common. Treatment options for vancomycin-resistant E. faecalis include linezolid and daptomycin, although ampicillin is preferred if the bacteria are susceptible. Quinupristin/dalfopristin can be used to treat Enterococcus faecium but not E. faecalis.

Historical
Prior to 1984, enterococci were members of the genus Streptococcus: thus E. faecalis was known as Streptococcus faecalis.