Two species, Enterococcus faecalis and Enterococcus faecium, are responsible for most human enterococcal infections. Enterococci cause wound infections, urinary tract infections, bacteremia, and endocarditis. Infections caused by penicillin-susceptible strains should be treated with ampicillin 2 g every 4 hours or penicillin 3–4 million units every 4 hours; if the patient is penicillin-allergic, vancomycin 15 mg/kg every 12 hours intravenously can be given. If the patient has endocarditis or meningitis, gentamicin 1 mg/kg every 8 hours intravenously should be added to the regimen for a duration of 2–3 weeks in order to achieve the bactericidal activity that is required to cure these infections. In cases of endocarditis, ceftriaxone 2 g every 12 hours may be given instead of gentamicin in combination with the ampicillin.
Resistance to vancomycin, penicillin, and gentamicin is common among enterococcal isolates, especially E faecium; it is essential to determine antimicrobial susceptibility of isolates. Infection control measures that may be indicated to limit their spread include isolation, barrier precautions, and avoidance of overuse of vancomycin and gentamicin. Consultation with an infectious diseases specialist is strongly advised when treating infections caused by resistant strains of enterococci. Quinupristin/dalfopristin and linezolid are approved by the FDA for treatment of infections caused by vancomycin-resistant strains of enterococci. Daptomycin, tigecycline, tedizolid, and oritavancin are not approved by the FDA for the treatment for vancomycin-resistant strains of enterococci, although they are frequently active in vitro.
Quinupristin/dalfopristin is not active against strains of E faecalis and should be used only for infections caused by E faecium. The dose is 7.5 mg/kg intravenously every 8–12 hours. Phlebitis and irritation at the infusion site (often requiring a central line) and an arthralgia-myalgia syndrome are relatively common side effects. Linezolid, an oxazolidinone, is active against both E faecalis and E faecium. The dose is 600 mg twice daily, and both intravenous and oral preparations are available. Its two principal side effects are thrombocytopenia and bone marrow suppression; however, peripheral neuropathy, optic neuritis, and lactic acidosis have been observed with prolonged use (typically greater than 6 weeks) due to mitochondrial toxicity. Emergence of resistance has occurred during therapy with both quinupristin/dalfopristin and linezolid.
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