Enterococcus spp.

Enterococcus species are gram-positive cocci arranged in pairs or chains, catalase-negative, and can grow in 6.5% NaCl and hydrolyze esculin in the presence of bile (bile-esculin test positive), distinguishing them from Streptococcus. They are part of the normal gut flora but act as opportunistic pathogens, especially in hospitalized or immunocompromised patients. Clinically important infections include: subacute bacterial endocarditis (classically following GI/GU procedures, associated with colon cancer when caused by S. gallolyticus but Enterococcus also implicated), urinary tract infections (especially catheter-associated), biliary tract infections, and intra-abdominal/pelvic abscesses often as part of polymicrobial infections. Enterococcus faecalis is more common overall, but E. faecium has higher rates of antibiotic resistance.

Key pharmacologic point: Enterococci are intrinsically resistant to cephalosporins, and have variable susceptibility to penicillin and ampicillin. Synergistic combination therapy (a cell wall-active agent like penicillin/ampicillin plus an aminoglycoside like gentamicin) is often required for serious infections like endocarditis, as the cell wall agent facilitates aminoglycoside uptake. Vancomycin-resistant enterococci (VRE) is a major concern in nosocomial infections, mediated by alteration of the D-ala-D-ala peptidoglycan precursor to D-ala-D-lactate, reducing vancomycin binding. Treatment options for VRE include linezolid and daptomycin.

Enterococcus is part of the differential for culture-negative or difficult-to-treat endocarditis and is a classic board topic tying together microbiology (bile-esculin test, growth in high salt), pharmacology (synergistic antibiotic therapy, VRE resistance mechanism), and clinical medicine (endocarditis, UTI in catheterized patients).