Enterococci

Enterococci are gram-positive cocci that grow in pairs and short chains and are catalase-negative, like streptococci. Once classified as group D streptococci, they were reassigned to their own genus. Two species account for most human disease: Enterococcus faecalis, historically the more common isolate, and Enterococcus faecium, which is more frequently drug-resistant.

Their laboratory profile is a favorite exam target. Enterococci are gamma-hemolytic (nonhemolytic) on blood agar, show group D Lancefield antigen, hydrolyze esculin in the presence of bile, and — unlike nonenterococcal group D organisms such as Streptococcus bovis — grow in 6.5% NaCl. The salt tolerance is the classic distinguishing test. This hardiness in bile and high salt reflects their natural niche in the gut and their persistence on hospital surfaces.

Clinically, enterococci are opportunists rather than aggressive primary pathogens. They cause urinary tract infections (especially catheter-associated), biliary tract infections, intra-abdominal and wound infections, and bacteremia, and they are a recognized cause of subacute infective endocarditis, particularly after genitourinary or gastrointestinal instrumentation. Antibiotic resistance is their defining problem: they are intrinsically resistant to cephalosporins and show low-level resistance to aminoglycosides, so serious infections have traditionally required a cell wall agent combined with an aminoglycoside for synergy. Vancomycin-resistant enterococci (VRE) arise when the peptidoglycan precursor terminus changes from D-Ala-D-Ala to D-Ala-D-Lac, eliminating vancomycin binding; linezolid and daptomycin are the usual alternatives.

USMLE Step 1 tests enterococci in the gram-positive cocci sequence, expecting you to place them on the identification algorithm — catalase-negative, gamma-hemolytic, bile-esculin positive, salt-tolerant — and to explain the molecular basis of vancomycin resistance and the rationale for combination therapy.