Gram Positive Cocci: Enterococci

Enterococci are Gram positive cocci arranged in pairs or short chains, catalase negative, and gamma-haemolytic, non-haemolytic, or occasionally weakly alpha-haemolytic.

They were formerly classified within the genus Streptococcus, Lancefield group D, and were reclassified as a separate genus given distinct genetic and clinical properties, most notably intrinsic antibiotic resistance and an inherently higher tolerance for adverse growth conditions.

Two species are of principal medical importance: Enterococcus faecalis, more common overall, and Enterococcus faecium, less common overall but disproportionately associated with resistant, healthcare-associated infection.

Bile and salt tolerance is a defining biochemical feature: enterococci grow in the presence of 6.5 percent sodium chloride and in bile, exploited diagnostically via bile-esculin agar and reflecting their natural habitat in the gastrointestinal tract.

Intrinsic antibiotic resistance is a foundational property: enterococci are intrinsically resistant to cephalosporins, all generations, and have only low-level intrinsic susceptibility to aminoglycosides when used alone, a foundational point for understanding why enterococcal infection requires specific treatment approaches distinct from streptococcal infection.

Environmental hardiness contributes to their efficiency as a healthcare-associated pathogen: enterococci survive well on environmental surfaces and tolerate a wide temperature and pH range.

Morphology. Gram positive cocci in pairs or short chains.

Important laboratory features. Catalase negative, bile-esculin positive, salt tolerant, 6.5 percent NaCl, gamma-haemolytic.

Reservoir. Normal gastrointestinal, colonic, flora, also found in the female genital tract in smaller numbers.

Transmission. Predominantly endogenous, the patient's own gut flora translocating into a normally sterile site. Person-to-person spread occurs in healthcare settings via contaminated hands or surfaces, relevant particularly for resistant strains.

Virulence factors. Relatively low intrinsic virulence compared to many other Gram positive pathogens. Adhesins, aggregation substance, and biofilm formation contribute to its ability to establish infection on damaged heart valves and medical devices. Cytolysin, in some strains, contributes to tissue damage.

Major diseases.

• Urinary tract infection, particularly catheter-associated and healthcare-associated.
• Intra-abdominal and pelvic infection, often polymicrobial, reflecting its GI origin.
• Infective endocarditis, typically subacute, affecting a previously abnormal valve. It arises in older patients, sometimes following a urological or gastrointestinal procedure rather than a dental one.
• Bacteraemia in hospitalised and immunocompromised patients.

Diagnosis. Culture from the relevant clinical specimen, urine, blood, wound, peritoneal fluid. Bile-esculin and salt tolerance tests support genus identification; species-level identification and susceptibility testing guide therapy.

Resistance patterns. Generally more antibiotic-susceptible than E. faecium. Ampicillin typically remains active against most E. faecalis isolates, an important practical distinction from E. faecium.

Morphology. Gram positive cocci in pairs or short chains, indistinguishable from E. faecalis on Gram stain.

Important laboratory features. Catalase negative, bile-esculin positive, salt tolerant, gamma-haemolytic, distinguished from E. faecalis by biochemical testing or molecular identification, not by microscopy alone.

Reservoir. Gastrointestinal flora, though present in smaller numbers than E. faecalis in most healthy individuals.

Transmission. As for E. faecalis, predominantly endogenous, with healthcare-associated person-to-person spread particularly relevant given this species' disproportionate association with multidrug resistance.

Virulence factors. Similar general mechanisms to E. faecalis, adhesion, biofilm formation, though E. faecium is clinically significant less for intrinsic virulence and more for its resistance profile and capacity for healthcare-associated outbreak spread.

Major diseases. The same general infection types as E. faecalis, urinary tract infection, intra-abdominal infection, bacteraemia, endocarditis, but disproportionately represented among healthcare-associated and resistant enterococcal infections, particularly in immunocompromised and critically ill patients.

Diagnosis. As for E. faecalis. Species-level distinction from E. faecalis is clinically important specifically because of the differing resistance patterns.

Resistance patterns. Ampicillin resistance is far more common in E. faecium than in E. faecalis, a key practical distinction guiding empirical therapy when an enterococcal infection is suspected before species identification is available.

The resistance mechanism involves acquisition of van resistance genes, most clinically important vanA and vanB, which alter the bacterial cell wall precursor target that vancomycin normally binds, preventing the drug from binding effectively.

Species distribution shows VRE is substantially more common among E. faecium isolates than E. faecalis isolates, compounding the already greater intrinsic antibiotic vulnerability of E. faecium to become resistant, and making vancomycin-resistant E. faecium a particularly difficult-to-treat healthcare-associated pathogen.

Transmission and infection control: VRE spreads readily in healthcare settings via contaminated hands, equipment, and surfaces, given the organism's environmental hardiness. Contact precautions and rigorous environmental cleaning are central to outbreak control.

Treatment implications: VRE infection typically requires alternative agents active against Gram positive organisms outside the standard beta-lactam and vancomycin options, for example linezolid or daptomycin, reflecting the loss of the single most reliable historical option for resistant Gram positive infection.

Clinical significance: VRE is frequently a marker of a broader pattern of multidrug resistance and healthcare exposure, and its detection, even as asymptomatic colonisation on screening, triggers infection control measures independent of whether active treatment is required at that time.