Taxonomy and Morphology
Bacteria of the genus Enterococcus belong to the family Enterococcaceae and comprise 28 species. Historically, they were classified as group D streptococci due to a shared group-specific polysaccharide antigen. The two species of greatest clinical importance are E. faecalis, which accounts for the vast majority of infections, and E. faecium.
Morphologically, they are spherical or ovoid Gram-positive bacteria. In smears, they appear in pairs or short chains. Enterococci do not form spores or capsules, although some species possess flagella and are motile.
Cultural Characteristics and Differentiation from Streptococci
Enterococci are chemoorganotrophs and facultative anaerobes. They are extremely undemanding regarding environmental conditions: they can multiply at temperatures ranging from 10 to 45 °C (optimum 37 °C), grow on standard media, and tolerate alkaline pH up to 9.6.
On blood agar, they form small (approx. 1 mm) greyish colonies; hemolysis is typically absent or appears as a zone of incomplete clearing. When inoculated into milk containing methylene blue, the bacteria decolorize the medium.
The main differential feature distinguishing them from streptococci is their high resistance to harsh chemical factors. Enterococci readily grow in the presence of 40% bile and high salt concentrations (6.5% NaCl). Like streptococci, they are catalase-negative, but they actively ferment many carbohydrates with acid production.
Pathogenesis and Clinical Presentation
Enterococci lack distinct organ tropism and can therefore infect various tissues. Virulence factors (well-studied in E. faecalis) include adhesins, invasion enzymes (protease, gelatinase, which facilitate the colonization of epithelium and heart valves), and the toxin cytolysin.
Disease occurs when host resistance decreases, leading to bacterial translocation from natural reservoirs to uncharacteristic cavities and tissues. This results in pyogenic inflammatory processes. The main clinical forms of opportunistic infections include:
- Urinary tract infections (UTIs): most commonly associated with urological procedures and catheterization.
- Bacteremia: elevated risk is observed in patients undergoing hemodialysis.
- Nosocomial infections: frequently develop as complications in postoperative patients.
- Endocarditis: predominantly affects elderly individuals, patients with prosthetic heart valves, and organ transplant recipients.
Microbiological Diagnostics and Treatment
Diagnostic specimens include blood, urine, and wound discharge (depending on the site of infection). The primary diagnostic method is bacterial culture. The sample is inoculated onto blood and selective media, after which pure cultures are isolated and identified based on biochemical and cultural properties. Molecular genetic methods such as PCR and ribotyping are also utilized.
Due to the high intrinsic resistance of enterococci to most antibiotics (especially cephalosporins), antimicrobial susceptibility testing (antibiogram) is strictly mandatory for diagnosis.
Treatment is prescribed exclusively based on susceptibility test results. Combinations of penicillins with $\beta$-lactamase inhibitors are often the drugs of choice (especially if $\beta$-lactamase-producing strains are identified). No specific prophylaxis exists; general sanitation and infection control measures are applied, similar to those for staphylococcal and streptococcal infections.