EnterococcusEdit
Enterococcus is a genus of Gram-positive, catalase-negative, facultatively anaerobic cocci that inhabit the gastrointestinal tracts of humans and many animals. While these bacteria are part of the normal microbiota, certain species can become opportunistic pathogens, causing a range of infections, particularly in hospitalized or immunocompromised individuals. The most clinically significant species are Enterococcus faecalis and Enterococcus faecium, which together account for the majority of enterococcal infections worldwide. Their hardy physiology and ecological versatility enable them to persist in the hospital environment and contribute to nosocomial infections as well as to community-acquired cases in some settings. Gram-positive bacteria, facultative anaerobe.
Enterococcus species display distinctive microbiological features that aid in their identification and understanding of their behavior in health and disease. They are typically small, Gram-positive cocci that arrange themselves in pairs or short chains. They can grow in the presence of bile and in high salt concentrations and can hydrolyze esculin in bile-containing media, a trait used in laboratory diagnostics such as the bile esculin test assay. They are generally non-motile and lack catalse activity, setting them apart from some other Gram-positive cocci. These organisms are intrinsically tolerant of a range of environmental stresses, including desiccation and fluctuating temperatures, which contributes to their persistence on hospital surfaces, equipment, and hands of health-care workers. This ecological plasticity helps explain both their commensal status in the gut and their capacity to cause infections when host defenses are compromised. Enterococcus faecalis, Enterococcus faecium.
Taxonomy and biology
The genus Enterococcus comprises multiple species, with E. faecalis and E. faecium being the most prominent in human disease. Taxonomic distinctions are based on genetic, biochemical, and phenotypic criteria, and modern laboratories rely on MALDI-TOF mass spectrometry and genetic methods to differentiate species. In human disease, the species differ in virulence traits, antimicrobial susceptibility, and epidemiology: E. faecalis is historically more common in community and some hospital-associated infections, whereas E. faecium has become increasingly prominent in hospital outbreaks and tends to harbor greater intrinsic resistance to many antibiotics. These organisms often reside in the gut as part of the normal flora but can translocate to cause urinary tract infections, bacteremia, endocarditis, meningitis (less commonly), intra-abdominal infections, and other serious conditions. Gram-positive; bacteremia; endocarditis; urinary tract infection.
Ecology and pathogenesis
As gut commensals, enterococci play a role in nutrient processing and microbial balance. Their ability to exchange genetic material via mobile elements facilitates rapid adaptation, including acquisition of resistance determinants. Pathogenic enterococci can form biofilms on medical devices such as catheters and prosthetic valves, complicating treatment and enabling persistent infections. The combination of host factors (age, comorbidities, invasive procedures) and pathogen traits (antibiotic resistance, virulence factors) underpins the burden of disease associated with enterococci in health care settings. biofilm; antibiotic resistance; horizontal gene transfer.
Clinical significance
Enterococcus species are notable causes of both community-onset and hospital-acquired infections. In the clinical setting, enterococci are implicated in urinary tract infections, biliary and intra-abdominal infections, bacteremia, endocarditis, and occasionally meningitis. They are particularly important in hospitalized patients, the elderly, and those with prior antibiotic exposure or indwelling devices. Treatment is complicated by distinctive resistance patterns and the bacterium’s capacity to reside in intracellular and biofilm-associated niches. Specific infections and manifestations include bacteremia and endocarditis, which can be challenging to eradicate due to resistance and the organism’s ability to persist in cardiac tissue and on prosthetic material. bacteremia; endocarditis; urinary tract infection.
Antibiotic resistance and treatment
Enterococci are intrinsically less susceptible to many antibiotics commonly used for Gram-positive infections, notably cephalosporins, and they show variable susceptibility to aminoglycosides. The synergistic effect between cell wall–active agents (e.g., ampicillin or vancomycin) and aminoglycosides historically enhanced killing of enterococci, but rising rates of high-level aminoglycoside resistance limit this approach. A major clinical concern is the emergence of vancomycin-resistant Enterococcus (VRE), primarily associated with vanA and vanB gene clusters that alter cell wall targets and reduce vancomycin binding. VRE poses treatment challenges and requires alternative agents such as linezolid, daptomycin, or certain combinations, with careful consideration of toxicity and resistance trends. Other therapeutic options include tigecycline in select scenarios and quinupristin/dalfopristin in limited contexts, though activity against Enterococcus is variable. Infection control and antimicrobial stewardship are essential to limit spread and preserve treatment options. antibiotic resistance; vancomycin-resistant Enterococcus; linezolid; daptomycin; tigecycline; Quinupristin-dalfopristin; antimicrobial stewardship.
Laboratory identification and diagnostic approach
Laboratories identify enterococci through a combination of phenotypic and genotypic methods. Routine culture on non-selective media, followed by tests such as bile esculin positivity and salt tolerance, supports genus identification. Species-level differentiation relies on biochemical panels, MALDI-TOF, and molecular assays. Clinically relevant isolates are further characterized for antimicrobial susceptibility to guide therapy, with particular attention to vancomycin susceptibility due to the public health implications of VRE. bile esculin test; MALDI-TOF; antibiotic susceptibility testing.
Prevention, control, and public health considerations
Because Enterococcus species are common inhabitants of hospital environments and can persist on surfaces, rigorous infection prevention practices are crucial. These include strict hand hygiene, environmental cleaning, isolation of infected or colonized patients when appropriate, and targeted surveillance in outbreaks. Antimicrobial stewardship programs aim to minimize selective pressures that drive resistance, including VRE, by optimizing antibiotic use and reviewing treatment protocols. The balance between treating infections effectively and reducing the emergence of resistance is a central concern for modern health care systems. infection control; nosocomial infection; antibiotic stewardship.