Enterococcus
Introduction
Enterococcus is a genus of Gram-positive bacteria. At first, scientists classified them as Group D Streptococcus, but later on, they recognized Enterococcus as a separate genus. In addition, these organisms naturally reside in the human gut, oral cavity, and female genital tract as part of the normal flora.
On the other hand, although harmless in most healthy individuals, they can behave as opportunistic pathogens. For example, they often cause hospital-acquired (nosocomial) infections such as urinary tract infections, bacteremia, endocarditis, and wound infections.
Furthermore, because of their rising resistance to antibiotics, Enterococcus species have become a significant concern in modern clinical microbiology. As a result, doctors and microbiologists closely monitor these bacteria to prevent and control outbreaks.
Enterococcus Species
Enterococcus faecalis is the more common species and typically remains susceptible to a wider range of antibiotics. In contrast, Enterococcus faecium shows higher resistance, particularly against vancomycin.
Morphology
Gram-positive oval cocci in short chains or pairs.Non-motile, non-sporing, catalase-negative (may show weak catalase).
Culture Media
Blood agar: γ-hemolysis (non-hemolytic) or α-hemolysis.
Bile esculin agar: Hydrolyzes esculin → black colonies.
Growth in 6.5% NaCl broth: Positive (important for differentiation).
Biochemical Reactions
Catalase: Negative (may be weak positive).
PYR test: Positive.
Bile-esculin test: Positive.
NaCl tolerance (6.5%): Growth (differentiates from non-enterococcal Group D streptococci).
Resistance
Intrinsic resistance to many β-lactams, aminoglycosides, cephalosporins.
Acquired resistance:
High-level aminoglycoside resistance.
Vancomycin resistance (VRE): Due to vanA, vanB genes → major hospital problem
Antigen Structure
Group D cell wall antigen (teichoic acid with D-alanine).
Surface adhesins, aggregation substance → adherence to host tissues.
Toxins & Enzyme
Not classical toxin-producers like S. aureus or S. pyogenes.
Produce cytolysin, gelatinase, hyaluronidase (aid virulence).
Pathogenesis
Opportunistic in nature, Enterococcus frequently causes infections in hospital settings (nosocomial infections).
Catheter-associated urinary tract infections are among the most common presentations.
It can also lead to bacteremia and endocarditis, particularly in patients with prosthetic heart valves.
In some cases, intra-abdominal and pelvic infections are linked to this bacterium.
Antibiotic Sensitivity
Ampicillin or penicillin + aminoglycoside (gentamicin/streptomycin) (synergy).Vancomycin – for resistant strains.Linezolid, daptomycin, tigecycline – for VRE.
Epidemiology
Reservoir
Enterococcus primarily resides in the human gut flora. Besides this, it may also be present in the oral cavity and female genital tract as part of the normal flora.
Transmission
Transmission most often occurs endogenously, when the bacteria spread from the gut to other parts of the body. Nevertheless, a large number of infections are hospital-acquired, especially in patients undergoing medical procedures.
Risk Factors
A variety of factors can increase the risk of Enterococcus infection. For example, prolonged hospitalization provides more opportunities for bacterial exposure. Additionally, indwelling devices such as catheters serve as potential entry points for infection. People with compromised immune systems remain particularly vulnerable. Lastly, the frequent use of broad-spectrum antibiotics disrupts the normal microbial balance, which makes it easier for Enterococcus to thrive.
Laboratory Diagnosis
Specimens: Urine, blood, wound swab, catheter tips.
Microscopy: Gram-positive cocci in pairs/chains.
Culture: Bile-esculin agar (black colonies), 6.5% NaCl tolerance.
Biochemical tests: PYR positive.
Antibiotic sensitivity: Disk diffusion, MIC, detection of van genes (PCR).
Treatment
Ampicillin + aminoglycoside (synergistic).
Vancomycin (if resistant to penicillin).
Linezolid, daptomycin for VRE.
Supportive: Device removal, infection control measures in hospitals.
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