Enterococci are catalase negative, gram-positive cocci in chains. Initially grouped under group D Streptococcus, they are reclassified later as a separate genus Enterococcus under a family Enterococcaceae. Enterococcus faecalis and Enterococcus faecium are the two most common species involved in infections. They are becoming increasingly important agents of human disease, especially in hospitalized patients. Enterococcus faecalis is an important cause of hospital-acquired urinary tract infections and endocarditis.
Habitat and Transmission
Enterococci are found in soil, food, water, and as normal flora of animals, birds, and humans. Enterococcus faecalis and Enterococcus faecium are normal flora of the human gastrointestinal tract (colon) and female genitourinary tract.
Transmission frequently occurs when endogenous strains gain access to sterile sites. For example; they may enter the bloodstream during gastrointestinal (GI) or genitourinary tract procedures and may infect other sites (e.g., endocarditis). Person-to-person transmission, directly or by contaminated medical equipment, allows nosocomial spread and colonization with multi-drug resistant strains. Once colonized, immunocompromised patients are at risk of developing infections.
Enterococci exhibit a number of virulence factors such as:
- Cytolysin/hemolysin: They lyse the sheep and human RBCs.
- Aggregation substances or pheromones: They help in the clumping of adjacent cells to facilitate plasmid exchange (transfers drug resistance).
- Extracellular surface protein (ESP): It helps in adhesion to the bladder mucosa.
- Common group D lipoteichoic acid antigen: It induces cytokine release such as tumor necrosis factor-α (TNF-α).
- Coccolysin: It inactivates endothelin, a vasoactive peptide.
Because of their intrinsic and increased drug resistance, Enterococci are mostly responsible for nosocomial infections. Enterococci cause various infections such as:
- Urinary tract infections (cystitis, urethritis, pyelonephritis, and prostatitis).
- Bacteremia and mitral valve endocarditis. Endocarditis is rare but life-threatening.
- Intra-abdominal, pelvic, and soft tissue infections
- Ocular infections
- Rarely, meningitis and respiratory tract infections.
Gram-stained smear and culture. Alpha, beta, or nonhemolytic colonies on blood agar. Grows in 6.5% NaCl and hydrolyze esculin in the presence of 40% bile. Serologic tests are not useful. Enterococci show the following characteristics that help in their identification:
- Enterococci are gram-positive oval cocci arranged in pairs; cocci in a pair are arranged at an angle to each other (spectacle-eyed appearance).
- Blood agar: It produces smooth, gray, non-hemolytic translucent colonies (rarely produces α or β hemolysis).
- MacConkey agar: It produces minute magenta pink colonies.
- Nutrient agar: It grows poorly.
- They can grow in presence of extreme conditions such as-6.5% NaCl, 40% bile, pH 9.6, 45°C, and 10°C.
- Heat tolerance test: They are relatively heat resistant, and can survive 60°C for 30 minutes.
- Groups: Enterococci can be divided into five groups-group I to V based on mannitol fermentation and arginine hydrolysis. E.faecalis and E. faecium belong to group II, which can be further differentiated by several biochemical properties.
Following biochemical tests are important for the differentiation and identification of Enterococcus faecalis.
|Name of the test||Enterococcus faecalis||Notes|
|Catalase test||Negative||To differentiate enterococci from staphylococci.|
|Motility test||Non-motile||E. gallinarum and E. casseliflavus are motile.|
|Pyrrolidonyl-β-naphthylamide (PYR) test||Positive||Presumptive identification of group A beta-hemolytic streptococci and enterococci.|
|Esculin and bile-esculin test||Positive (Growth, black precipitate)||To differentiate enterococci and non-enterococcus group D streptococci.|
|Bile solubility test||Insoluble||S. pneumoniae is bile soluble.|
|LAP test||Positive||Identification of catalase-negative, gram-positive cocci.|
|Pyruvate broth||Positive||To differentiate E. faecalis (positive) from E. faecium (negative).|
|Salt tolerance test||Positive||To differentiate enterococci from non-enterococci.|
|Features||E. faecalis||E. faecium|
Enterococci show intrinsic resistance to cephalosporins and cotrimoxazole. Most strains of enterococci are resistant to penicillins, aminoglycosides, and sulfonamides. Resistance is overcome by combination therapy with penicillin (or ampicillin) and aminoglycoside such as gentamicin (due to synergistic effect) and this remains the standard therapy for life-threatening enterococcal infections. Aminoglycoside alone is ineffective because it cannot penetrate. Penicillin or vancomycin weakens the cell wall, allowing the aminoglycoside to penetrate.
Vancomycin is usually indicated in resistant cases but resistance to vancomycin has also been reported. Linezolid can be used to treat vancomycin-resistant enterococci (VRE).
Penicillin and gentamicin should be given to patients with damaged heart valves prior to intestinal or urinary tract procedures. No vaccine is available.