Enterococcus faecalis: Properties, Pathogenesis, Lab Diagnosis

Last updated on June 25th, 2021

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 in gram-stain

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 and female genitourinary tract.

Transmission frequently occurs when endogenous strains gain access to sterile sites. For example; they may enter 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.

Virulence Factors

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.

Clinical manifestations

Because of their intrinsic and increased drug resistance, Enterococci are mostly responsible for nosocomial infections. Enterococci causes various infections such as:

  • Urinary tract infections (cystitis, urethritis, pyelonephritis, and prostatitis).
  • Bacteremia and mitral valve endocarditis. Endocarditis rare but life-threatening.
  • Intra-abdominal, pelvic, and soft tissue infections
  • Ocular infections
  • Rarely, meningitis and respiratory tract infections.

Laboratory Diagnosis

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 not useful. Enterococci show the following characteristics that help in their identification:

Enterococcus faecalis in blood agar
Gamma-hemolytic colonies of Enterococcus faecalis.  (Image source)
  1. Enterococci are gram-positive oval cocci arranged in pairs; cocci in a pair are arranged at an angle to each other (spectacle-eyed appearance).
  2. Blood agar: It produces smooth, gray, non-hemolytic translucent colonies (rarely produces α or β hemolysis).
  3. MacConkey agar: It produces minute magenta pink colonies.
  4. Nutrient agar: It grows poorly.
  5. They can grow in presence of extreme conditions such as-6.5% NaCl, 40% bile, pH 9.6, 45°C, and 10°C.
  6. Heat tolerance test: They are relatively heat resistant, can survive 60°C for 30 minutes.
  7. 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.

Biochemical tests

Following biochemical tests are important for the differentiation and identification of Enterococcus faecalis.

Name of the testEnterococcus faecalisNotes
Catalase testNegativeTo differentiate enterococci from staphylococci.
HemolysisNon-hemlytic 
Motility testNon-motileE. gallinarum and E. casseliflavus are motile.
Pyrrolidonyl-β-naphthylamide (PYR) testPositivePresumptive identification of group A beta-hemolytic streptococci and enterococci.
Esculin and bile-esculin testPositiveTo differentiate enterococci and non-enterococcus group D streptococci.
Bile solubility testNegativeS. pneumoniae is bile soluble.
LAP testPositive Identification of catalase-negative, gram-positive cocci.
Pyruvate brothPositiveTo differentiate E. faecalis (positive) from E. faecium (negative).
Salt tolerance testPositiveTo differentiate enterococci from non-enteroocci.
Flow chart for the identification of Enterococcus
FeaturesE. faecalisE. faecium
ArabinoseNot fermentedFermented
SorbitolFermentedNot fermented
PyruvateFermentedNot fermented

Treatment

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).

Prevention

Penicillin and gentamicin should be given to patients with damaged heart valves prior to intestinal or urinary tract procedures. No vaccine is available.

About Nisha Rijal 46 Articles
I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.

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