Enterococcus Identification Chart
Pinpoint colony on growth medium
Gram Stain: Gram-positive cocci in chains
Catalase test: Negative
Bile Esculin Test: Positive
Growth in 6.5% NaCl ( may also grow @ 10, 45 and even 60°C)
Pyruvate Fermentation Test
Positive: Enterococcus faecalis
Negative: Enterococcus faecium
Gamma Hemolytic Colony on Blood Agar of Enterococcus
5% sheep blood agar (BAP) is with gamma (γ) hemolytic i.e. non-hemolytic colony of Enterococcus faecalis as shown above picture. Enterococci are small (less than 1 mm) or pinpoint colony and catalase test negative.
Classification of Enterococcus
Species: Enterococcus faecalis and E. faecium
Introduction of Enterococcus
Streptococci belonging to group D are classified as enterococci and they are usually non-hemolytic while some strains may be alpha or beta-hemolytic. This enterococci group has been classified in 1984 as a separate genus called Enterococcus based on nucleic acid hybridization studies that showed a more distant relationship to streptococci. The genus Enterococcus contains over 50 species. They are normal flora of the human intestinal tract. E. faecalis and E. faecium are medically important species.
Morphology of Enterococcus
Gram-positive cocci in pairs and short chains. They are non-motile (except, E. gallinarum and E. casseliflavus) non-capsulated, and non- sporing.
Blood agar: They are usually non-hemolytic or gamma ( as shown above image) whereas some strains may be alpha or beta-hemolytic.
MacConkey agar: On MacConkey agar without crystal violet they grow as tiny deep pink colonies.
Note: Types of MacConkey agar are as follows-
MacConkey agar without bile salt and crystal violet: for both Gram-positive and negative bacteria
MacConkey medium with bile salt: For mainly Gram-negative bacteria but Enterococci may also be grown.
MacConkey medium with both bile salt and crystal violet: Only for Gram-negative rods
- They are able to grow in the presence of 40% bile and 6.5% sodium chloride.
- Similarly, their ability to grow at 45°C and at pH 9.6.
- They are PYR test positive.
- E. faecalis is the most commonly isolated Enterococcus from human sources. Other enterococci are E. faecium and E. durans. E. faeclais can be identified by fermentation of mannitol, sucrose, sorbitol, and aesculin and by producing black colonies when grown on tellurite blood agar.
They have the ability to grow over wide-temperature and pH ranges, survive desiccation and grow in the presence of 6.5% NaCl and 40% bile salts.
Their ability to cause –
- Urinary Tract Infection
- Surgical site infections
- Root canal failure
- Immune evasion
- Tissue Damage
- Toxic Metabolites
Lab Diagnosis of Enterococci
Enterococci can be isolated as follow-
- Pinpoint gamma i.e. non-hemolytic colonies on blood agar except some
- Catalase test : Negative
- Bile esculin test: Positive
- Growth in 6.5% sodium chloride and at pH 9.6
- Growth at 10°C and 45°C
- Pyruvate fermentation test
- Acetoin production test
- Litmus milk decolorization test
- PYRase test
- Reduces potassium tellurite test
- Reduces triphenyl tetrazolium chloride (TTC) test
Enterococci resistance to penicillin and other antibiotics occurs frequently. Vancomycin is the primary alternative drug to penicillin for treating enterococcal infections. Vancomycin-Resistant Enterococcus (VRE) has also been isolated. Resistance is most common in E. faecium but vanocomycin resistant strains of E. faecalis also occur.
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