ENTEROCOCCUS: Morphology, Lab Diagnosis and Control

Enterococcus/Group D Streptococcus:

Group D Streptococci

  • Former Lancefield Group D is classified in 1980 into two groups.

Enterococci

  • Fecal Streptococci has been reclassified as separate Genus called Enterococcus- containing different species, such as Enterococcus faecalis, Enterococcus faecium.
Non-Enterococcal Group D
Streptococcus bovis
Enterococci
  • Enterococci contain cell wall polysaccharide that reacts with group D antisera. Therefore, in the past, they were considered group D streptococci. Today, DNA analysis and other properties have placed them in their own genus, Enterococcus.
  • The clinically most important species are E.faecalis and E. faecium. Enterococci can be Alpha, Beta, or nonhemolytic. As a rule, enterococci are not very virulent, but they have become prominent as a cause of nosocomial infections due to their multiple antibiotic resistance.

Enterococcus faecalis

  • Enterococcus faecalis is the main pathogen in the genus Enterococcus, causing about 95% of enterococcal infection.

Morphology

  • They are Gram‐positive cocci arranged in pairs or short chains. They are non-capsulated.

Pathogenesis

  • Enterococci are a commensal organism and do not have potent toxin and well-defined virulence factors.
  • Their greatest significance is their resistance to many commonly used antibiotics.

Disease:

It produces:
  1. Urinary tract infection
  2. Biliary tract infection
  3. Ulcers (e.g. bed sores)
  4. Wound infection (particularly abdominal)
  5. Occasionally endocarditis and meningitis

Laboratory diagnosis

Specimen collection: Possible pathological specimens are
  1. Urine
  2. Pus swab
  3. Blood
  4. CSF.
Microscopic examination:
  • Microscopic examinations of Gram‐staining smear shows Gram‐positive cocci in pairs or in short chains.
Culture characters: 
  • Enterococci are aerobic; organisms are capable of growing a wide range of temperature (10‐45°C). They can withstand heat at 60°C for 30 minutes. Grow at pH 9.6, and 6.5 % NaCl broth. Grow at 40 % Bile.
Blood agar: 
  • Enterococci are generally non-hemolytic but some strains show alpha or beta hemolysis.
Fig: Enterococcus facalis in blood agar
MacConkey agar: 
  • E. faecalis ferments lactose, produce small, dark‐red colonies.
Bile Esculin Agar:
  • Produces black colored colonies due to hydrolysis of esculin to esculetin which forms the black precipitate.
Cysteine lactose electrolytedeficient agar (CLED): 
  • It produces small yellow colonies on CLED agar.
TU Notes: ENTEROCOCCUS: Morphology, Lab Diagnosis and Control

Biochemical reactions

  • Enterococci are distinguished from the non‐Group D streptococci by their ability to survive in the presence of bile and to hydrolyze the polysaccharide esculin.
  • Unlike non-enterococcal group D streptococci, enterococci grow in 6.5 percent NaCl and yield a positive PYR test.
  • E. faecalis can be distinguished from E. faecium by their fermentation patterns, which are commonly evaluated in clinical laboratories.

Treatment

  • Most Enterococci are sensitive to ampicillin and resistant to cephalosporin.

Antibiotic Resistance

  • A major problem with the enterococci is that they can be very resistant to antibiotics. E faecium is usually much more antibiotic‐resistant than E. faecalis.
  • Enterococci are naturally resistant to BetaIactam antibiotics and aminoglycosides but are sensitive to the synergistic action of a combination of these classes. In the past, the initial regimens of choice were penicillin + streptomycin, or ampicillin + gentamicin.

Vancomycin Resistance

  • The glycopeptide vancomycin is the primary alternative drug to a penicillin (plus an aminoglycoside) for treating enterococcal infections. These enterococci are not synergistically susceptible to vancomycin plus an aminoglycoside. Vancomycin resistance has been most common in E faecium, but vancomycin‐resistant strains of E. faecalis also occur.
  • The gene encoding for the enterococcal Beta‐lactamase is the same gene as found in Staphylococcus aureus. The gene is constitutively expressed in enterococci and inducible in staphylococci. Because enterococci may produce small amounts of the enzyme, they may appear to be susceptible to penicillin and ampicillin by routine susceptibility tests

 NON-ENTEROCOCCAL GROUP D STREPTOCOCCI

  • Streptococcus bovis is the most clinically important of the non-enterococcus group D streptococci. Part of normal fecal flora, they are either alpha‐ or nonhemolytic. S. bovis occasionally causes urinary tract infections and subacute bacterial endocarditis, especially in association with bowel malignancy. 
  • The organism is bile‐ and esculin positive, but is PYR‐negative, and does not grow in 6.5 percent salt (unlike the enterococci). It tends to be sensitive to penicillin and other antibiotics.

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