Laboratory Features of Escherichia coli

Laboratory Features of Escherichia coli

Specimen:

Specimen include urine, pus, faeces, cerebrospinal fluid (Infants), and blood for culture or depending upon site of infections.

Morphology:

E.coli is a gram negative usually motile rod. A minority of strains are capsulate.

Cultural Characteristics:

Escherichia coli is an aerobe and facultative anaerobe. Optimum Temperature for the growth of E.coli is 36 – 37 ºC with most strains growing over the range 18 – 44 ºC.

Blood Agar:

Colonies of E.coli on blood agar 1 – 4 mm diameter after overnight incubation. Colonies may also appear mucoid and some strains of E.coli is hemolytic.

E.coli Growth on Blood Agar
E.coli Growth on Blood Agar

MacConkey agar:

Escherichia coli ferment lactose. They produced smooth pink color colonies on MacConkey Agar. Some Strains of E.coli are late or non-lactose fermenting.

E.coli Growth on MacConkey Agar
E.coli Growth on MacConkey Agar

CLED Agar:

They produced yellow colonies on Cystine lactose electrolyte deficient (CLED) agar.

E.coli Growth on CLED Agar
E.coli Growth on CLED Agar

Sorbitol MacConkey Agar:

All strains of E.coli and other enterobacteria ferment sorbitol except E.coli (VTEC) 0157 in non-sorbitol fermenting. E.coli (VTEC) 0157 identified by latex reagent testing.

E.coli Growth on Sorbitol MacConkey Agar
E.coli Growth on Sorbitol MacConkey Agar

XLD Agar:

E.coli strains produced yellow color colonies on Xylose Lysine Deoxycholate agar (XLD agar).

E.coli Growth on XLD Agar
E.coli Growth on XLD Agar

KIA (Kligler Iron Agar):

E.coli produce and acid deep and an acid slope with gas production and no H2S blacking.

E.coli on KIA

Biochemical Reactions:

Most Strains of E.coli:

  • Lysine decarboxylase (LDC) positive.
  • Indole Positive
  • Beta-glucuronidase (PGUA) positive ( E.coli 0157 is PGUA negative).
  • Reduce nitrate to nitrite, urine nitrite test positive.
  • Citrate negative.
  • H2S negative.

Antimicrobial susceptibility:

E.coli strains are sensitive to:

  • Sulphonamides
  • Trimethoprim
  • Cotrimoxazole
  • Nalidixic acid
  • Nitrofurantoin
  • Tetracycline
  • Ampicillin
  • Amoxycillin
  • Cephalosporins
  • Aminoglycosides
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