Specimen include urine, pus, faeces, cerebrospinal fluid (Infants), and blood for culture or depending upon site of infections.
E.coli is a gram negative usually motile rod. A minority of strains are capsulate.
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.
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.
Escherichia coli ferment lactose. They produced smooth pink color colonies on MacConkey Agar. Some Strains of E.coli are late or non-lactose fermenting.
They produced yellow colonies on Cystine lactose electrolyte deficient (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 strains produced yellow color colonies on Xylose Lysine Deoxycholate agar (XLD agar).
KIA (Kligler Iron Agar):
E.coli produce and acid deep and an acid slope with gas production and no H2S blacking.
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.
E.coli strains are sensitive to:
- Nalidixic acid