O157 Case Study

Doyle and Schoeni (1984) reported 37 0C as the optimum temperature for growth of Escherichia coli 0157:H7 in Trypticase Soy Broth. This grew poorly in the temperature range of 44 to 45.5 °C which is generally used for recovery of E. coli from foods.
Bettelheim (1998) designed Rainbow Agar O157 for the isolation and identification of EHEC. In this medium E. coli O157 was characterised by black colonies whereas O113 and some other EHEC strains were mauve, red or pink and indistinguishable from other strains of E. coli.
Manafi and Kremsmaier (2001) evaluated the efficiency of different media for detecting Escherichia coli O157:H7. They found that SMAC agar had a sensitivity and specificity of 94.1 per cent and 91.6 per cent, Fluorocult HC agar
(2010) reported that 6 h enrichment instead of 24 h in medium described by Posse et al. (2008) had no significant effect on the recovery rate of EHEC. Using direct plating after enrichment, recovery rates were influenced by the inoculated serotype and the inoculation level of the organism. Using immunomagnetic separation, recovery of serotype O157 was significantly improved, whereas for O111, O26, O103 and O145 no significant effect was observed.
For the isolation and identification of EHEC from stool samples, Jigna and Pratibha (2012) streaked samples on MacConkey agar plates and typical colonies suspected to be of E.coli were selected and further streaked on the EMB agar plate. The colonies showing Metallic sheen and characteristic biochemical tests for E. coli were identified and streaked on to MUG Sorbitol MacConkey agar plates. Non sorbitol fermenting and nonfluorescent colonies were identified as EHEC.
Tzschoppe et al. (2012) carried out enrichment of spiked samples of ready-to-eat salads in Brilliant Green Bile Lactose Broth (BRILA) for 6 h. After enrichment, samples were plated on to selective Chromocult Tryptone Bile X-Glucuronide agar (TBX) and incubated at 44 °C overnight. This was sufficient to detect 1 to 10 EHEC from spiked
(2013) reported a method for isolation of E. coli O157 from meat sample. Twenty five grams of meat sample was blended with 225 ml of sterile modified tryptone soya broth containing vancomycin (40 μg/ml) followed by incubation at 37 °C for 18 h. Enriched culture was plated onto sorbitol MacConkey agar supplemented with cefixime and potassium tellurite. The plates were incubated at 37 °C for 24 h. E. coli O157 positive strains were further tested for sorbitol fermentation, glucuronidase activity, and enterohemolysin production. D-Sorbitol fermentation was examined in the tube test, while the β-glucuronidase activity was tested in 4-methylumbelliferyl-p-D-glucuronide containing Fluorocult BRILA broth. Detection of the enterohemolytic phenotype was performed on blood agar plates containing 5 per cent of washed sheep erythrocytes. Appearance of a narrow turbid zone of hemolysis within 18–24 h incubation at 37 °C was regarded as a positive result for E. coli