Abstract
Growth and survival of verotoxigenic Escherichia coli(VTEC) were studied in tryptic soy broth (TSB), brain–heart infusion (BHI), and whole milk at 12, 9.5, 8.5, 7.5, 6.5, and 5.5°C (±0.2°C). Colony forming units (cfu) were enumerated by surface plating on eosin methylene blue (EMB) agar plates and incubating at 37°C for 24 h. Growth curves were plotted and lag and generation times were determined. Results indicated that the average generation times for each strain was similar in each medium (4.6±0.9 h) at 12°C. However, at 9.5°C the average generation time differed significantly among the media ( P<0.05). The average generation time was longer in BHI compared to TSB and whole milk at 9.5°C (14 h in BHI vs 11 h in milk and TSB). Also, the generation time of individual strains in BHI varied from 9.5–22 h at 9.5°C. Decrease of temperature to 8.5°C resulted in lack of growth of two strains in BHI. At 7.5°C none of the strains grew in BHI, one grew in TSB, whereas all grew in milk. A further decrease of temperature to 6.5°C did not allow growth of any of the strains in TSB or BHI whereas all grew in milk. Addition of 5% lactose to TSB and BHI enabled survival and growth of all strains at 6.5°C. The results of this study demonstrate the influence of growth medium composition on the minimum growth temperature of verotoxigenic E. coli.
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