Influence of substrate and low temperature on growth and survival of verotoxigenic Escherichia coli

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 degrees C (+/-0.2 degrees C). Colony forming units (cfu) were enumerated by surface plating on eosin methylene blue (EMB) agar plates and incubating at 37 degrees 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 degrees C. However, at 9.5 degrees 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 degrees 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 degrees C. Decrease of temperature to 8.5 degrees C resulted in lack of growth of two strains in BHI. At 7.5 degrees 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 degrees 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 degrees C. The results of this study demonstrate the influence of growth medium composition on the minimum growth temperature of verotoxigenic E. coli. (C) 1996 Academic Press Limited