Studies on the growth of Escherichia coli O157:H7 strains at 45oC

The objectives of the present report were to examine the ability of 18 strains of Escherichia coli O157:H7 to grow in EC broth at 42.4, 43.5, 44.5, and 45.5°C, and to document the incidence of phenotypic variants present in low numbers that are capable of growth at 45.5°C in EC broth. Among the 18 strains of E. coli O157:H7 studied, only 3 were capable of producing turbid growth with gas formation in EC broth at 45.5°C with 1 × 102 initial CFU/ml. Higher initial densities of CFU resulted in turbid growth and gas formation in EC broth at 45.5° C with all strains. The presence of bile salts #3 in EC broth was found to be inhibitory at 45.5°C. All 18 strains were found to be capable of growth at 45.5°C in nonselective media. The ability of at least one sensitive strain to grow in EC broth at 45.5°C was found to be dependent on the initial number of CFU/ml. Prior growth of cells of a sensitive strain in EC broth at 45.5°C from a cell density of 2.0 × 107 to 8.0 × 107 CFU/ml followed by removal of cells and reinoculation at a cell density of 2.0 × 106 CFU/ml resulted in growth at 45.5°C that did not occur without such conditioning of the inhibitory medium. These results indicate that the ability of most strains of E. coli O157:H7 to grow in EC broth at 45.5°C is dependent on the initial density of CFU and that at low densities of CFU the ability to initiate growth is dependent on either low numbers of phenotypic variants tolerant to the presence of bile salts #3 in EC broth at 45.5°C or to conditioning of the medium with prior elevated numbers of cells