Variation among Escherichia coli O157:H7 strains relative to their growth, survival, thermal inactivation, and toxin production in broth

To estimate the potential outcomes of food processing on the fate of foodborne pathogens, variations in microbial parameters such as growth rate, survival time, thermal inactivation time, and toxin production must be known. Previous microbial studies using single strains or cocktails provide error estimates for the uncertainty of the experimental and statistical procedures, but not for variations among strains. In this study, the behavior of 17 strains of Escherichia coli O157:H7 were followed when placed in synthetic media that permitted growth, survival, or thermal inactivation. The parameter values were not rejected as being normal, lognormal, gamma, or Weibull distributions. The ratio of the standard deviation to mean (normal distribution) for the exponential growth rate was 0.16 and for the lag phase duration, it was 0.38. The ratios of times to achieve a 4−log10 reduction at two survival conditions were 0.39 and 0.46; ratios of thermal D values at 55 and 60 °C were 0.42 and 0.33, respectively. The ratio of the negative log10 of toxin production was 0.24. These distributions are larger than the coefficient of variations observed for experimental errors in single strain and cocktail experiments. This indicates the limitations in precision that predictions of future population numbers can have when the potential presence of all strains needs to be considered. This variation among strains is applicable whether predictions are made by traditional subjective and point estimates or by using models and risk assessments.