| The objectives of this study are (i) to determine the effect of environmental stress such as low pH on the survival of lineage I and lineage II isolates Escherichia coli O157:H7 and (ii) to investigate the effect of starvation, acid and heat on the distribution of individual lag time (tau) of E. coli O157:H7. Results showed that the survival of non-acid adapted lineage II isolates was significantly higher (P < 0.05) than that of lineage I isolates either at room temperature or 4°C in acid TSB (pH3.5) or at 37°C in synthetic gastric fluid (SGF, pH 2.5). However, following acid adaptation, the difference in the survival rate of the two lineages in SGF at 37°C was not significant (P > 0.05). Both the mean and standard deviation of tau value increased with the duration of stresses tested. The distributions of tau could be described as PearsonV, PearsonVI, Extreme Value, Lognormal, Lognorm(2), and InverseGaussian. These were the top 6 ranked distributions among the 21 types of continuous distribution available in Bestfit. However, the rank order of these distributions varied with the duration of stress treatments. Extreme Value tends to be ranked the 1st for the most severely stressed cells. Overall, Lognormal appears to be the most appropriate to characterize individual cell tau. When acid challenged, the more acid tolerant the strain, the less injury occurs in cells, and the shorter the subsequent lag times are. |
