Growth of Strains of the Major Non-O157 Shiga Toxin–Producing Escherichia coli Serogroups Is Not Different from Growth of Escherichia coli O157:H7 in Neutral Broth (pH 7.4) and Acidified Broth (pH 5.6) at 10°C

Abstract

Understanding the survival and growth of non-O157 Shiga toxin–producing Escherichia coli (STEC) strains under cold temperatures may be important for protecting public health. The aim of this study was to compare the growth of three strains of each of the major non-O157 STEC serogroups (O26, O45, O103, O111, O121, and O145) with the growth of six O157:H7 STEC strains in broth at 10°C. Brain heart infusion broth (BHIB; pH 7.4) was inoculated with a single strain of stationary-phase STEC culture to produce a starting inoculum of ~106 CFU/ml and stored at 10°C for up to 96 h (three trials per strain). Populations over time were fitted to the Baranyi and Roberts model, and lag-phase duration (LPD) and growth rate were calculated for each strain per trial. Average LPD ranged from 9.2 to 32.8 h for non-O157 STEC and from 10.5 to 17.2 h for O157 STEC. One strain of O26 STEC had a significantly longer LPD (P < 0.05) than did the other strains (32.8 h); otherwise, no significant differences were noted (P > 0.05). Growth rate ranged from 0.031 to 0.060 log CFU/ml/h for non-O157 STEC strains and from 0.034 to 0.046 log CFU/ml/h for O157 STEC strains. No significant difference in growth rate was noted among strains in BHIB at pH 7.4 and 10°C. In subsequent trials, growth of a single strain of each of the non-O157 STEC serogroups was compared with growth of four acid-tolerant O157 STEC strains in BHIB acidified to pH 5.6 with lactic acid. Acidification generally increased LPD and decreased the growth rate for strains, although the effect was variable and not significant. These findings suggest that growth patterns for strains of non-O157 STEC are similar to those for strains of O157 STEC in neutral and pH 5.6 BHIB at 10°C. Further research is needed to determine whether strains behave similarly in meat systems.