Variation in acid resistance among enterohaemorrhagic Escherichia coli in a simulated gastric environment

Aims: To compare survival of enterohaemorrhagic Escherichia coli (EHEC) strains of two clonal groups in a simulated gastric environment and to quantify the effect of storage in an acidic food, apple juice, on subsequent survival of EHEC in the simulated gastric environment. Methods and Results: To characterize acid resistance of EHEC under conditions simulating the gastric environment, survival was measured in a model stomach system (MSS) for two clonal groups of EHEC: 14 EHEC 1 strains of serotype O157:H7 and 12 EHEC 2 strains of serotypes O26:H11 and O111:H8. There were significant differences between the two EHEC groups, with the average survival rate of O157 strains in the MSS twice as great as the O26/O111 strains. Strains of the two groups also differed in the quantity of injured cells in MSS and in the transcript levels of the glutamate decarboxylase genes (measured by quantitative PCR) in stationary phase before cultures entered the MSS. Conclusions: The results indicate that E. coli O157:H7 strains have superior ability to survive simulated gastric acidity compared with the non-O157 EHEC. Significance and Impact of the Study: E. coli O157:H7 becomes acid resistant rapidly upon entry into stationary phase, which may underlie the low infectious dose of this pathogen.