Survival of enterohemorrhagic Escherichia coli in food and the gastric environment

Pathogenic strains of Escherichia coli, such as E. coli O157:H7, have been shown to be unusually acid resistant, surviving acidic conditions typically expected to inactivate bacteria. Adaptations for survival in low pH environments may also contribute to risk of foodborne illness. These adaptations could account for the low infectious dose (10 to 100 cells) and ability to contaminate acidic foods, such as apple cider. Recently, Enterohemorrhagic E. coli (EHEC) strains of serotypes other than O157:H7 have been implicated in outbreaks of foodborne disease. Acid resistance (AR) studies in acidic foods and in defined media have revealed considerable variation in survival rates among strains of E. coli O157:H7. The basis of this research is to: (1) measure AR ability of strains representing 3 major clonal groups of EHEC, (2) assess the effect of incubation in food matrices on subsequent AR in a simulated gastric environment, and (3) observe the genome-wide transcriptional response of E. coli O157:H7 to stressful conditions. To address these goals, phenotypic AR assays were conducted in two different environments: a minimal acidic environment designed to isolate one of three known AR mechanisms, and a complex acidic environment that simulates gastric conditions. Whole genome transcription profiling was conducted to assess the response of O157:H7 to growth transitions in a minimal medium. The extent to which the known AR mechanisms of E. coli contribute to overall survival and the AR response in naturally acidic foods and the gastric environments was then determined. Variation in survival rates under distinct acidic conditions will provide data for the development of more accurate risk assessment models. Observed differences in survival among the clonal groups of EHEC in this study will allow further studies to characterize the genetic response of EHEC induced by food matrices.