Survival to different acid challenges and outer membrane protein profiles of pathogenic Escherichia coli strains isolated from pozol, a Mexican typical maize fermented food.

In this study, the acid resistance and the changes in outer membrane protein (Omps) profiles of Escherichia coli strains isolated from pozol, an acid-fermented maize beverage consumed in Southeastern Mexico, were determined. Results showed that adaptation to acid by these E. coli strains significantly enhances their survival in acid conditions. Changes in Omp profiles were found in non-adapted acid challenged cells compared with non-challenged cells that had not been adapted to acid. Challenged adapted cells showed no significant changes in these profiles when compared with the acid adapted non-challenged strains. N-terminal sequences of some of the Omps were determined. The intensity of the main porins OmpC and OmpA was lower in the acid challenged strains, than in the non-challenged ones. The OmpF porin was identified in non-challenged K12 strain, but did not appear in adapted or non-adapted pozol strains nor in E. coli O157:H7. A protein band with an approximate molecular mass of 22 kDa corresponds to OmpW and its expression decreased in pozol strains challenged with HCl and lactic acid. OmpX was one of the main proteins expressed when strains were acid challenged with organic acids. Seventy out of seventy-three E. coli strains isolated from pozol in a previous work [Sainz, T., Wacher, C., Espinoza, J., Centurion, D., Navarro, A., Molina, J., Cravioto, A., Eslava, C., 2001. Survival and characterization of Escherichia coli strains in a typical Mexican acid-fermented food. International Journal of Food Microbiology 71, 169-176] carry this gene and belong to a reported pathogenic class of E. coli strains, or have virulence factors or survived at pH values less than 4.8. We suggest this protein could be involved in survival to stress conditions.