Survival of Escherichia coli O157:H7 and Salmonella spp. in Acidic Condiments

This study examined the effects of adaptation to acid and storage temperature on the survival of a nonpathogenic Escherichia coli strain, three strains of E. coli O157:H7, and three strains of Salmonella spp. in ketchup, mustard, and sweet pickle relish. Stationary-phase cells were adapted to acidic conditions in pH 5.0 Trypticase soy broth for 4 h at 37°C. Samples inoculated with individual strains (10⁵ CFU/g for E. coli and 10⁶ CFU/g for Salmonella spp.) were stored at 5 and 23°C. Numbers of surviving cells were determined by plating on Trypticase soy agar. Acid adaptation enhanced the survival of all three Salmonella strains (ATCC 6962, 13311, and 25957) and all three E. coli O157:H7 strains (ATCC 43889, 43894, and 43895), with the magnitude of the effect depending on the strain and storage temperature. Acid adaptation enhanced survival of the nonpathogenic E. coli strain at 5°C, but not at 23°C. Cells of all tested strains survived longer at 5°C than at 23°C. In general, the E. coli O157:H7 strains survived longer than the Salmonella and nonpathogenic E. coli strains. Nonadapted E. coli O157:H7 cells were recovered from ketchup stored for I to 7 days at 5°C, whereas only low numbers of nonadapted Salmonella strain ATCC 25957 were recovered after 1 day at 5°C and no nonadapted Salmonella strains survived 2 days at 5°C. All of the E. coli and Salmonella strains died within 1 h in mustard and sweet pickle relish stored at 5 and 23°C. This study showed that adaptation to acid and low temperature enhanced the survival of E. coli O157:H7 and Salmonella strains in ketchup but not in mustard or sweet pickle relish.