The culturability of acid-tolerant Salmonella in mayonnaise, a raw egg-based sauce

Abstract

Salmonella is one of the most common bacterial foodborne pathogens and is frequently found as a contaminant of raw egg-based foods. Food safety regulations recommend the use of food acids to mitigate the risk of Salmonella persistence in raw egg-based foods. Salmonella, however, can become tolerant to acidic environments and it is not known how this can affect bacterial persistence in food. This study investigated whether acid-tolerant strains of Salmonella persisted longer in mayonnaise compared with sensitive strains. Isolates of S. typhimurium, S. infantis, S. enteritidis, and S.hessarek were used in this project. Acid-tolerant Salmonella strains were generated using a three-day step-down method where pH was decreased every 24 h from pH 7 to pH 5. Growth curves were determined for both acid-sensitive and acid-tolerant strains. Time-kill experiments were conducted to compare the survivability of acid-sensitive and acid-tolerant Salmonella serotypes in mayonnaise stored at either 5 °C or 25 °C for 72 h. Salmonella exhibited an extended lag phase with increased acid concentration, and acid-tolerant strains recovered faster in media compared with acid-sensitive strains. Elevated biofilm formation was found in acid-habituated strains compared to sensitive strains, and this varied between serotypes. The culturability of Salmonella in mayonnaise stored at 5 °C declined slower than when stored at 25 °C. Acid-tolerant strains persisted longer in mayonnaise and there was a statistically significant difference in culturability (P < 0.05). In conclusion, the current safe food recommendations to control Salmonella in raw egg-based foods are not effective in eliminating it.