The acid tolerance responses of the Salmonella strains isolated from beef processing plants

The development of the stationary-phase, low-pH-inducible acid tolerance response (ATR) in the Salmonella contaminant of beef during the processing arises food safety concerns, because it may evoke bacterial coping mechanisms against bactericidal insults and alter gene expression that contribute to pathogen virulence. However, information on the development of the ATR and the stability (defined as the capacity to maintain the acquired acid tolerance after induction) in the Salmonella during the production and distribution of beef is limited. After adaptation overnight, ATRs in the 79 strains of Salmonella isolated from beef processing plants were investigated by comparing the log reduction in the 2-h acid challenge trials at pH 3.0. Six representative strains were selected to further estimate the effect of three factors in the incubation period on the development of the ATR, including adapted pH values (5.0, 5.4, 6.0, and 7.0), temperatures (10 °C and 37 °C), and the adaptation media (meat extract and brain heart infusion media). The stability of acid tolerance during the long-time chilled storage (4 °C for 13 days) was also observed on two strains of serotypes S. Derby and S. Meleagridis. All the strains isolated from beef processing plants exhibited an enhanced acid tolerance indicating the widespread existence of ATR. The results also revealed that strain variability was present in the development of ATR. Significant tolerance to lethal acidic environments (pH 3.0) was found when the Salmonella strains had been acid-adapted in meat extract at pH 5.0, pH 5.4, or pH 6.0, which indicated the possible induction of ATR during beef production. After the acid adaptations, the population reduction after the acid challenge (BHI, pH = 3) in the strains was significantly lower than the non-induced at the 1d, 7 day and 13 day's storage in meat extract media at 4 °C, which revealed the persistence of ATR during beef distribution. Compared to 37 °C, adaptation in lower temperature (10 °C) significantly reduced the ATR and no ATR was developed when adapted in 4 °C. This emphasizes the importance of keeping a low temperature of beef throughout the supply chains of beef industry.