The Acid Tolerance Responses of the Salmonella Strains Isolated from Beef Processing Plants

Abstract

The development of acid tolerance response (ATR) in the Salmonella contaminant of beef during the processing of the beef arises food safety concerns. However, information on the development and the inducing and the inhibiting factors of the ATR in the Salmonella during the production and the distribution of beef is limited. We investigated the ATRs in the 79 strains of Salmonella that were isolated from beef processing plants and selected the representative strains to estimate the influences of the pH values (5.0, 5.4, 6.0, and 7.0), temperatures (10 °C and 37 °C), the acid-adaptation and the acid-challenge media (meat extract and brain heart infusion media), and chilled storage (4 °C for 13 days) on the induction and the persistence of the ATR. Four types of ATR were found in the 80 Salmonella strains (one standard strain, Salmonella Typhimurium ATCC 14028, was included as a reference strain), which indicated the prevalence of ATR in the Salmonella strains isolated from beef processing plants. Moreover, the strain-specificity of the type of ATR was established. 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 further confirmed the possible induction of ATR during beef production. After the acid adaptations (i.e., ATR inductions), the ATRs persisted during the 13-day storages of the bacterial cultures in meat extract media at 4 °C, which revealed the plausible 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.