Effect of organic acids and temperature on survival of Shigella flexneri in broth at pH 4

The survival of bacterial pathogens in acidified foods depends not only on the hydrogen ion concentration, but also on the type of acid and the storage temperature. Shigella flexneri is a foodborne pathogen that is acid tolerant. The survival of S. flexneri 5348 in brain heart infusion broth supplemented with 0.04 M acetic, citric, lactic, malic, or tartaric acid and adjusted to pH 4 with HCl or NaOH was studied. The control medium was brain heart infusion broth adjusted to pH 4 with HCl. Stationary-phase cells were inoculated into media at initial populations of 6 to 7 log(10) CFU/ml and incubated at 4, 19, 28, and 37degreesC. A two-phase linear inactivation model was applied to plate count data to derive lag times (t(L)) and slopes of the curves, from which D-values and time required for a 4-log(10) decrease in population (T-4D) were calculated. In all cases, survival increased with decreasing temperature. For each acid, t(L), the D-value, and T-4D increased with decreasing temperature. All acids inhibited S. flexneri to some extent but to differing degrees as follows: lactic acid, acetic acid > citric acid, malic acid, tartaric acid > HCl. The T-4D values for the control medium and for media containing acetic, citric, lactic, malic, and tartaric acids were 64, 47, 50, 34, 58, and 52 h, respectively, at 37degreesC and 2,607, 1,498, 1,905, 1,346, 1,726, and 2,134 h, respectively, at 4degreesC. The results of this study indicate that organic acids may aid in the inactivation of Shigella. However, these data also suggest that foods stored at or below room temperature containing low levels (<1%) of acids could cause illness if contaminated with Shigella.