The effect of temperature and low pH on survival of Shigella flexneri in broth.

The survival characteristics of Shigella fiexneri strain 5348 were determined in brain heart infusion broth as a function of low pH (2 to 5) and temperature (4 to 37 degrees C). Stationary-phase cells were inoculated into sterile media to give initial populations of 6 to 7 log10 CFU/ml. Bacterial populations were determined periodically by aerobic plate counts. Survivor curves were fitted from plate count data using a two-phase linear model to derive lag times and slopes of the curves, from which D-values and times to a 4-D (99.99%) inactivation (T4D) were calculated. In general, survival increased as temperature decreased and as pH increased. Bacterial populations reached undetectable levels (<1.3 log10 CFU/ml) at 37, 28, 19, 12, and 4 degrees C in media adjusted to pH 4 after 5, 15, 23, 85, and 85 days, respectively, and in media adjusted to pH 3 after 1, 7, 9, 16, and 29 days, respectively. In media adjusted to pH 2, bacterial populations were stable for 2 to 12 h at temperatures of 19 degrees C or lower and reached undetectable levels after 1 to 3 days, while at 28 and 37 degrees C, the bacteria were undetectable after 8 and 2 h, respectively. In media adjusted to pH 5, bacterial levels decreased only 0.5 to 1.5 log10 CFU/ml after 75 days at 4 degrees C and decreased to undetectable levels after 135 days at 12 degrees C, while growth occurred at higher temperatures. These results indicate that S. flexneri is acid resistant and that acidic foods may serve as vehicles for infection.