The effect of NaCl on survival of Shigella flexneri in broth as affected by temperature and pH

Abstract

Shigella, a major foodborne pathogen, survives well in salt-containing environments. However, systematic data are scarce. We studied the behavior of Shigella flexneri 5348 in brain heart infusion broth (pH 4 to 6) containing 0.5 to 8% NaCl. Stationary-phase cells were inoculated into sterile media at initial concentrations of 6 to 7 log10 CFU/ml and incubated at 12 to 37 degrees C. Bacterial population sizes were determined periodically by plate counts. Survivor curves were derived from plate count data by using a two-phase linear model to determine lag times and slopes of the curves, from which decimal reduction times (D-values) and times to a 4-log10 inactivation (T4D) were calculated. In media of pH 6, the bacteria grew in the presence of < or = 6% NaCl at 19 and 37 degrees C and in the presence of < or = 7% NaCl at 28 degrees C. In media of pH 5, growth was observed in the presence of < or = 2, < or = 4, < or = 4, and 0.5% NaCl at 37, 28, 19, and 12 degrees C, respectively. Growth did not occur and bacterial populations gradually declined in media of pH 4. While NaCl had a major effect on growth, bacterial survival was affected to a lesser extent. Lag times decreased with increasing NaCl levels; however, the effect on D-values and T4D values was less pronounced. The average T4D values for media of pH 4 containing 0.5 to 6% NaCl were 4, 13, 23, and 61 days at 37, 28, 19, and 12 degrees C, respectively. These results show that S. flexneri is salt tolerant and suggest that salty foods may serve as vehicles for infection with this bacterium.