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°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 ≤6% NaCl at 19 and 37°C and in the presence of ≤7% NaCl at 28°C. In media of pH 5, growth was observed in the presence of ≤2, ≤4, ≤4, and 0.5% NaCl at 37, 28, 19, and 12°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°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.