Effect of Sodium Nitrite on Growth of Shigella flexneri

A partial factorial design study of the effect of NaNO₂ (0, 100, 200, 1000 ppm) in combination with NaCl (0.5, 2.5, 4.0%), pH (7.5, 6.5, 5.5), and temperature (37, 28, 19°C) on growth of Shigella flexneri is reported. Experiments were done aerobically in brain-heart infusion medium, using an inoculum of 1 × 10³ CFU/ml. Growth curves were fitted from plate count data by the Gompertz equation; exponential growth rates, lag times, generation times, and maximum populations were derived for all variable combinations. In the absence of nitrite, the organism grew well under all test conditions at 37 and 28°C but did not grow at 19°C at pH 5.5 nor at pH 7.5 with 4% NaCl. Nitrite did not affect growth in media of pH 7.5 at 37 and 28°C. At pH 6.5 growth was inhibited by 1000 ppm NaNO₂. The organism failed to grow at 19°C at all nitrite levels in the presence of 2.5 or 4.0% NaCl. The inhibitory effect of nitrite was much greater in media of pH 5.5 and increased with increasing salt levels. More inhibition was apparent at 28 than at 37°C. While lack of growth was used as a paradigm of the effect of nitrite on S. flexneri , nitrite also increased the lag and generation times and decreased the exponential growth rate. Results indicated that NaNO₂ in combinations with low temperature, low pH, and high salt content can effectively inhibit the growth of S. flexneri .