Model for the combined effects of temperature, initial pH, sodium chloride and sodium nitrite concentrations on anaerobic growth of Shigella flexneri.

A fractional factorial design was used to measure the effects and interactions of temperature (12-37 degrees C), initial pH (5.5-7.5), NaCl (0.5-4.0%) and NaNO2 (0-1000 ppm) on the anaerobic growth kinetics of Shigella flexneri in Brain-Heart Infusion broth. Anaerobic conditions were established by flushing the culture flasks with N2. A total of 375 cultures representing 124 variable combinations were analyzed, with growth curves being generated using the Gompertz equation. Growth rates decreased with decreasing temperature, decreasing pH and increasing NaCl level. NaNO2 in combination with low temperature, low pH and high NaCl content effectively inhibited S. flexneri. Response surface analysis was used to obtain models for estimating the growth of S. flexneri in terms of temperature, initial pH, and NaCl and NaNO2 concentrations. A third-order equation using the natural logarithm transformations for the Gompertz B and M terms gave reasonable estimates of bacterial growth in response to any combination of the variables studied within the specified ranges.