Model for the survival of Staphylococcus aureus in nongrowth environments

A model was developed to estimate the survival times of Staphylococcus aureus in nongrowth environments. A four strain mixture of S. aureus was inoculated into BHI broth that had a lactate buffer with various combinations of pH (3-7) and lactate (0-1%), NaCl (0.5-20%) and NaNO2 (0-200 ppm) and stored at different temperatures (4-42 degrees C). At appropriate times the survivors were enumerated by sampling and spreading on TSA plates. The survival curves were modeled with two forms of a logistic equation and the D values were determined. Polynomial regression equations were then calculated to predict the effect of the environmental factors on the D values. Survival times were increased with higher pH values, lower temperatures, and lower nitrite and lactate concentrations. Added salt increased survival times until the salt concentrations exceeded that of most foods.