RESPONSE-SURFACE MODELS FOR THE EFFECTS OF TEMPERATURE, PH, SODIUM-CHLORIDE, AND SODIUM-NITRITE ON THE AEROBIC AND ANAEROBIC GROWTH OF STAPHYLOCOCCUS-AUREUS 196E

The effects and interactions of temperature (12-45C), initial pH (4.5-9.0), NaCl (0.5-165%), and sodium nitrite (1-200 mug/ml) on the aerobic and anaerobic growth of Staphylococcus aureus 196E were studied using 50 ml portions of Brain Heart Infusion Broth in 250ml unsealed and sealed trypsinizing flasks, respectively. The flasks were inoculated to a level of approximately 10(3) cfu/ml, incubated on a rotary shaker, sampled periodically, and enumerated on Tryptic Soy Agar. Growth curves were generated by fitting the data to the Gompertz function using nonlinear regression analysis. The general growth characteristics of S. aureus in response to the five environmental variables were similar to those observed by other investigators including (1) enhanced growth in the presence of oxygen, (3) ability to grow at high sodium chloride concentrations, and (3) dependence of the bacteriostatic activity of sodium nitrite on pH and oxygen availability. Supplemental studies indicated that growth kinetics were independent of inoculum size, which allowed the Gompertz A term to be treated as a constant. However, the maximum population density (MPD) achieved by the cultures was dependent on the independent variables, requiring that it be modeled in addition to the Gompertz B and M terms. The MPD was then used to calculate the Gompertz C term. Quadratic and cubic response surface models were generated using various data transformations. Quadratic models using and LN-transformation provided reasonable predictions of the effects of the four variables on the growth kinetics of S. aureus, and should prove useful for providing initial estimates of the behavior of S. aureus in foods.