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 aerobi c and anaerobic growth of Staphylococcus aureus 196E were studied usin g 50 ml portions of Brain Heart Infusion Broth in 250ml unsealed and s ealed 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 curve s were generated by fitting the data to the Gompertz function using no nlinear 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 i n the presence of oxygen, (3) ability to grow at high sodium chloride concentrations, and (3) dependence of the bacteriostatic activity of s odium nitrite on pH and oxygen availability. Supplemental studies indi cated that growth kinetics were independent of inoculum size, which al lowed the Gompertz A term to be treated as a constant. However, the ma ximum 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 gen erated using various data transformations. Quadratic models using and LN-transformation provided reasonable predictions of the effects of th e four variables on the growth kinetics of S. aureus, and should prove useful for providing initial estimates of the behavior of S. aureus i n foods.