Initiation of staphylococcal growth in laboratory media

The effects of pH and NaCl concentration on the probability of aerobic growth initiation in Brain Heart Infusion broth at 30 C by five staphylococcal strains producing enterotoxins A, B, C, and D were studied in a factorial design experiment. Statistical analysis of the data indicated: (i) significant effects of pH, NaCl, and strain on the probability of growth; (ii) diverse effects of NaCl with various pH levels and strains; (iii) essentially a linear relationship between NaCl concentration and probability of growth initiation when data for all strains were pooled; (iv) the relationship between NaCl concentration and probability of growth initiation varies from linear to sigmoid, depending on the pH of the broth, when the statistically untreated (raw) data are plotted for each strain. Equations were derived which relate the decimal reductions of the number of cells of a staphylococcal population to the concentration of NaCl and pH of broth to which the population was exposed. From the equations, the probability that one cell is capable of initiating growth can be calculated. The impact of these types of studies on the development of realistic staphylococcal standards in foods is discussed.