Effect of pH and sodium chloride on growth of Bacillus cereus in laboratory media and certain foods.

The effects of NaCl concentration, pH, and water activity (aw) on the ability of vegetative cells of Bacillus cereus to initiate aerobic growth in brain heart infusion broth at 30 C were studied in a factorial design experiment. By using multiple regression techniques, equations were derived which related the decimal reduction of the bacterial population to the concentration of NaCl and pH of broth to which the population was exposed. From these equations, the percentage of inoculated cells capable of initiating growth could be calculated. The reliability of these equations in foods was tested in laboratory-processed meat and rice media. The foods were less inhibitory than the broths, so that accurate prediction of growth initiation in foods was not possible by using the developed formulas. The impact of this type of quantitative study on the development of specific microbial standards for foods is discussed. When the NaCl concentration is increased, the aw is decreased and, with increased deviation of pH from optimum, more concentrated inoculum of B. cereus cells is needed to assure initiation of growth in culture media and foods.