Representative strains of 15 species of lactic acid bacteria were examined for their ability to influence growth of Staphylococcus aureus and production of enterotoxin in associative culture. Among the organisms used as effectors the streptococci were-most inhibitory, followed by Pediococcus cerevisiae. The lactobacilli and Leuconostoc citrovorum were not inhibitory to growth and only slightly inhibitory to production of enterotoxin. Enterotoxin was detected in all cultures in which the population of S. aureus reached 8 x 107 per ml. At lower S. aureus populations no enterotoxin was detected after incubation for 48 h. Mechanisms of inhibition of growth and enterotoxin production by S. aureus strain 243 grown in association with Streptococcus lactis A64 or P. cerevisiae 10791 in APT broth were investigated. Competition for vital nutrients, especially niacin and biotin, and probably production of hydrogen peroxide contribute to inhibition. Production of lactic acid appears to inhibit growth of S. aureus in the early but not the late stages of incubation. MATERIALS AND METHODS Cultures. S. aureus 243 produces enterotoxin B and was obtained from the late E. P. Casman of the Federal Food and Drug Administration, Washington, D.C. S. lactis A62, A64, A254, G18; S. cremoris E8; Pediococcus cerevisiae FBB39 and FBB63; Lactobacillus plantarum FBB12; and Lactobacillus casei 54 were stock cultures from the Food Microbiology Laboratory at Michigan State University. Streptococcus thermophilus W, Leuconostoc citrovorum

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Representative strains of 15 species of lactic acid bacteria were examined for their ability to influence growth of Staphylococcus aureus and production of enterotoxin in associative culture. Among the organisms used as effectors the streptococci were most inhibitory, followed by Pediococcus cerevisiae . The lactobacilli and Leuconostoc citrovorum were not inhibitory to growth and only slightly inhibitory to production of enterotoxin. Enterotoxin was detected in all cultures in which the population of S. aureus reached 8 × 10 7 per ml. At lower S. aureus populations no enterotoxin was detected after incubation for 48 h. Mechanisms of inhibition of growth and enterotoxin production by S. aureus strain 243 grown in association with Streptococcus lactis A64 or P. cerevisiae 10791 in APT broth were investigated. Competition for vital nutrients, especially niacin and biotin, and probably production of hydrogen peroxide contribute to inhibition. Production of lactic acid appears to inhibit growth of S. aureus in the early but not the late stages of incubation.

The effects of pH, temperature, proportion of Staphylococcus aureus in the inoculum, various strains of effector organism, and various strains of S. aureus were examined for their influence on interactions between staphylococci and effector organisms in associative culture. In general, small changes in pH had little effect upon either growth of S. aureus or production of enterotoxin in associative culture. Inhibition of growth of S. aureus caused by effector organisms was much greater at 25 than at 30 C. Proportion of S. aureus in the inoculum greatly affected both growth of the staphylococci and production of enterotoxin. Only slight differences were found between strains of either effector organism or S. aureus which affected the interactions in associative culture. Previous work in this laboratory has shown that several species of lactic acid bacteria, particularly Streptococcus lactis and Pediococcus cerevisiae, were inhibitory to growth and enterotoxin production by Staphylococcus aureus when grown in association with S. aureus at temperatures favorable to the organisms concerned. Several reports (2-8) have indicated that certain conditions of incubation may significantly affect the influence of the competing organisms upon growth of S. aureus, but none report any effect on enterotoxin production. The investigation reported herein was conducted to determine the effect of variations in some conditions upon the ability of S. lactis and P. cerevisiae to inhibit growth and production of enterotoxin by S. aureus. MATERIALS AND METHODS Cultures. S. lactis strains A62, A64, A254, and G18 and P. cerevisiae strains FBB39 and FBB63 used as effector organisms were from the stock cultures in the Food Microbiology Laboratory at Michigan State University. S. Iactis W was obtained from E. H. Marth of the University of Wisconsin. P. cerevisiae 10791 was obtained from the American Type Culture Collection, Rockville, Md. P. cerevisiae "Accel" is a commercial starter culture from Merck and Co.,