Four strains of Staphylococcus aureus producing enterotoxin types A, B, C, and D were inoculated into buffered Brain Heart Infusion (BHI) broth at a concentration of 106 cells/ml and incubated with agitation at temperatures ranging from 7 to 50 C for intervals extending to 4 weeks. At 45 C, strains 265 (type A), 493 (type C), and 315 (type D) grew and produced enterotoxin, but there was a rapid decrease in viable cells and no enterotoxin produced by strain 243 (type B). In the range of 19 to 39 C, all strains grew and produced enterotoxin. At 13 C, strains 265, 243, and 493 grew but only strain 243 produced enterotoxin. Strain 315 did not grow at 13 C and none of the strains grew at 7 or 50 C. The population of S. aureus varied from 1.3 × 107 to 5.5 × 108 cells/ml when enterotoxin was first detected.

S. aureus strain 243 was inoculated at a concentration of 108 cells/ml into BHI broth with and without 0.2 M sodium phosphate buffer and incubated at 37 C for time intervals extending to 172 hr. The initial pH of the broth was adjusted to various values from 3.62 to 9.84. Growth occurred when the initial pH of the broth was 4.96 to 9.02 and a slight increase in population was noted at pH 4.76 and 9.40. Production of detectable amounts of enterotoxin was restricted to pH 5.15 to 9.02. Detectable enterotoxin was produced in 4 to 6 hr in non-buffered broth, but a minimum of 9 hr was required in buffered broth.

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Author notes

1Michigan Agricultural Experiment Station Journal Article No. 6200.

2Present address: 4625 N. Green Bay Road, Racine, Wisconsin 53404.