Influence of pH on the heat inactivation of staphylococcal enterotoxin A as determined by monkey feeding and serological assay.

The effect of pH on the thermal inactivation of staphylococcal enterotoxin A was investigated. Analysis of heated toxin by immunodiffusion in gel indicated that enterotoxin A in beef bouillon was inactivated faster at pH 5.3 than at pH 6.2. The z values (slopes) for the heat inactivation curves at pH 6.2 and 5.3 were 49.5 and 55 F (about 27 and 30 C), respectively. Enterotoxin produced and heated in dialyzed Casamino Acids medium and assayed by monkey feeding was more easily inactivated by heat at pH 5.3 than at pH 7.8. Thermal inactivation curves for enterotoxin A in beef bouillon (5 mug/ml, pH 5.3) were determined by two methods, monkey feeding and serological assay. The z values for the curves obtained by these two methods were both 55 F, although loss of biological or toxic activity of the enterotoxin occurred before loss of serological activity.

Effect of toxin concentration on the heat inactivation of staphylococcal enterotoxin A in beef bouillon and in phosphate buffer.

The slope of the thermal inactivation curve of enterotoxin A in beef bouillon (initial pH 6.2) was found to be approximately 27.8 C (50 F) with three different concentrations of toxin. Inactivation by heat was dependent on the concentration of enterotoxin A heated. Enterotoxin A was inactivated by less heat in a pH 7.2 phosphate buffer than in beef bouillon as detected by serology. Results indicate that natural (usually low) levels of toxin rather than concentrated enterotoxin A should be used in heat inactivation studies.