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Factors Influencing the Production of Staphylococcal Enterotoxin A in Milk^1,2,3,

Department of Food Science and Industries, University of Minnesota, St. Paul 55101

J. C. Olson, Jr. and E. P. Casman

and Division of Microbiology, Bureau of Science, Food and Drug Administration, Washington, D.C. 20204

ABSTRACT

The influences of the initial bacterial count and heat treatment of raw whole milk and the initial pH of sterile reconstituted nonfat dry milk on growth and enterotoxin A production by Staphylococcus aureus were evaluated. In milk relatively free from competing microorganisms, an S. aureus population of 2 to 3 million per milliliter was associated with detectable enterotoxin A. It was detected after 4 to 6 hours in low-count raw milk or sterile reconstituted nonfat dry milk inoculated initially with 10³ to 10⁵ S. aureus per milliliter. Though S. aureus reached 2 to 3 million per milliliter in commercial raw milks, no enterotoxin A was detected. Growth inhibition of S. aureus in raw milks may or may not always result in simultaneous inhibition of enterotoxin A production. Good growth and detectable enterotoxin A resulted in heat treated raw milk (pasteurized or heated at 65.6 C for 16 sec). Enterotoxin A was produced in sterile reconstituted nonfat dry milk at initial pH's of 4.5 to 6.5 (adjusted with HCl). At pH 4.5 lactic acid was bactericidal to S. aureus.

¹ Paper 7384, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul.

² Taken from data submitted to the Graduate School faculty at the University of Minnesota by the senior author in partial fulfillment of the requirements for the Ph.D. degree.

³ This research project was supported in part by grant (UI00269) from the National Center for Urban and Industrial Health, USPHS.

⁴ Present address: Department of Botany and Microbiology, Montana State University, Bozeman 59715.