Factors Influencing the Production of Staphylococcal Enterotoxin A in Milk

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 10 S 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 see). Enterotoxin A was produced in sterile reconstituted nonfat dry milk at initial pH's of 4.5 to 6.5 (adjusted with HC1). At pH 4.5 lactic acid was bactericidal to S. aureus.