The dairy product in which you would be most likely to enumerate Staphylococcus aureus is which?

• A. Pasteurized fluid milk
• B. Yogurt
• C. Raw milk
• D. Nonfat dry milk

Answer: (C)

Staphylococcus aureus in dairy is most likely to be found in a product that hasn’t been heat-treated, because heat treatment such as pasteurization dramatically reduces or eliminates viable bacteria. Raw milk skips that step, so any S. aureus present in the cow’s milk or introduced during milking can remain at detectable levels, especially if there is bovine mastitis or contamination. Pasteurized milk should have had its bacteria killed, and while post-pasteurization contamination can occur, it’s less likely to reach detectable counts. Yogurt creates an acidic, competitive environment from fermentation that inhibits S. aureus growth, making enumeration unlikely. Nonfat dry milk has low water activity, which also limits bacterial growth and survival, further reducing the chance of detectable counts. Thus raw milk is the dairy product most likely to yield measurable S. aureus.