Growth of Staphylococcus aureus in Milks and Creams with Various Amounts of Milk Fat

Abstract

Staphylococcus aureus strains 100-A, 196-E, 254, 473, 505, and 521 generally grew better in skim milk and whole milk (3.5% milk fat) than in light cream (18% milk fat) and heavy whipping cream (36% milk fat) at 25 and 37°C. S. aureus strains grew to greater than 106/ml from an inoculum of ca. 103/ml in skim and whole milk held at 37°C. From the same level of inoculum, strains grew to 106–107/ml in light cream held at 37°C. A 10 to 100-fold increase in population was observed for most strains in heavy whipping cream inoculated with ca. 104S. aureus/ml and held at 37°C. Less than a 10-fold increase in population was noted for strain 254 in heavy whipping cream held at 37°C. When strains of S. aureus were inoculated into milks and creams and held at 25°C, populations of the pathogen increased approximately 100-fold in skim and whole milk and 10-fold or less in light cream and heavy whipping cream. S. aureus was enumerated using Chapman Stone, Mannitol Salt, Baird-Parker, and Vogel-Johnson agars; best results were obtained with Baird-Parker and Vogel-Johnson agars, particularly when high-fat fluid products were tested. Milk fat content, incubation temperature, and strain variation influenced the ability of S. aureus to grow in various dairy fluids.