Effects of milk on activity of antimicrobics against Staphylococcus aureus isolated from bovine udders

Addition of milk to Mueller-Hinton susceptibility test medium permitted measurement of milk effect on agar disc diffusion zone diameters obtained from Staphylococcus aureus herd isolates and stock strains. Milk reduced zone diameters for all antimicrobics tested when compared with zones obtained by standard methods. Antimicrobics most affected included novobiocin, streptomycin, gentamycin, tetracycline, and vancomycin. Growth curves of a reference strain of Staphylococcus aureus in Mueller-Hinton broth with and without milk indicated no effect on growth rate of the organism, suggesting the observed effect was due to the action of milk on the antimicrobics being tested. The described method offers a simple in vitro test for milk effects on antimicrobic activity.