Effect of selected antibiotics on Staphylococcus aureus present in milk from infected mammary glands

Milk from a mammary gland infected with Staphylococcus aureus Newbould 305 was collected and dispensed into sterile tubes. Selected antibiotics were added at 2, 4, or 32 times the MIC, and the percentage survival of S. aureus at various times through 24 h after addition of each antibiotic was determined. Percentage survival of the same S. aureus strain grown in vitro and added to 1) Mueller-Hinton broth and 2) ultrahigh temperature pasteurized milk with the same concentration of antibiotics was also determined. Percentage survival observed after 24 h in milk from the infected quarter at 2, 4, and 32 times MIC, respectively, were: penicillin: 12, 9.4, and 13%; cephapirin 43, 50, and 30%; erythromycin: 120, 95, and 82%; pirlimycin: > 1000, 148, and 38%; tilmicosin: > 1000, > 1000, and 9%; ciprofloxacin: 458, 6, and 3%; norfloxacin: 40, 10, and < 1%; rifampicin: < 1, < 1, and < 1%; and novobiocin: 20, 41, and 5%. By comparison, percentage survival observed after 24 h for in vitro grown S. aureus tested in Mueller-Hinton broth at 2, 4, and 32 times MIC, respectively, were < 1% for penicillin, cephapirin, pirlimycin, ciprofloxacin, norfloxacin, and rifampicin, and < 10% for erythromycin, novobiocin, and tilmicosin. UHT milk had minimal effect on percentage survival compared to Mueller-Hinton broth for S. aureus 305 when tested against penicillin or cephapirin. Increased survival was noted in UHT milk for erythromycin, pirlimycin, and tilmicosin at all antibiotic concentrations and for ciprofloxacin, norfloxacin, rifampicin, and novobiocin at 2 times the MIC.(ABSTRACT TRUNCATED AT 250 WORDS)