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 ted and dispensed into sterile tubes. Selected anti biotics were added at 2, 4, or 32 times the MIC, and the percentage su rvival of S. aureus at various times through 24 h after addition of ea ch antibiotic was determined. Percentage survival of the same S. aureu s strain grown in vitro and added to 1) Mueller-Hinton broth and 2) ul trahigh temperature pasturized milk with the same concentration of ant ibiotics was also determined. Percentage survival observed after 24 h in milk from the infected quarter at 2, 4, and 32 times MIC, respectiv ely, were: penicillin: 12, 9.4, and 13 %; cephapirin 43, 50, and 30 %; erythromycin: 120, 95, ind 82 pirlimycin: >1000, 148, and 38 %; tilmi cosin: >1000, >1000, and 9 %; ciprofloxacin: 458, 6, and 3 %; norfloxa cin: 40, 10, and <1 %; rifampicin: <1, <1, and <1 %; and novobiocin: 2 0, 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 w as noted in UHT milk for erythromycin, pirlimycin, and tilmicosin at a ll antibiotic concentrations and for ciprofloxacin, norfloxacin, rifam picin, and novobiocin at 2 times the MIC. The lack of correlation betw een in vitro susceptibility testing and antibiotic activity in milk ag ainst organisms grown in vivo is discussed.