Relevance of Sensitivity Testings (MIC) of S. aureus to Predict the Antibacterial Action in Milk

Bacterial susceptibility testings were carried out in parallel Iso‐sensitest broth (ISB) and bovine milk cultures using 16 antibacterials and 4 sensitive strains of mastitic isolates of Staphylococcus aureus. Bacterial activities were analyzed by continuous turbidity monitoring (broth cultures), continuous fluorometric monitoring of the resazurin‐reducing redox activity, and by analyzing the triphenyltetra‐zolium (TTC)‐reducing capacity at the end of the incubation period. To obtain an equipotent bacteria‐suppressing activity, milk cultures required in general several times more antibiotic than the respective ISB cultures. Antibacterial activities of sulfadoxine‐trimethoprim, vancomycin, novobiocin, macrolides, aminoglycosides and oxytetracycline were most effectively suppressed by milk. Aminoglycosides suffered additionally from reduction of oxygen in the incubation environment. The ß‐lactams (penicillin G, oxacillin, cephalothin, ceftiofur, ampicillin, ampicillin‐clavulanic acid), gentamicin and enrofloxacin showed extremely variable sensitivity results depending on the S. aureus/milk combination.