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 triphenyltetrazolium (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 beta-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.