In vitro determination in milk of the Minimal Inhibitory and Bactericidal Concentrations of celquinone, marbofloxacin, tylosin and spiramycin for bovine mastitis pathogens.

The authors compared the Minimal Inhibitory Concentrations (MIC) and the Minimal Bactericidal Concentrations (MBC) of four antimicrobial agents used for treatment of bovine mastitis, obtained by dilution procedure in Mueller-Hinton broth and milk (UHT sterilized low-fat milk; fat: 1.5%, pH: 6.6) respectively. Effects of milk on in vitro activity of cefquinome, marbofloxacin, spiramycin and tylosin were tested against five strains of Staphylococcus aureus, Streptococcus uberis and Streptococcus dysga-lactiae. Effects of milk on the activity of cefquinome and marbofloxacin were also tested against five strains of Escherichia coli. MICs and MBCs were multiplied in milk by a factor 1 to 2 for cefquinome, 2 to 4 for marbofloxacin, 1 to 2 for tylosin and 2 to 8 for spiramycin. After discussion of the results, the authors suggest to take these data into account for the interpretation of in vitro susceptibility tests for bacteria isolated from mastitis.