Potency of antibacterial drugs in milk as analysed by beta-glucuronidase-based fluorometry

The potency of selected antibacterials on mastitis-causing Escherichia coli, Streptococcus agalactiae and Streptococcus uberis in milk, whey and Iso-sensitest broth (ISB) was compared, based on the suppression of bacterial beta-glucuronidase production. The beta-glucuronidase activity in the samples was analysed by substrate-defined fluorometry where the turbidity of milk does not disturb the assay. In ISB, all four E. coli strains were susceptible to enrofloxacin and gentamicin, sulfadoxin-trimethoprim and tetracycline. S. agalactiae and S. uberis strains were susceptible in ISB to most of the antibacterials tested. The antibacterial potency of sulfadoxin-trimethoprim, tetracycline, novobiocin, gentamicin and enrofloxacin on E. coli and S. agalactiae were considerably decreased in milk as compared with that in ISB, However, S. uberis seemed to be more susceptible to antibacterials in milk or whey than in ISB. Regression analysis of the sigmoidal dose-response curves of sulfadoxin-trimethoprim showed that slopes of the linearized lines seemed to become less steep in milk than in the artificial broth medium, indicating a shift of the bactericidic effect in ISB towards a bacteriostatic effect in milk.