THE ACTIVITY OF A COMBINATION OF PENICILLIN AND NOVOBIOCIN AGAINST BOVINE MASTITIS PATHOGENS - DEVELOPMENT OF A DISK DIFFUSION TEST

The combination of penicillin and novobiocin is currently available fo r the treatment of bovine mastitis, but methods are not available for susceptibility testing of the combination by veterinary diagnostic lab oratories. The minimum inhibitory concentration (MIG) and disk diffusi on data were determined for penicillin, novobiocin, and a combination of the two in a 1:2 ratio for 225 staphylococcal, streptococcal, and G ram-negative isolates from bovine intramammary infections. Based on th e drug concentrations in milk following infusion, linear regression an alysis, and error rate bounding, the interpretive zone diameters selec ted were less than or equal to 16 mm for resistant isolates and greate r than or equal to 17 mm for susceptible isolates with a disk containi ng 10 U of penicillin and 30 mu g of novobiocin. Additionally, MIC bre akpoints of less than or equal to 2 mu g/ml of penicillin and 4 mu g/m l of novobiocin were selected to categorize isolates as susceptible an d greater than or equal to 4 mu g/ml of penicillin and 8 mu g/ml of no vobiocin were selected to categorize isolates as resistant. The MIC an d disk diffusion results, as well as studies to monitor bacterial kill ing by antimicrobial agents over time, indicated that the combination of penicillin and novobiocin in a 1:2 ratio was more active than were the individual drugs. Kinetics of the kill curves with the penicillin and novobiocin combination (1:2 ratio) showed that the combination was bactericidal for Staphylococcus aureus and Staphylococcus xylosus.