Antibacterial susceptibility of bovine-mastitis pathogens tested directly in milk from infected quarters

Antibacterial susceptibilities of bovine-mastitis pathogens were analysed directly in 57 mastitic milk samples without inoculation with exogenous organisms. Aseptically collected milk was mixed with serial dilutions of antibacterials and the growth was observed using 2,3,5-triphenyltetrazolium chloride (TTC) reduction the following day. The results were compared with those obtained by using calibrated bacterial inocula in turbidimetric minimum-inhibitory-concentration (MIC) determination in broth cultures, and in TTC-broth culture-test and TTC-normal milk-test. The results of different methods all correlated positively when the entire data was used. However, taking the direct test in mastitic milk as the 'true' result, the total discrepancies varied from 34.7% to 48.8%. Antibacterial activities of the trimethoprim-sulphadoxine combination, and of spiramycin and ampicillin, decreased significantly when nutrient broth was replaced by milk as the test medium. The efficacy of trimethoprim-sulphadoxine as an antibacterial agent was also dependent on the source of milk.