INTERACTIONS OF ANTIMICROBIALS IN MILK - CONSEQUENCES FOR THE DETECTION BY A MICROBIAL INHIBITOR TEST

Detection limits of microbial inhibitor tests are indicated individual ly for each antimicrobial in milk. Mastitis treatment with drugs consi sting of a combination of different antimicrobials or mixing of milk o f cows treated with different drugs can lead to the occurrence of more than one inhibitory substance in a milk sample. In model trials with the Delvotest SP a marked synergistic effect was observed between beta -lactam-antibiotics. Due to the presence of 0.5 and 1.0 mu g penicilli n/kg respectively the detection limits of ampicillin decreased from 3 to 1.5 and 0.5 mu g/kg and of cloxacillin from 20 to 12.5 and 5 mu g/k g, respectively. The synergistic effect was less pronounced between th e tested sulfonamides, trimethoprim and dihydrostreptomycin, respectiv ely, and penicillin. The influence of penicillin on the detection limi ts of neomycin, oxytetracyline and spiramycin was insignificant. There was no evidence of antagonistic effects of the tested combinations on the Delvotest SP resulting in increased detection limits.