Interactions of antimicrobials in milk and their detection by the disk diffusion method and Delvotest SP.

The combination of more than 2 different microbials might show interactions with various effects (synergistic, additive, antagonistic, or indifferent) on target microorgnisms. An objective of this paper was to evaluate the possible interactions of several antimicrobials--those used most frequently in the treatment of mastitis in clinical veterinary practice (beta-lactam antibiotics, aminoglycosides, peptides, other antibiotics, and sulfonamides)--and their consequences on detection limits. In the model experiment with milk artificially altered by means of Delvotest SP and the disk diffusion method with Bacillus stearothermophilus var. calidolactis C 953, we observed the synergistic effect between all the antimicrobials tested. The results show that Delvotest SP is more sensitive (approximately 7.5- to 40-fold) than the disk diffusion method in estimating the detection limits of cephalosporin antibiotics.