Mastitis-causing Escherichia coli: serum sensitivity and susceptibility to selected antibacterials in milk.

A total of 169 Escherichia coli strains were isolated from cows with cases of clinical mastitis. beta-Glucuronidase production, serum sensitivity, and susceptibility to selected antibacterials were analyzed using the fluorometric beta-glucuronidase assay. About 89% (150 of 169) of the isolates tested positive for beta-glucuronidase. Of these isolates producing beta-glucuronidase, 102 (68%) were resistant or moderately resistant to bovine serum. The antibacterial susceptibility of 96 isolates was tested in broth and milk. There was a significant shift from lower fluorometric minimum inhibitory concentration for tetracycline, sulfadoxin-trimethoprim, enrofloxacin, and gentamicin in broth to higher fluorometric minimum inhibitory concentration in milk. Serum sensitivity and susceptibility to tested antibacterials in broth or in milk were not related. Gentamicin and sulfadoxin-trimethoprim seemed to be more potent in mastitic milk than in normal milk, suggesting a possible synergistic effect between these exogenous antibacterials and the indigenous antibacterial agents in mastitic milk.