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.{3-Glucuronidase production, serum sensitivity, and susceptibility to selected antibacterials were analyzed using the fluorometric {3-glucuronidase assay.About 89% (150 of 169) of the isolates tested positive for {3glucuronidase.Of these isolates producing {3glucuronidase, 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 sulfadoxintrimethoprim 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.(Key words: Escherichia coli, (3-glucuronidase, antibacterial susceptibility, mastitis) Abbreviation key: FMIC =fluorometric MIC, ICoo = drug concentrations exhibiting 90% inhibition of fluorescence, ISB = Iso-Sensitest® broth, MIC = minimum inhibitory concentration, 4-MUG = 4-methylumbelliferyl-{3-D-glucuronide.