One hundred and twenty seven strains of Gram-negative rods (72 E. coli, 45 Klebsieila pneumoniae, 8 Enterobacter spp. and 2 Pseudomonas aeruginosa) isolated from bovine mastitis were examined for resistance to ampicilin, carbenicillin and cetazolin, β-lactamase activity and transferable β-lactamase plasmids.
Stains resistant to ampicillin were 13.9% in E. coli, 93.3% in Klebsiella pneumoniae, 87.5% in Enterobacter. spp. and all in Pseudomonas aeruginosa, Resistance of E. coli, Klebsiella pneumoniae and Enterobacter spp. to ampicillin was due to the β-lactamases, but all Pseudomonas aeruginosa exhibited a high level of the non-enzymic resistance.
Transferable plasmid-mediated β-lactamase synthesis was demonstrated in 61.9% of Klebsiella pneumoniae, 50% of E. coli and 42.9% of Enterobacter spp. The same β-lactamase plasmids specified different resistance to various β-lactam antibiotics in different recipients.
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