Inducibility of β-lactamase activity by cefoxitin was examined in 626 gram-negative clinical isolates selected for amoxycillin and cephalothin resistance. The results indicated that precise identification and cefoxitin sensitivity or resistance could be used to predict the inducibility of β-lactamase. Of 326 organisms from species capable of β-lactamase induction, induction was shown in 68% and was predictable from the cefoxitin-sensitivity and identification data. No induction of β-lactamase occurred in the remaining species. A comparison of β-lactamase activities against cefotaxime, cefoperazone and latamoxef showed that induction of enzyme activity against cefotaxime and cefoperazone occurred at similar rates. Induction of activity against latamoxef did not occur or was minimal with three bacterial species. The data show that of 119 strains of Enterobacteriaceae displaying inducible β-lactamase, 113 would have been reported as unequivocally sensitive to cefotaxime, 109 as sensitive to cefoperazone and 116 as sensitive to latamoxef if the disk-diffusion technique alone had been used. The majority of strains examined produced inducible enzyme and they were more resistant to the three cephalosporins tested than were the Enterobacteriaceae.


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