Cefoxitin sensitivity as a marker for inducible beta-lactamases

Inducibility of beta-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 beta-lactamase. Of 326 organisms from species capable of beta-lactamase induction, induction was shown in 68% and was predictable from the cefoxitin-sensitivity and identification data. No induction of beta-lactamase occurred in the remaining species. A comparison of beta-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 beta-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 Pseudomonas strains examined produced inducible enzyme and they were more resistant to the three cephalosporins tested than were the Enterobacteriaceae.