SUMMARY. The mechanisms of resistance to beta-lactam antibiotics in 191 isolates of were examined. These represented the most resistant organisms of 1866 isolates collected during a national survey of antibiotic resistance in this species. One hundred and seventy-two isolates were selected because they were resistant to carbenicillin (MIC > 128 mg/L) and 19 because the MICs of cefotaxime were greater than the MICs of carbenicillin. Of the carbenicillin-resistant isolates, 35 produced plasmid-mediated beta-lactamases known to be active against carbenicillin and seven produced unusual beta-lactamases; in 131 strains, resistance could not be attributed to beta-lacta-mase production and was considered to be intrinsic. The unusual antibiogram in which the MIC of cefotaxime was greater than the MIC of carbenicillin was associated with overproduction of the chromoso-mally-determined Sabath and Abrahams' beta-lactamase. Selection of strains with this last mechanism represents a novel resistance problem and one which may increase with increased use of newer antipseudo-monal beta-lactams.


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