Beta-lactamase in gram-negative rods: the relationship between penicillinase and R plasmids in gram-negative rods.

Concomitant with the extensive use of antibiotics, the number of multiple antibiotic-resistant strains has been increasing. Since resistance is mainly mediated by R plasmids, we undertook to investigate the characteristics of R plasmid-determined beta-lactamase in 6 Gram-negative rods. The beta-lactamase produced by each organism was classified by its substrate: type P which attacks penicillins, type C which attacks cephalosporins, and type C/P which attacks both penicillins and cephalosporins. Though the chromosomally mediated beta-lactamase of almost all Gram-negative rods is classified as type C, R plasmid-mediated beta-lactamase is almost equally active against both penicillins and cephalosporins. Therefore, we suggest that type C/P beta-lactamase was mediated by R plasmids in Gram-negative rods which already produced chromosomally mediated type C beta-lactamase. The strains which produced type C/P beta-lactamase tended to be more resistant to antibiotics than the other beta-lactamase producing strains. Among type C/P strains, the sensitivity to cephalosporins varied with the bacterial species, whereas all these strains were highly resistant to penicillins. Even for piperacillin, which is stable to cephalosporinase, the MIC at which the cumulative percentage of strains inhibited was 50% (MIC50) was over 50 micrograms/ml in all strains tested.