VARIABILITY OF CHROMOSOMALLY ENCODED BETA-LACTAMASES FROM KLEBSIELLA-OXYTOCA

The beta-lactamase genes of Klebsiella oxytoca were previously divided into two main groups: bla(OXY-1) and bla(OXY-2). The two beta-lactama se groups were each represented by beta-lactamases with four different pIs, In each group, one form of beta-lactamase is more frequent than the others combined, The beta-lactamase gene of each representative be ta-lactamase with a different pi that was not yet sequenced (pIs 5.7, 6.8 [OXY-2], 7.1, 8.2, and 8.8 [OXY-1]) was cloned and sequenced, The susceptibility patterns as well as relative rates and kinetic paramete rs for beta-lactam hydrolysis revealed that OXY-2 enzymes hydrolyzed s everal of the beta-lactams that were examined (carbenicillin, cephalot hin, cefamandole, ceftriaxone, and aztreonam) at a greater rate than t he OXY-1 enzymes did, Comparison of K, oxytoca beta-lactamases,vith pl asmid-mediated extended-spectrum beta-lactamases MEN-1 and TOHO-1 impl ied that the threonine at position 168 present in OXY-2 beta-lactamase instead of the alanine in OXY-1 could be responsible for its modified substrate hydrolysis. In each group, the Rho-lactamase with a variant pi differs from the main form of beta-lactamase by one to five amino acid substitutions, The substrate profile and the 50% inhibitory conce ntrations revealed that all substitutions differing from the main form of beta-lactamase were neutral except one difference in the OXY-1 gro up. This substitution of an Ala to a Gly at position 237 increases the hydrolysis of some beta-lactams, particularly aztreonam; decreases th e hydrolysis of benzylpenicillin, cephaloridine, and cefamandole, and decreases the susceptibility to clavulanic acid (fivefold increase in the 50% inhibitory concentration).