INHIBITOR-RESISTANT OXY-2-DERIVED BETA-LACTAMASE PRODUCED BY KLEBSIELLA-OXYTOCA

Klebsiella oxytoca strains are generally moderately resistant to amoxi cillin and ticarcillin due to the activities of the chromosomally enco ded OXY-1 and OXY-2 class A P-lactamase families. These enzymes have t he ability to hydrolyze not only penicillins but also cephalosporins, including cefuroxime, ceftriaxone, and aztreonam, and are inhibited by clavulanic acid. A Klebsiella oxytoca strain was isolated from a cult ure of blood from a patient who had been treated with amoxicillin-clav ulanate (3 g/day) for 10 days 1 month earlier. This strain harbored an unusual phenotype characterized by resistance to amoxicillin-clavulan ate. It produced an OXY-2-type p-lactamase (pI 6.3), as confirmed by P CR amplification with primers specific for the OXY-2-encoding gene. Ge ne sequencing revealed a point mutation (A --> G) corresponding to the amino acid substitution Ser-->Gly at position 130. This mutant enzyme was poorly inhibited by inhibitors, and its kinetic constants compare d to those of the parent enzyme were characterized by an increased K-m value for ticarcillin, with a drastically reduced activity against ce phalosporins, as is observed with inhibitor-resistant TEM enzymes. The substitution Ser --> Gly-130 was previously described in the inhibito r-resistant P-lactamase SHV-10 derived from an SHV-5 variant, but this is the first report of such a mutant in OXY enzymes from K. oxytoca.