OXA-16, a further extended-spectrum variant of OXA-10 beta-lactamase, fromtwo Pseudomonas aeruginosa isolates

Two extended-spectrum mutants of the class D beta-lactamase OXA-10 (PSE-2) from Pseudomonas aeruginosa isolates obtained in Ankara, Turkey, were descr ibed previously and were designated OXA-11 and -14. P. aeruginosa 906 and 9 61, isolated at the same hospital, were highly resistant to ceftazidime (MI C greater than or equal to 128 mu g/ml) and produced a p-lactamase with a p i of 6.2. The MICs of ceftriaxone, cefoperazone, cefsulodin, and cefepime w ere 4- to 16-fold above the typical values for P. aeruginosa, whereas the M ICs of penicillins and cefotaxime were raised only marginally. Ceftazidime MICs were not significantly reduced by clavulanate or tazobactam at 4 mu g/ ml. Ceftazidime resistance did not transfer conjugatively but was mobilized to P. aeruginosa PU21 by plasmid pUZ8. Both isolates gave similar DNA rest riction patterns, suggesting that they were replicates; moreover, they yiel ded identically sized BamHI fragments that hybridized with a bla(OXA-10) pr obe. DNA sequencing revealed that both isolates had the same new beta-lacta mase, designated OXA-16, which differed from OXA-10 in having threonine ins tead of alanine at position 124 and aspartate instead of glycine at positio n 157. The latter change is also present in OXA-11 and -14 and seems critic al to ceftazidime resistance. Kinetic parameters showed that OXA-16 enzyme was very active against penicillins, cephaloridine, cefotaxime, and ceftria xone, but hydrolysis of ceftazidime was not detected despite the ability of the enzyme to confer resistance.