Laboratory mutants of OXA-10 beta-lactamase giving ceftazidime resistance in Pseudomonas aeruginosa

Several extended-spectrum beta-lactamases (ESBLs) belonging to molecular Cl ass D have been described from Pseudomonas aeruginosa isolates collected in Turkey. Four of these, OXA-11, -14, -16 and -17, are derivatives of OXA-10 beta-lactamase. We tried to select similar mutants in vitro from OXA-10-pr oducing transconjugants of P. aeruginosa, using a multistep method on cefta zidime-containing agars. Forty-four such mutants were obtained; all had inc reased resistance to ceftriaxone, cefsulodin, cefepime, cefpirome, latamoxe f, aztreonam and, especially, ceftazidime whereas MICs of piperacillin, car benicillin, cefotaxime, cefoperazone and carbapenems were little altered. G enes related to bla(OXA-10) were sequenced from five mutants. One mutant en zyme had aspartate instead of glycine at position 157, and corresponded exa ctly to natural OXA-14 beta-lactamase. Another mutant strain appeared to ha ve both OXA-14 and a new pi 6.2 enzyme, designated OXA-M102, with serine in stead of alanine at position 124 and aspartate instead of glycine at positi on 157. This latter variant resembled natural OXA-16 enzyme, which has thre onine at position 124 and aspartate at position 157. The remaining three mu tant enzymes differed from any so far found in wild-type isolates. Two had leucine replacing tryptophan at position 154 (this enzyme was named OXA-M10 1) while the third (OXA-M103) had a pi of 7.6, and had lysine instead of as paragine at position 143. A different mutation at this position was previou sly found in OXA-11, a wild-type OXA-10 mutant. Thus, some of the ESBL muta nts selected (OXA-14 and OXA-M102) correspond exactly or almost exactly to ESBLs found in wild-types, whereas others (OXA-M101 and OXA-M103) were tota lly new.