OXA-17, a further extended-spectrum variant of OXA-10 beta-lactamase, isolated from Pseudomonas aeruginosa

Pseudomonas aeruginosa isolates 871 and 873 were isolated at Hacettepe Univ ersity Hospital in Ankara and were highly resistant to ceftazidime (MIC, 12 8 mu g/ml). Each produced three beta-lactamases, with pIs of 5.3, 6.1, and 7.9. The beta-lactamase with a pI of 5.3 was previously shown to be PER-1 e nzyme, The antibiograms of the isolates were not entirely explained by prod uction of PER-1 enzyme, insofar as ceftazidime resistance was incompletely reversed by clavulanate. The enzymes with pIs of 6.1 and 7.9 were therefore investigated. The enzyme with a pI of 6.1 proved to be a novel mutant of O XA-10, which we designated OXA-17, and had asparagine changed to serine at position 73 of the protein. When cloned into Escherichia coli XL1-blue, OXA -17 enzyme conferred greater resistance to cefotaxime, latamoxef, and cefep ime than did OXA-10, but it had only a marginal (two- to fourfold) effect o n the MIC of ceftazidime. This behavior contrasted with that of previous OX A-10 mutants, specifically OXA-11, -14, and -16, which predominately compro mise ceftazidime. Extracted OXA-17 enzyme had relatively greater activity t han OXA-10 against oxacillin, cloxacillin, and cefotaxime but, in terms of k(cat)/K-m, it had lower catalytic efficiency against most beta-lactams, Th e enzyme with a pI of 7.9 was shown by gene sequencing to be OXA-2.