Detection of a new extended-spectrum oxacillinase in Pseudomonas aeruginosa

Sir, Since the early 1990s, several extended-spectrum oxacillinases (ES-OXA) have been reported in clinical strains of Pseudomonas aeruginosa, mainly from Turkey and France.1 Most of these enzymes, which inactivate ceftazidime and cefepime, differ from the narrow-spectrum penicillinases OXA-2 (e.g. OXA-15 and -32) and OXA-10 (e.g. OXA-11, -13, -14, -16, -17, -19 and -28) by one or more amino acid substitutions.2 With the notable exception of OXA-18, they are not significantly antagonized in their hydrolytic activities by clavulanic acid, an inhibitor widely used in routine testing to detect extended-spectrum β-lactamases (ESBLs). As a result, their identification in the medical laboratory is often challenging for clinical microbiologists.1 In this paper, we describe the emergence in a patient under therapy of a novel ES-OXA, named OXA-147, deriving from OXA-35. In January 2009, two isolates of P. aeruginosa (4747 and 4759) exhibiting different susceptibilities to ceftazidime were isolated from the infected foot of a diabetic patient admitted to Hospital Jean Minjoz in Besançon (France). Table 1 shows that while being more resistant to ceftazidime and aztreonam, isolate 4747 was more susceptible than isolate 4759 to ticarcillin ± clavulanic acid, piperacillin ± tazobactam, cefotaxime and ceftriaxone. At the time these isolates were recovered, the patient was under treatment with ceftriaxone plus rifampicin for an infectious endocarditis due to Proteus mirabilis. A subsequent pulsed-field gel electrophoresis experiment demonstrated that the two strains shared an identical DraI banding pattern and, thus, were closely related (data not shown).