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Antimicrobial Agents and Chemotherapy, June 2004, p. 2043-2048, Vol. 48, No. 6
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.6.2043-2048.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Biochemical Characterization of the Naturally Occurring Oxacillinase OXA-50 of Pseudomonas aeruginosa

Delphine Girlich, Thierry Naas, and Patrice Nordmann*

Service de Bactériologie-Virologie, Université Paris XI, Hôpital de Bicêtre, Assistance Publique/Hôpitaux de Paris, Faculté de Médecine Paris-Sud, 94275 Le Kremlin-Bicêtre, France

Received 1 August 2003/ Returned for modification 9 November 2003/ Accepted 12 February 2004

The blaOXA-50 gene (formerly known as the PA5514 gene) is an oxacillinase gene identified in silico in the genome of Pseudomonas aeruginosa PAO1. By using a mutant strain of P. aeruginosa PAO1 that had an inactivated blaAmpC cephalosporinase gene, the blaOXA-50 gene was shown to be expressed constitutively in P. aeruginosa. This ß-lactamase gene was cloned onto a multicopy plasmid and expressed in P. aeruginosa and Escherichia coli. It conferred decreased susceptibility to ampicillin and ticarcillin and, interestingly, to moxalactam and meropenem in P. aeruginosa but not in E. coli. Overexpression and purification enabled us to determine the molecular mass (25 kDa), the pI value (8.6), and the hydrolysis spectrum of the OXA-50 ß-lactamase. It is a narrow-spectrum oxacillinase that uncommonly hydrolyzes imipenem, although at a low level. Very similar oxacillinase genes were identified in all P. aeruginosa isolates from various geographical origins tested. The weak variability of the nucleotide sequence of this gene (0 to 2%) corresponded to that found for the naturally occurring blaAmpC cephalosporinase gene of P. aeruginosa. The study indicated that P. aeruginosa harbors two naturally encoded ß-lactamase genes, one of which encodes an inducible cephalosporinase and the other of which encodes a constitutively expressed oxacillinase.

* Corresponding author. Mailing address: Service de Bactériologie-Virologie, Hôpital de Bicêtre, 78 rue du Général Leclerc, 94275 Le Kremlin-Bicêtre cedex, France. Phone: 33-1-45-21-36-32. Fax: 33-1-45-21-63-40. E-mail: nordmann.patrice{at}bct.ap-hop-paris.fr.

Antimicrobial Agents and Chemotherapy, June 2004, p. 2043-2048, Vol. 48, No. 6
0066-4804/04/$08.00+0     DOI: 10.1128/AAC.48.6.2043-2048.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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