Characterization of poxB, a chromosomal-encoded Pseudomonas aeruginosa oxacillinase

Pseudomonas aeruginosa is the major pathogen associated with morbidity and mortality of patients with cystic fibrosis. One of the reasons for the failure of beta-lactam antibiotic regimens appears to be mediated by de-regulation of the ampC gene, encoding the chromosomal Ambler's Class C beta-lactamase. Currently, the AmpC is the only known chromosomal beta-lactamase whose expression is regulated by a transcriptional regulator, AmpR. We generated an ampC mutation in the prototypic P. aeruginosa strain PAO1. The mutation in ampC did not abolish the beta-lactamase activity entirely suggesting the expression of yet another unreported beta-lactamase. Our genomic analysis revealed the presence of an open reading frame encoding a protein with high homology to the Class D beta-lactamases, commonly known as oxacillinases. The gene was named poxB for Pseudomonas oxacillinase. Cloning and expression of poxB in Escherichia coli conferred beta-lactam resistance to the host. We detected the presence of poxB both in clinical and environmental isolates. Our studies show that P. aeruginosa possesses two beta-lactamases, AmpC and PoxB, which contribute to its resistance against a wide spectrum of beta-lactam antibiotics.