Characterization of a Carbapenem-Hydrolyzing Enzyme, PoxB, in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is an opportunistic pathogen often associated with severe and life-threatening infections that are highly impervious to treatment. This microbe readily exhibits intrinsic and acquired resistance to varied antimicrobial drugs. Resistance to penicillin-like compounds is commonplace and provided by the chromosomal AmpC β-lactamase. A second, chromosomally encoded β-lactamase, PoxB, has previously been reported inP. aeruginosa . In the present work, the contribution of this class D enzyme was investigated using a series of clean in-frameampC ,poxB , andoprD deletions, as well as complementation by expression under the control of an inducible promoter. WhilepoxB deletions failed to alter β-lactam sensitivities, expression ofpoxB inampC -deficient backgrounds decreased susceptibility to both meropenem and doripenem but had no effect on imipenem, penicillin, and cephalosporin MICs. However, when expressed in anampCpoxB -deficient background, that additionally lacked the outer membrane porin-encoding geneoprD , PoxB significantly increased the imipenem as well as the meropenem and doripenem MICs. Like other class D carbapenem-hydrolyzing β-lactamases, PoxB was only poorly inhibited by class A enzyme inhibitors, but a novel non-β-lactam compound, avibactam, was a slightly better inhibitor of PoxB activity.In vitro susceptibility testing with a clinical concentration of avibactam, however, failed to reduce PoxB activity against the carbapenems. In addition,poxB was found to be cotranscribed with an upstream open reading frame,poxA , which itself was shown to encode a 32-kDa protein of yet unknown function.