Metallo-β-Lactamase Production byPseudomonas otitidis: a Species-Related Trait

Abstract

ABSTRACTSusceptibility to several β-lactams and β-lactamase production was investigated in a collection of 20 strains ofPseudomonas otitidis, a newPseudomonasspecies that has been recently recognized in association with otic infections in humans. All strains appeared to be susceptible to piperacillin, cefotaxime, ceftazidime, and aztreonam, while resistance or decreased susceptibility to carbapenems was occasionally observed. All strains were found to express metallo-β-lactamase (MBL) activity and to carry a new subclass B3 MBL gene, namedblaPOM, that appeared to be highly conserved in this species.P. otitidis, therefore, is the first example of a pathogenicPseudomonasspecies endowed with a resident MBL. The POM-1 protein fromP. otitidistype strain MCC10330 exhibits the closest similarity (60 to 64%) to the L1 MBL ofStenotrophomonas maltophilia. Expression inEscherichia coliandPseudomonas aeruginosarevealed that, similar to L1 and other subclass B3 MBLs, POM-1 confers decreased susceptibility or resistance to carbapenems, penicillins, and cephalosporins but not to aztreonam. Expression of the POM MBL inP. otitidisis apparently constitutive and, in most strains, does not confer a carbapenem-resistant phenotype. However, a strong inoculum size effect was observed for carbapenem MICs, and carbapenem-resistant mutants could be readily selected upon exposure to imipenem, suggesting that carbapenem-based regimens should be considered with caution forP. otitidisinfections.