Tn2011, a new transposon encoding oxacillin-hydrolyzing beta-lactamase

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 in P. aeruginosa. In the present work, the contribution of this class D enzyme was investigated using a series of clean in-frame ampC, poxB, and oprD deletions, as well as complementation by expression under the control of an inducible promoter. While poxB deletions failed to alter β-lactam sensitivities, expression of poxB in ampC-deficient backgrounds decreased susceptibility to both meropenem and doripenem but had no effect on imipenem, penicillin, and cephalosporin MICs. However, when expressed in an ampCpoxB-deficient background, that additionally lacked the outer membrane porin-encoding gene oprD, 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.

The genetics and biochemistry of mercury resistance

The ability of bacteria to detoxify mercurial compounds by reduction and volatilization is conferred by mer genes, which are usually plasmid located. The narrow spectrum (Hg2+ detoxifying) Tn501 and R100 determinants have been subjected to molecular genetic and DNA sequence analysis. Biochemical studies on the flavoprotein mercuric reductase have elucidated the mechanism of reduction of Hg2+ to Hg0. The mer genes have been mapped and sequenced and their protein products studied in minicells. Based on the deduced amino acid sequences, these proteins have been assigned a role in a mechanistic scheme for mercury flux in resistant bacteria. The mer genes are inducible, with regulatory control being exerted at the transcriptional level both positively and negatively. Attention is now focusing on broad-spectrum resistance involving detoxification of organomercurials by an additional enzyme, organomercurial lyase. Lyase genes have recently been cloned and sequencing studies are in progress.

Analysis by using DNA probes of the OXA-1 beta-lactamase gene and its transposon

From recombinant clone pTY27, encoding an OXA-1 beta-lactamase gene, we performed subcloning experiments to more precisely delimit the gene. We describe the use as probes of six different restriction fragments known from subcloning experiments to be within the structural gene or part of the transposable element, Tn2603, flanking the OXA-1 determinant. We showed that the OXA-1 structural gene is slightly related to the OXA-2 determinant and also that sequences within Tn2603 are common to all the OXA- and PSE-producing strains tested. For epidemiological purposes, we began nucleotide sequencing of the OXA-1 determinant, and from preliminary sequence data we synthesized an oligonucleotide 15 bases in length, corresponding to a sequence within the OXA-1 gene. This oligonucleotide was found to be specific for the OXA-1 determinant, because it hybridized only to bacteria producing that beta-lactamase.