Plasmid-mediated resistance to quinolones is increasingly reported in studies of Enterobacteriaceae . Using a PCR-based strategy, a series of gram-negative species were screened for qnrA -like genes. Shewanella algae , an environmental species from marine and fresh water, was identified as its reservoir. This is a one of the very few examples of progenitor identification of an acquired antibiotic resistance gene.

The contributions of different mechanisms of resistance to carbapenems among a collection of imipenem- and meropenem-nonsusceptible Pseudomonas aeruginosa isolates were investigated. This screening included the recently reported extended-spectrum cephalosporinases (ESACs) weakly hydrolyzing carbapenems. Eighty-seven percent of the studied isolates were resistant to imipenem. Genes encoding metallo-β-lactamases or carbapenem-hydrolyzing oxacillinases were not identified. The main mechanism associated with imipenem resistance was the loss of outer membrane protein OprD. Identification of overexpressed ESACs and loss of OprD were observed for 65% of the isolates, all being fully resistant to imipenem. Resistance to meropenem was observed in 78% of the isolates, with all but one also being resistant to imipenem. Overexpression of the MexAB-OprM, MexXY-OprM, or MexCD-OprJ efflux systems was observed in 60% of the isolates, suggesting the contribution of efflux mechanisms in resistance to meropenem. The loss of porin OprD and the overproduction of ESACs were observed in 100% and 92% of the meropenem-resistant isolates, respectively. P. aeruginosa can very often accumulate different resistance mechanisms, including ESAC production, leading to carbapenem resistance.

The characterization of AmpC-type β-lactamases was performed in a collection of 32 clinical Pseudomonas aeruginosa isolates with intermediate susceptibility or resistance to imipenem and ceftazidime. Twenty-one out of those 32 isolates overexpressed AmpC β-lactamase, and the MICs of ceftazidime and imipenem were reduced after cloxacillin addition. Cloning and sequencing identified 10 AmpC β-lactamase variants. Reduced susceptibility to imipenem, ceftazidime, and cefepime was observed only with recombinant P. aeruginosa strains expressing an AmpC β-lactamase that had an alanine residue at position 105. The catalytic efficiencies ( k _cat / K _m ) of the AmpC variants possessing this residue were increased against oxyiminocephalosporins and imipenem. In addition, we show here that those AmpC variants constitute a favorable background for the in vitro selection of imipenem-resistant strains. This report identified a novel resistance mechanism that may contribute to imipenem resistance in P. aeruginosa .

To gain insight into the functionality of qnr-like genes of several bacterial species of Vibrionaceae that may encode quinolone resistance determinants. A PCR-based strategy was used to obtain qnr-like genes of reference strains of Vibrio vulnificus CIP103196, Vibrio parahaemolyticus CIP71.2 and Photobacterium profundum CIP106289 that were sequenced, cloned and expressed in Escherichia coli. MICs of quinolones were determined for these reference strains and an E. coli reference strain harbouring recombinant plasmids. The Qnr-like proteins of these Vibrionaceae bacterial species shared 40-67% amino acid identity with the plasmid-mediated quinolone resistance determinants QnrA, QnrB and QnrS with a series of highly conserved residues. Once cloned in E. coli they conferred reduced susceptibility to quinolones. This work provides further evidence that water-borne Vibrionaceae may constitute a reservoir for Qnr-like quinolone resistance determinants.