1. Int J Antimicrob Agents. 2016 Dec;48(6):740-743. doi:
10.1016/j.ijantimicag.2016.09.012. Epub 2016 Oct 19.

High-level resistance to meropenem in clinical isolates of Pseudomonas aeruginosa
in the absence of carbapenemases: role of active efflux and porin alterations.

Chalhoub H(1), Sáenz Y(2), Rodriguez-Villalobos H(3), Denis O(4), Kahl BC(5),
Tulkens PM(1), Van Bambeke F(6).

Author information: 
(1)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain, Brussels, Belgium. (2)Centro de Investigación
Biomédica de La Rioja (CIBIR), Logroño, Spain. (3)Laboratoire de microbiologie,
Cliniques universitaires Saint-Luc, Brussels, Belgium. (4)Laboratoire de
microbiologie, Hôpital Erasme, Université libre de Bruxelles, Brussels, Belgium. 
(5)University Hospital Münster, Münster, Germany. (6)Pharmacologie cellulaire et 
moléculaire, Louvain Drug Research Institute, Université catholique de Louvain,
Brussels, Belgium. Electronic address: francoise.vanbambeke@uclouvain.be.

High-level carbapenem resistance is worryingly increasing in clinical isolates
and is often attributed to carbapenemase expression. This study aimed to
determine the mechanisms leading to high-level meropenem resistance in six
carbapenemase-negative Pseudomonas aeruginosa isolated from cystic fibrosis (CF) 
patients and seven carbapenemase-positive isolates from patients suffering from
hospital-acquired pneumonia (HAP). MICs were determined in the absence or
presence of l-arginine or glycine-glutamate as competitive substrates for OprD
(OccD1) or OpdP (OccD3), respectively, or the efflux pump inhibitor Phe-Arg
β-naphthylamide (PAβN). β-Lactamases were screened by phenotypic tests and/or
PCR. The oprD gene and its promoter were sequenced; protein expression was
evidenced by SDS-PAGE. mexA, mexX, mexC and mexE transcripts were evaluated by
real-time and semiquantitative PCR. Meropenem/imipenem MICs were
64-128/16-32 mg/L and 128/128-256 mg/L in CF and HAP isolates, respectively; PAβN
reduced meropenem MICs to 4-16 mg/L only and specifically in CF isolates; porin
competitors had no effect on MICs. All isolates showed an increase in
transcription levels of mexA, mexX and/or mexC and mutations in oprD leading to
production of truncated proteins. AmpC-type cephalosporinases were overexpressed 
in CF isolates and VIM-2 was expressed in HAP isolates. Antibiotic exclusion from
bacteria by concomitant efflux and reduced uptake is sufficient to confer
high-level resistance to meropenem in isolates overexpressing AmpC-type
cephalosporinases. As efflux is preponderant in these isolates, it confers a
paradoxical phenotype where meropenem is less active than imipenem. Concomitant
susceptibility testing of both carbapenems and rapid elucidation of the most
probable resistance mechanisms is thus warranted.

Copyright © 2016 Elsevier B.V. and International Society of Chemotherapy. All
rights reserved.

DOI: 10.1016/j.ijantimicag.2016.09.012 
PMID: 28128097  [PubMed - in process]