1. Sci Rep. 2017 Jan 16;7:40208. doi: 10.1038/srep40208.

Mechanisms of intrinsic resistance and acquired susceptibility of Pseudomonas
aeruginosa isolated from cystic fibrosis patients to temocillin, a revived

Chalhoub H(1), Pletzer D(2), Weingart H(2), Braun Y(2), Tunney MM(3), Elborn
JS(3), Rodriguez-Villalobos H(4), Plésiat P(5), Kahl BC(6), Denis O(7),
Winterhalter M(2), Tulkens PM(1), Van Bambeke F(1).

Author information: 
(1)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain, Brussels, Belgium. (2)Life Sciences, School of 
Engineering and Science, Jacobs University, Bremen, Germany. (3)CF &Airways
Microbiology Research Group, Queen's University Belfast, Belfast, UK.
(4)Laboratoire de microbiologie, Cliniques Universitaires Saint-Luc, Université
catholique de Louvain, Brussels, Belgium. (5)Laboratoire de bactériologie,
Hôpital Jean Minjoz, Besançon, France. (6)University Hospital Münster, Münster,
Germany. (7)Laboratoire de microbiologie, Hôpital Erasme, Université libre de
Bruxelles, Brussels, Belgium.

The β-lactam antibiotic temocillin (6-α-methoxy-ticarcillin) shows stability to
most extended spectrum β-lactamases, but is considered inactive against
Pseudomonas aeruginosa. Mutations in the MexAB-OprM efflux system, naturally
occurring in cystic fibrosis (CF) isolates, have been previously shown to reverse
this intrinsic resistance. In the present study, we measured temocillin activity 
in a large collection (n = 333) of P. aeruginosa CF isolates. 29% of the isolates
had MICs ≤ 16 mg/L (proposed clinical breakpoint for temocillin). Mutations were 
observed in mexA or mexB in isolates for which temocillin MIC was ≤512 mg/L
(nucleotide insertions or deletions, premature termination, tandem repeat,
nonstop, and missense mutations). A correlation was observed between temocillin
MICs and efflux rate of N-phenyl-1-naphthylamine (MexAB-OprM fluorescent
substrate) and extracellular exopolysaccharide abundance (contributing to a
mucoid phenotype). OpdK or OpdF anion-specific porins expression decreased
temocillin MIC by ~1 two-fold dilution only. Contrarily to the common assumption 
that temocillin is inactive on P. aeruginosa, we show here clinically-exploitable
MICs on a non-negligible proportion of CF isolates, explained by a wide diversity
of mutations in mexA and/or mexB. In a broader context, this work contributes to 
increase our understanding of MexAB-OprM functionality and help delineating how
antibiotics interact with MexA and MexB.

DOI: 10.1038/srep40208 
PMID: 28091521  [PubMed - in process]