1. Int J Antimicrob Agents. 2018 Nov;52(5):697-701. doi:
10.1016/j.ijantimicag.2018.07.027. Epub 2018 Aug 3.

Loss of activity of ceftazidime-avibactam due to MexAB-OprM efflux and
overproduction of AmpC cephalosporinase in Pseudomonas aeruginosa isolated from
patients suffering from cystic fibrosis.

Chalhoub H(1), Sáenz Y(2), Nichols WW(3), Tulkens PM(4), Van Bambeke F(1).

Author information: 
(1)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain, Brussels, Belgium.
(2)Área de Microbiología Molecular, Centro de Investigación Biomédica de La Rioja
(CIBIR), Logroño, Spain.
(3)Consultant Microbiologist, Cambridge, MA, USA.
(4)Pharmacologie cellulaire et moléculaire, Louvain Drug Research Institute,
Université catholique de Louvain, Brussels, Belgium. Electronic address:

In Pseudomonas aeruginosa (P. aeruginosa) collected from cystic fibrosis (CF)
patients, 24% resistance to ceftazidime-avibactam in isolates negative for
carbapenemases and extended-spectrum β-lactamases (ESBLs) has previously been
observed. The current study aimed to unravel the underlying mechanism(s). Using
the laboratory strain PAO1 and derivatives thereof, with ampC expression induced 
by a sub-minimum inhibitory concentration (MIC) of imipenem, a higher MIC of
ceftazidime-avibactam was found for those overexpressing MexAB-OprM (quantitative
polymerase chain reaction (PCR) of mexA) and, to a lesser extent, MexEF-OprN (PCR
of mexE), or without OprD expression (SDS-Page and Coomassie blue staining). This
was ascribed to (i) an efflux of avibactam (efflux mutants) and (ii) a lack of
avibactam penetration (OprD mutants), respectively. We then used 10 CF clinical
isolates resistant to ceftazidime (MIC ≥ 128 mg/L) and with (i) variable basal
levels of ampC overexpression, (ii) mutations in mexA or mexB inactivating to
variable extent the MexAB-OprM transport capacity (assessed by extrusion of
N-phenyl-1-naphthylamine [NPN]), and (iii) expression or not of mexE and of OprD 
porin. The reduction of ceftazidime MIC in the presence of avibactam was
partially lost for isolates with large efflux activity of MexAB-OprM and/or
increased ampC expression, but not significantly with mexE expression or lack of 
OprD (non-parametric and parametric tests). This identified MexAB-OprM as a main 
avibactam efflux transporter in P. aeruginosa that, together with ampC
overexpression, reduced avibactam potency. Since about 30% of CF isolates show
mutations in MexAB-OprM compromising efflux (Chalhoub, et al. Sci Reports
2017;7:40208), routine susceptibility testing of CF P. aeruginosa with
ceftazidime-avibactam is warranted.

Copyright © 2018 Elsevier Ltd. All rights reserved.

DOI: 10.1016/j.ijantimicag.2018.07.027 
PMID: 30081137