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J Antimicrob Chemother. 2013 Mar;68(3):619-30. doi: 10.1093/jac/dks439. Epub 2012 Nov 9.

Challenges for accurate susceptibility testing, detection and interpretation of β-lactam resistance phenotypes in Pseudomonas aeruginosa: results from a Spanish multicentre study.

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  • 1Servicio de Microbiología, Hospital Son Espases, Palma de Mallorca, Spain.



To evaluate the proficiency of Spanish laboratories regarding accurate susceptibility testing, detection and interpretation of Pseudomonas aeruginosa β-lactam resistance phenotypes.


Thirteen characterized strains were sent to 54 participating centres: clinical strains producing horizontally acquired β-lactamases [extended-spectrum β-lactamases (ESBLs; PER-1 and OXA-161) and class A (GES-5) and B (VIM-2) carbapenemases] and mutants with combinations of chromosomal mechanisms (AmpC, OprD and/or efflux). The centres were requested to evaluate six antipseudomonal β-lactams, provide raw/interpreted clinical categories and detect/infer the resistance mechanisms. Consensus results from reference centres were used to assign minor, major or very major errors (mEs, MEs or VMEs).


Vitek2, MicroScan WalkAway and Wider were the most used devices (25%-30% each). CLSI/EUCAST breakpoints were used in 86%/14% of the determinations. Discrepancies exclusively due to the differential application of breakpoints were highest for aztreonam, followed by piperacillin/tazobactam. The lowest percentage of VMEs was for Vitek2, followed by Wider. The highest percentages of VMEs (6%) were for the AmpC-hyperproducing OprD(-) strain and for the GES-5 producer, while among antibiotics the highest percentage of VMEs (22%) involved piperacillin/tazobactam. Appropriate inference of resistance mechanisms was high for the VIM-2-producing strain (83%), but low (<40%) for strains producing ESBLs or non-metallo-β-lactamase carbapenemases.


The use of different breakpoints and devices, the complexity of mutation-driven resistance mechanisms and the lack of unequivocal tests to detect ESBLs or carbapenemases in P. aeruginosa leads to extraordinary variability and low accuracy in susceptibility testing, which may have consequences for the treatment and control of nosocomial infections.

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