JAC Advance Access originally published online on September 6, 2005
Journal of Antimicrobial Chemotherapy 2005 56(4):657-664; doi:10.1093/jac/dki303
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Published by Oxford University Press 2005.
Simple method to determine ß-lactam resistance phenotypes in Pseudomonas aeruginosa using the disc agar diffusion test
Laboratoire de Bactériologie (Pr Claire Poyart) Groupe Hospitalier Cochin Saint-Vincent-de-Paul La Roche-Guyon, 27, rue du Faubourg Saint-Jacques, 75679 Paris Cedex 14, France
Received 4 April 2005; returned 10 May 2005; revised 15 June 2005; accepted 2 August 2005
* Service de Bacteriologie, Groupe Hospitalier Cochin Saint-Vincent-de-Paul La Roche Guyon, 75679 Paris, France. Tel: +33-1-58-41-15-44; Fax: 33-1-58-41-15-48; E-mail: gerard.vedel{at}cch.ap-hop-paris.fr
Background: Pseudomonas aeruginosa is a major opportunistic bacterial pathogen in nosocomial infections because of the increasing prevalence of resistance to many of the commonly used antibiotics. To ensure optimal efficiency of antibiotic treatment against this species, antibiotic susceptibility tests must be interpreted with caution. Most microbiologists now consider it essential to characterize the antibiotic resistance expressed by isolates. Particular resistance mechanisms may be suspected when the bacterium is resistant to several antibiotics in the same family (for example ß-lactam agents).
Methods: Using the disc agar diffusion test, a simple method was developed to distinguish between the common ß-lactam resistance phenotypes of P. aeruginosa and, consequently, the possible resistance mechanism(s). Over a period of 5 years, we analysed 6300 P. aeruginosa strains isolated from various pathological specimens collected from different wards of Cochin Port-Royal Hospital, and reference and collection strains. Each strain had the wild-type phenotype or an acquired resistance phenotype. Eight anti-pseudomonal ß-lactams (ticarcillin, cefotaxime or moxalactam, cefepime or cefpirome, imipenem, ceftazidime, aztreonam, cefsulodin and ticarcillin + clavulanic acid) were used as phenotypic markers.
Results: The following markers were sufficient to distinguish between the wild-type phenotype and the various acquired resistance phenotypes: ß-lactamase synthesis, reduced cell wall permeability and/or increased expression of efflux transporters (active efflux). Detection of resistance phenotypes allows ‘interpretive reading’ of antibiotic susceptibility tests.
Conclusions: Clearly, improved interpretation of antibiotic susceptibility tests is important for a better appreciation of the effect of antimicrobial agents on bacteria such as P. aeruginosa.
Keywords: resistance phenotypes , resistance mechanisms interpretive reading , zone diameters
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