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Journal of Clinical Microbiology, August 2003, p. 3712-3718, Vol. 41, No. 8
0095-1137/03/$08.00+0     DOI: 10.1128/JCM.41.8.3712-3718.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Evaluation of the Osiris Expert System for Identification of ß-Lactam Phenotypes in Isolates of Pseudomonas aeruginosa

Service de Microbiologie, Hôpital Beaujon, 92110 Clichy,¹ Service de Microbiologie, Hôpital Tenon, 75020 Paris,² Bio-Rad, 92430 Marnes-la-Coquette,³ Service de Microbiologie, Faculté de Pharmacie, Université de Bordeaux 2, 33076 Bordeaux cedex, France⁴

Received 25 April 2002/ Returned for modification 25 September 2002/ Accepted 17 March 2003

Osiris is a video zone size reader for disk diffusion tests featuring a built-in extended expert system (EES). The efficacy of the EES for the identification of the ß-lactam susceptibility phenotypes of Pseudomonas aeruginosa isolates was evaluated. Thirteen ß-lactams were tested in four laboratories by the disk diffusion test with 53 strains with well-characterized resistance mechanisms, including the production of 12 extended-spectrum ß-lactamases (ESBLs). The plates were read with the Osiris system and the results were interpreted with the ESS, and then the phenotype identified by the EES was compared to the resistance mechanism. The strains were also screened for the presence of ESBL production by a double-disk synergy test by placing the strains between an extended-spectrum cephalosporin-containing disk and a clavulanic acid-containing disk at distances of 30, 20, 15, and 10 mm from each other. Overall, the EES accurately identified the phenotypes of 88.2% of the strains and indicated an association with several mechanisms for 3.8% of the strains. No phenotype was identified in four strains with low levels of penicillinase production. Misidentifications were observed for two penicillinase-producing strains: one strain with partially derepressed cephalosporinase production and one strain overexpressing the MexA-MexB-OprM efflux system. The production of only four ESBLs was detected by the standard synergy test with a 30-mm distance between the disks. The production of five further ESBLs was identified by reducing the distance to 20 mm, and the production of the last three ESBLs was detected only at a distance of 15 or 10 mm. Our results indicate that the Osiris EES is an effective tool for the identification of P. aeruginosa ß-lactam phenotypes. A specific double-disk synergy test with reduced disk distances is necessary for the detection of ESBL production by this organism.

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