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Journal of Clinical Microbiology, April 2005, p. 1846-1850, Vol. 43, No. 4
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.4.1846-1850.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Evaluation and Updating of the Osiris Expert System for Identification of Escherichia coli ß-Lactam Resistance Phenotypes

Service de Microbiologie, Hôpital Beaujon, Clichy,¹ Bio-Rad, Marnes-la-Coquette,² Service de Microbiologie, Hôpital Tenon, Paris, France³

Received 31 August 2004/ Returned for modification 17 October 2004/ Accepted 13 November 2004

Osiris is a video zone size reader for disk diffusion tests that includes a built-in extended expert system (EES). We evaluated the efficacy of the Osiris EES for the identification of ß-lactam susceptibility phenotypes of Escherichia coli isolates. Fifteen ß-lactam agents and three ß-lactam-ß-lactamase inhibitor combinations were tested by the disk diffusion method against 50 E. coli strains with well-characterized resistance mechanisms. The strains were screened for the production of extended-spectrum ß-lactamase (ESBL) by the double-disk synergy test using a disk of amoxicillin-clavulanic acid with disks of the extended-spectrum cephalosporins and aztreonam. Overall, the EES accurately identified the phenotype for 78% of the strains, indicated an inexact phenotype for 17% of the strains, and could not find a matching phenotype for the remaining 5% of the strains. The percentage of correct identification for each resistance mechanism was 100% for inhibitor-resistant TEM and for TEM plus cephalosporinase, 88.9% for TEM and for ESBL, 70.8% for cephalosporinase overproduction, and 25% for oxacillinase. The main cause of discrepancy was the misidentification of oxacillinase as inhibitor-resistant TEM. The conventional double-disk synergy test failed to detect ESBL production in two strains (one producing VEB-1 and one producing CTX-M-14), but synergy between cefepime and amoxicillin-clavulanic acid was visible after the distance between the disks was reduced to 20 mm. After the interpretative guidelines of the EES were updated according to our results, the percentage of correct phenotype identification increased from 78 to 96%.