Evaluation and updating of the Osiris expert system for identification of Escherichia coli beta-lactam resistance phenotypes.

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 beta-lactam susceptibility phenotypes of Escherichia coli isolates. Fifteen beta-lactam agents and three beta-lactam-beta-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 beta-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%.

Evaluation of the Osiris expert system for identification of beta-lactam phenotypes in isolates of Pseudomonas aeruginosa.

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 beta-lactam susceptibility phenotypes of Pseudomonas aeruginosa isolates was evaluated. Thirteen beta-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 beta-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 beta-lactam phenotypes. A specific double-disk synergy test with reduced disk distances is necessary for the detection of ESBL production by this organism.