Ability of the VITEK 2 advanced expert system to identify β-lactam phenotypes in isolates of Enterobacteriaceae and Pseudomonas aeruginosa

The Advanced Expert System (AES) was used in conjunction with the VITEK 2 automated antimicrobial susceptibility test system to ascertain the p-lactam phenotypes of 196 isolates of the family Enterobacteriaeae and the species Pseudomonas aeruginosa. These isolates represented a panel of strains that had been collected from laboratories worldwide and whose beta-lactam phenotypes had been characterized by biochemical and molecular techniques. The antimicrobial susceptibility of each isolate tvas determined with the VITEK 2 instrument, and the results were analyzed with the AES to ascertain the beta-lactam phenotype. The results were then compared to the beta-lactam resistance mechanism determined by biochemical and molecular techniques. Overall, the AES was able to ascertain a beta-lactam phenotype for 183 of the 196 (93.4%) isolates tested. For 111 of these 183 (60.7%) isolates, the correct beta-lactam phenotype was identified definitively in a single choice by the AES, while for an additional 46 isolates (25.1%), the AES identified the correct beta-lactam phenotype provisionally within two or more choices. For the remaining 26 isolates (14.2%), the beta-lactam phenotype identified by the AES was incorrect. However, for a number of these isolates, the error nas due to remediable problems. These results suggest that the AES is capable of accurate identification of the beta-lactam phenotypes of gram-negative isolates and that certain modifications can improve its performance even further.