Comparison of commonly used antimicrobial susceptibility testing methods for evaluating susceptibilities of clinical isolates of Enterobacteriaceae and nonfermentative Gram-negative bacilli to cefoperazonee-sulbactam

Background/Purpose: The aim of this study was to investigate the cefoperazone esulbactam (CFPeSUL) susceptibilities of important Gram-negative bacteria (GNB) by agar dilution (reference method), disk diffusion, and two automated methods. Methods: A total of 799 GNB isolates, including Enterobacteriaceae (n Z 500) and nonfermentative GNB (NFGNB, n Z 299), were recovered from various clinical specimens collected at National Taiwan University Hospital, Taipei, Taiwan from November 2013 to December 2014. The agar dilution method, disk diffusion method, and two automated susceptibility systems (Phoenix and Vitek 2) were used for testing susceptibility of the isolates to CFPeSUL. Categories of susceptibility (susceptible, intermediate, or resistant) to CFPeSUL yielded from each method were interpreted according to CFPeSUL interpretive breakpoints proposed previously. The results of categorical agreement and errors obtained between the agar dilution method and the other three methods were analyzed. Results: The Vitek 2 system had the highest error rates against Escherichia coli (n Z 150) and Enterobacter cloacae (n Z 77) isolates, i. e., 6.7% and 11.7% minor errors, 8.5% and 1.7% major errors, and 40% and 20% very major errors, respectively. Additionally, the Vitek 2 system was also found to have a significantly lower sensitivity (44.4%) and lower positive predictive value (18.2%) for detecting CFPeSUL nonsusceptible E. coli isolates than other methods. For carbapenem-nonsusceptible Enterobacteriaceae isolates, the Vitek 2 system failed to detect correct susceptibility to CFPeSUL. The three methods failed to correctly detect CFPeSUL susceptibility categories against all NFGNB isolates except Pseudomonas aeruginosa. Conclusion: The Vitek 2 system is a suboptimal method in correctly detecting CFPeSUL susceptibility categories for E. coli, E. cloacae, and carbapenem-nonsusceptible Enterobacteriaceae isolates. Copyright | 2015, Taiwan Society of Microbiology. Published by Elsevier Taiwan LLC.