Background: The aim of this study was to evaluate the Vitek system and the disk diffusion method for susceptibility testing of Enterobacteriaceae against piperacillin-tazobactam (TZP) and methicillin-resistant Staphylococcus aureus(MRSA) against trimethoprim-sulfamethoxazole (SXT) using the broth microdilution method as the reference.

Methods: Using the Vitek system and the disk diffusion method, we tested 96 isolates of Enterobacteriaceae (48 Escherichia coli, 26 Klebsiella pneumoniae, 8 Serratia marcescens, 6 Enterobacter cloacae, 2 E. aerogenes, 2 K. oxytoca, 2 Citrobacter freundii, 2 Pantoea agglomerans) and 61 isolates of MRSA for susceptibity against TZP and SXT, respectively; the broth microdilution of National Committee for Clinical Laboratory Standards was used as the reference method.

Results: In the susceptibility testing of Enterobacteriaceae against TZP, Vitek system yielded 10 (10%) minor errors, and the disk diffusion method one (1%) very major and 13 (14%) minor errors. For the MRSA against SXT, the rate of categorical agreement between the reference method and the Vitek or the disk diffusion method was both 100%. The rates of agreement between the reference method and the Vitek system in term of MICs (within ±1 dilution) were 93% and 98% in the susceptibility testing of Enterobacteriaceae against TZP and MRSA against SXT, respectively.

Conclusion: Both Vitek system and disk diffusion method showed an acceptable level of accuracy for the susceptibility test of Enterobacteriaceae against TZP and MRSA against SXT.(Korean J Clin Microbiol 2005;8(2):160-164)