Identification and Antimicrobial Susceptibility Testing of Microorganisms From Positive blood Cultures by a Combined Lysis-centrifugation Method with MALDI-TOF MS and VITEK2 System

Background: Rapid identification and the application of antimicrobial susceptibility testing (AST) to microorganisms causing bloodstream infections is pivotal to guide antimicrobial therapy. This study aims to: 1) utilize the Lysis-Centrifugation Method (LCM) not only for identification of microorganisms from positive blood culture bottles, but also for direct AST full panel by Vitek®2 system (bioMérieux, Inc. France) and by disc diffusion plate (Kirby Bauer Method) and 2) analyze the accuracy of these combined methods.Methods: 124 mono-microbial positive blood culture bottles were included in this study. An aliquot was subjected to LCM and used for the identification by the MALDI-TOF System. Moreover the microbial pellet was used for direct AST testing full panel by VITEK®2 system and by Kirby Bauer Method.Results: 123 isolates were correctly identified to the species level and 1 isolate was identified to the genus level. Comparing the two utilized AST methods, it was observed that Gram-positive isolates showed an agreement rate of 96.6% (58/60). Enterococcus faecalis was the only microorganism with a major error rate of 0.6% (2/324) related to erythromycin. Among the Gram-negative, the overall agreement rate was 93.3 (56/60). Klebsiella pneumoniae, Escherichia coli and Enterobacter spp. were the major cause of minor error rates (0.6%, 4/709) and major error rates (1.1%, 8/709). Among the yeasts, results showed an agreement rate of 100% (4/4).Conclusions: Our simple and cost-effective sample preparation method is very useful for rapid identification as well as AST of microorganisms directly from positive blood culture bottles in a clinical setting