Original Article

Ann Lab Med 2016; 36(2): 117-123

Published online March 1, 2016

Copyright © Korean Society for Laboratory Medicine.

Direct Identification and Antimicrobial Susceptibility Testing of Bacteria From Positive Blood Culture Bottles by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry and the Vitek 2 System

Sung Jin Jo, M.D.*, Kang Gyun Park, M.T.*, Kyungja Han, M.D., Dong Jin Park, M.D., and Yeon-Joon Park, M.D.

Department of Laboratory Medicine, College of Medicine, The Catholic University of Korea, Seoul, Korea

Correspondence to: Yeon-Joon Park
Department of Laboratory Medicine, School of Medicine, The Catholic University of Korea, Seoul St. Mary’s Hospital, 222 Banpo-daero, Seocho-gu, Seoul 06591, Korea
Tel: +82-2-2258-1640
Fax: +82-2-2258-1719

*Sung Jin Jo and Kang Gyun Park contributed equally to this paper.

Received: July 14, 2015; Revised: August 27, 2015; Accepted: November 16, 2015


Background: We evaluated the reliability and accuracy of the combined use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) bacterial identification and Vitek 2 antimicrobial susceptibility testing (AST) for bacteria from positive blood culture bottles.
Methods: Direct identification and AST were performed in parallel to the standard methods in monomicrobial positive blood culture bottles. In total, 254 isolates grown on aerobic and/or anaerobic bottles were identified with MALDI-TOF Vitek MS (bioMérieux, France), and 1,978 microorganism/antimicrobial agent combinations were assessed. For isolates from anaerobic bottles, an aliquot of the culture broth was centrifuged, washed, and filtered through a nylon mesh. For isolates from aerobic/pediatric bottles, a lysis step using 9.26% ammonium chloride solution and 2% saponin solution was included.
Results: The overall correct identification rate was 81.8% (208/254) and that for gram-positive/gram-negative isolates was 73.9%/92.6%, respectively, and it was 81.8%, 87.6%, and 57.9% for isolates from aerobic, anaerobic, and pediatric bottles, respectively. Identification was not possible in 45 cases, and most of these isolates were streptococci (N=14) and coagulase-negative staphylococci (N=11). Misidentification occurred only in one case. Compared with standard methods, direct AST showed 97.9% (1,936/1,978) agreement with very major error of 0.25%, major error of 0.05%, and minor error of 1.8%.
Conclusions: This simple and cost-effective sample preparation method gives reliable results for the direct identification and AST of bacteria. For the identification of streptococci and coagulase-negative staphylococci, the method should be further improved.

Keywords: Direct identification, MALDI-TOF MS, Vitek MS, Vitek 2, Positive blood culture