Background: Bloodstream infections are the most serious infectious diseases and one of the leading causes of morbidity and mortality worldwide. Rapid bacterial identification (ID) and antimicrobial susceptibility test (AST) could lead to quicker microbiological diagnosis and appropriate antimicrobial therapy.

Objective: To compare the performance of ID and AST between bacteria directly from positive blood culture broths by using differential centrifugation method (direct method) and conventional culture-based methods.

Materials and Methods: The prospective descriptive study on 349 positive monobacterial blood culture bottles was conducted between November 15, 2016 and January 15, 2017 at the Vajira Hospital, Bangkok, Thailand. The differential centrifugation method was used to isolate and concentrate bacterial cells directly from positive blood culture broths. This sample preparation process takes only 15 to 20 minutes, and the bacteria cell pellet was used as a sample for ID by MALDI Biotyper system (direct ID method) and AST by disc diffusion assay (direct AST method). Results were then compared with those obtained from isolated colonies grown overnight on agar plates, as conventional methods.

Results: For direct ID, correct species ID was obtained for 297 (85.10%) isolates and 49 isolates (14.04%) were not identified. Two isolates (0.57%) were identified to genus only, and only one isolate (0.29%) was misidentified. There were limitations in the MALDI-TOF technology. A chi-square test showed that there was no statistically significant difference between the two methods in the ID of Gram-negative isolates, Staphylococcus aureus, and Enterococci. For direct AST, overall categorical agreement was 98.20% with no false resistance or susceptibility.

Conclusion: The present data suggest that the direct ID was useful for preliminary ID of bacteria in positive blood culture, especially for Gram-negative bacteria, S. aureus, and Enterococci. Results can be reported within 30 to 45 minutes after the positive signal. The direct AST provided excellent susceptibility testing results with both Gram-positive and Gram-negative bacteria, and yielded results 18 to 24 hours earlier than conventional AST method. These direct ID and AST methods were simple, rapid, and inexpensive.

Keywords: Identification, Antimicrobial susceptibility testing, Matrix-assisted laser desorption ionization-time of flight mass spectrometry, MALDI-TOF MS, Direct identification method, Conventional culture-based method

Received 29 May 2019 | Revised 30 July 2019 | Accepted 2 Aug 2019