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Direct Bacterial Identification and Antimicrobial Susceptibility Test Evaluated

By LabMedica International staff writers
Posted on 01 Jul 2021
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Image: The MicroIDSys Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) system (Photo courtesy of ASTA Corp)
Image: The MicroIDSys Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) system (Photo courtesy of ASTA Corp)
Rapid and accurate microbial identification and antimicrobial susceptibility testing (AST) are essential for timely use of appropriate antimicrobial agents for bloodstream infection. Rapid, accurate diagnostic tests, and innovative treatments constitute the key for improvement of bloodstream infection outcome.

The conventional method to detect infectious agents from the positive blood cultures usually takes up to several days due to sub-culturing onto solid agar plates and biochemical tests. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), which can identify the microorganisms from the colonies within minutes, has proven over the years to be a rapid and accurate method for identification of microorganisms.

Medical Laboratory Scientists at The Catholic University of Korea (Seoul, Korea) prospectively evaluated 124 blood cultures flagged as positive for bacterial growth from August to September 2020. When automated blood culture system showed positive signal, an aliquot from positive blood cultures was subjected to Gram staining and then subcultured. After overnight incubation, a pure colony from the agar plate was used for MALDI-TOF MS analysis with MicroIDSys system (ASTA Corp, Suwon, Korea) and AST by VITEK-2 system (bioMérieux, Marcy l'Etoile, France). For cases showing single morphotype by Gram stain, direct bacterial identification was performed using ASTA Corp’s SepsiPrep kit.

The investigators reported that compared to the conventional method using pure colony, correct direct identification rate was 96.5% and 98.5% for 57 gram-positive isolates and 67 gram-negative isolates, respectively. For direct AST, among the 55 gram-positive isolates, the categorical agreement (CA) for staphylococci, streptococci, and enterococci was 96.7%, 98.4%, and 94.1%, respectively. For 66 gram-negative isolates, the CA for Enterobacterales and non-fermentative gram-negative rods was 99.0% and 96.6%, respectively. A total of three isolates showed “invalid identification”; Streptococcus mitis/oralis, Enterococcus faecium, and Klebsiella oxytoca.

The authors concluded that their study demonstrated the effectiveness of the blood culture pellet prepared with SepsiPrep kit for direct AST as well as rapid and accurate identification. The ASTA SepsiPrep kit was very easy to use because the lysis buffer is freeze-dried and contained in one tube and only two washing steps were needed. Combining use of MicroIDSys Elite system and VITEK-2 system with blood culture pellet provided reliable identification and AST results in same day that the blood culture bottles flagged positive. The study was published in the June, 2021 issue of the Journal of Clinical Laboratory Analysis.

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