Abstract
We used an in-house saponin-based extraction method to evaluate the performance of the Bruker Biotyper matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS) system for the identification of bacteria and fungi in 405 positively flagged blood culture bottles. Results obtained from MALDI-TOF/MS were compared with those obtained using conventional phenotypic identification methods. Of the 405 positively flagged blood culture bottles, 365 showed monomicrobal growth and were correctly identified to the species (72.1%) or genus (89.6%) level using the Bruker Biotyper system. The remaining 40 positively flagged blood culture bottles showed polymicrobial growth. Of them, 82.5% (n = 33) of the isolates were correctly identified to the species level and 92.5% (n = 37) to the genus level using the Bruker Biotyper system. The overall accuracy of identification to the genus level in flagged blood cultures was 89.5% for Gram-positive organisms, 93.5% for Gram-negative pathogens and 71.9% for fungi. Confidence scores were ≥1.500 for 307 (75.8%) bottles, ≥1.700 for 249 (61.5%) bottles and ≥2.000 for 142 (35.1%) bottles. None of the yeast cultures yielded scores ≥1.700. Using an identification-score cutoff of ≥1.500, the MALDI Biotyper correctly identified 99.2% of Gram-positive bacteria, 97.6% of Gram-negative bacteria and 100% of yeast isolates to the genus level and 77.6% of Gram-positive bacteria, 87.1% of Gram-negative bacteria and 100.0% of yeast isolates to the species level. The overall rate of identification using our protocol was 89.9% (364/405) for genus level identification and 73.1% (296/405) for species level identification. Yeast isolates yielded the lowest confidence scores, which compromised the accuracy of identification. Further optimization of the protein extraction procedure in positive blood cultures is needed to improve the rate of identification.
Highlights
Bloodstream infections are a leading cause of admission to intensive care units and carry a high mortality rate
Further optimization of the protein extraction procedure in positive blood cultures is needed to improve the rate of identification
All positive cultures preserved in Bactec Plus Aerobic/F bottles or Bactec Anaerobic Lytic/10 bottles (Becton-Dickinson Microbiology Systems, Sparks, MD, USA) that had been obtained from patients treated for bloodstream infections at the National Taiwan University Hospital (NTUH) during the period October 13, 2014 to January 15, 2015 were evaluated by conventional phenotypic methods and the MALDI TOF-MS Biotyper system
Summary
Bloodstream infections are a leading cause of admission to intensive care units and carry a high mortality rate. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/MS) techniques are routinely used for the direct identification of microorganisms from agar cultures and positively flagged blood cultures (Moussaoui et al, 2010; Lagace-Wiens et al, 2012; Martiny et al, 2012) These techniques provide definitive identification of pathogens causing bloodstream infections 18–48 h earlier than conventional methods. A number of purification methods are available to prepare samples for MALDI-TOF/MS analysis such as differential centrifugation, lysis centrifugation, pre-incubation on sold media and the SepsiTyperTM kit (Schubert et al, 2011; Saffert et al, 2012; March-Rossello et al, 2013) Of those methods, processing of specimens using the SepsiTyperTM kit has been shown to result in highly accurate identification rates; the kit is not widely used because of its high cost (Buchan et al, 2012; Lagace-Wiens et al, 2012)
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