Direct Identification of Bacteria in Positive Blood Culture Bottles by Matrix-Assisted Laser Desorption Ionisation Time-of-Flight Mass Spectrometry

Bernard La Scola,Didier Raoult,Stefan Bereswill

doi:10.1371/journal.pone.0008041

Abstract

BackgroundWith long delays observed between sampling and availability of results, the usefulness of blood cultures in the context of emergency infectious diseases has recently been questioned. Among methods that allow quicker bacterial identification from growing colonies, matrix-assisted laser desorption ionisation time-of-flight (MALDI-TOF) mass spectrometry was demonstrated to accurately identify bacteria routinely isolated in a clinical biology laboratory. In order to speed up the identification process, in the present work we attempted bacterial identification directly from blood culture bottles detected positive by the automate.Methodology/Principal FindingsWe prospectively analysed routine MALDI-TOF identification of bacteria detected in blood culture by two different protocols involving successive centrifugations and then lysis by trifluoroacetic acid or formic acid. Of the 562 blood culture broths detected as positive by the automate and containing one bacterial species, 370 (66%) were correctly identified. Changing the protocol from trifluoroacetic acid to formic acid improved identification of Staphylococci, and overall correct identification increased from 59% to 76%. Lack of identification was observed mostly with viridans streptococci, and only one false positive was observed. In the 22 positive blood culture broths that contained two or more different species, only one of the species was identified in 18 samples, no species were identified in two samples and false species identifications were obtained in two cases. The positive predictive value of bacterial identification using this procedure was 99.2%.Conclusions/SignificanceMALDI-TOF MS is an efficient method for direct routine identification of bacterial isolates in blood culture, with the exception of polymicrobial samples and viridans streptococci. It may replace routine identification performed on colonies, provided improvement for the specificity of blood culture broths growing viridans streptococci is obtained in the near future.

Highlights

Blood culture is likely the most significant specimen type used for the diagnosis of bacterial infections, especially for bloodstream infections
We evaluated the performance of MALDI-TOF mass spectrometry for direct identification of bacteria in blood culture bottles detected as positive by the automate
The efficiency of mass spectrometry to identify crude bacteria was first established in non-medical microbiology laboratories [24] and was recently confirmed as the most promising technique for routine bacterial identification in clinical microbiology laboratories [19,25,26]

Summary

Introduction

Blood culture is likely the most significant specimen type used for the diagnosis of bacterial infections, especially for bloodstream infections. When a blood culture bottle is identified as growing bacteria the automate, presumptive identification is based on Gram staining, which allows classification of bacteria as either cocci or bacilli and as Gram-positive or Gram-negative. This information is given to clinicians in order to adapt presumptive antibiotic therapy. In order to speed up the identification process, in the present work we attempted bacterial identification directly from blood culture bottles detected positive by the automate

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Results

Conclusion