Identification of Aerobic Gram-Positive Bacilli by Use of Vitek MS

Abstract

The accuracy of Vitek MS mass spectrometric identifications was assessed for 206 clinically significant isolates of aerobic Grampositive bacilli representing 20 genera and 38 species. The Vitek MS identifications were correct for 85% of the isolates (56.3% to the species level, 28.6% limited to the genus level), with misidentifications occurring for 7.3% of the isolates. I dentification of aerobic, Gram-positive bacilli (GPB) to the species level is challenging. Biochemical methods are slow and labor-intensive and may not provide reliable results. Molecular sequencing techniques have become the gold standard for identification of GPB (1-3) but are too expensive for routine use by the clinical microbiology laboratory. As a consequence, GPB identifications are often limited to the genus level or descriptive results (e.g., diphtheroid).Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) is a new technology offering accurate, fast, and affordable identification for bacteria and yeasts (4-7). Mass spectrometry results are graphed as peaks based on the mass/charge ratio of microbial proteins that have been ionized from intact cells obtained from pure culture. Identification is achieved by comparison of the mass spectral profile to a reference database.The purpose of this study was to evaluate the accuracy of MALDI-TOF MS identifications generated by the Vitek MS system (v2.0; bioMérieux, Durham, NC) for aerobic GPB isolates commonly encountered in the clinical microbiology laboratory. The Vitek MS results were compared to identifications previously reported that were based on conventional biochemical or molecular methods. Sequencing of 16S rRNA or rpoB genes was performed to arbitrate discordant results. Although the Vitek MS system recently received FDA 510(k) de novo clearance, only a few of the species analyzed in this current study were included in the FDA clinical trials and data submission.A total of 206 GPB isolates recovered from specimens submitted for culture from 31 December 2007 to 21 August 2012 were selected from the frozen stock database of the Cleveland Clinic Clinical Microbiology Laboratory for inclusion in the study. Duplicate isolates of the same species from a single patient were excluded. Approval was received by the institutional review board prior to initiation of the study. These clinical isolates had been previously identified using biochemical testing or molecular methods (sequencing of 16S rRNA or rpoB genes). The frozen isolates were plated on 5% sheep blood agar and incubated overnight at 35°C in 5% CO 2 . Isolates were subcultured a second time before Vitek MS testing was performed. The manufacturer's instructions were followed for Vitek MS testing.A thin film of fresh colony growth was applied directly to the target slide. Each spot was covered with 1 l of matrix solution (␣-cyano-4-hydroxycinnamic acid; bioMérieux), allowed to dry, and analyzed on the Vitek MS instrument. If two attempts at direct colony testing failed to provide an acceptable si...

Corynebacterium striatum: an emerging respiratory pathogen

“…Many studies find multi-drug resistant strains, though [14]. A recent study which included a large number of isolates (n = 179) found 72% isolates to be resistant to all oral antimicrobials [15]. All our isolates were multidrug resistant sensitive to only Linezolid and vancomycin.…”

Short Communication: Identification and phylogenetic analysis of Corynebacterium diphtheriae isolates from Jakarta, Indonesia based on partial rpoB gene

“…Another VITEK MS-Automated Testing also showed misidentifications of two C. afermentans isolates as Lactobacillus species (Navas et al 2014). Moreover, C. ureicelerivorans and C. minutissimum were also misidentified as C. mucifaciens and C. aurimucosum, respectively (Navas et al 2014). To the best of our knowledge, there is no report about misidentifications of 2 C. argentoratense isolates as C. diphtheriae by VITEK 2 ANC-Automated Testing.…”

“…A multicenter study reported that identification by VITEK 2 using clinical isolates showed 4.9% low discrimination, 4.6% incorrect identification, and 0.3% unidentified isolates, and the results of Corynebacterium identification showed misidentification of C. urealyticum strain from 51 Corynebacterium species (not included C. argentoratense) (Rennie et al 2008). Another VITEK MS-Automated Testing also showed misidentifications of two C. afermentans isolates as Lactobacillus species (Navas et al 2014). Moreover, C. ureicelerivorans and C. minutissimum were also misidentified as C. mucifaciens and C. aurimucosum, respectively (Navas et al 2014).…”

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry: a powerful tool for identification of Corynebacterium species

“…As MALDI-TOF MS is reported to be widely used in routine laboratories (20)(21)(22)(23). The most prominent advantages are its speed and low running costs provided that a quality-controlled database of reference spectra including all relevant microorganisms is available (3).…”