Abstract
Viridans Group Streptococci (VGS) species-level identification is fundamental for patients management. Matrix-assisted laser desorption ionization—time of flight mass spectrometry (MALDI-TOF MS) has been used for VGS identification but discrimination within the Mitis group resulted difficult. In this study, VGS identifications with two MALDI-TOF instruments, the Biotyper (Bruker) and the VITEK MS (bioMérieux) have been compared to those derived from tuf, soda and rpoB genes sequencing. VGS isolates were clustered and a dendrogram constructed using the Biotyper 3.0 software (Bruker). RpoB gene sequencing resulted the most sensitive and specific molecular method for S. pneumonia identification and was used as reference method. The sensitivity and the specificity of the VITEK MS in S. pneumonia identification were 100%, while the Biotyper resulted less specific (92.4%). In non pneumococcal VGS strains, the group-level correlation between rpoB and the Biotyper was 100%, while the species-level correlation was 61% after database upgrading (than 37% before upgrading). The group-level correlation between rpoB and the VITEK MS was 100%, while the species-level correlation was 36% and increases at 69% if isolates identified as S. mitis/S. oralis are included. The less accurate performance of the VITEK MS in VGS identification within the Mitis group was due to the inability to discriminate between S. mitis and S. oralis. Conversely, the Biotyper, after the release of the upgraded database, was able to discriminate between the two species. In the dendrogram, VGS strains from the same group were grouped into the same cluster and had a good correspondence with the gene-based clustering reported by other authors, thus confirming the validity of the upgraded version of the database. Data from this study demonstrated that MALDI-TOF technique can represent a rapid and cost saving method for VGS identification even within the Mitis group but improvements of spectra database are still recommended.
Highlights
The Viridans Group Streptococci (VGS) constitutes a heterogeneous group of bacteria
The specificity of the MALDI-TOF Biotyper system for S. pneumoniae identification was 92.4% (10 non pneumococcal isolates were misidentified as S. pneumoniae) after the last upgrade of the database (January 2014) and 49% with the precedent version (67 non pneumococcal isolates were misidentified as S. pneumoniae) (Table 2)
The VITEK Compact system correctly identified 23/26 (88.5%) S. pneumoniae isolates, these three misidentified S. pneumoniae were in two cases identified as S.pneumoniae/S. mits at the same score and in one case as S. gordonii
Summary
The Viridans Group Streptococci (VGS) constitutes a heterogeneous group of bacteria. Most species are part of the normal flora in different tracts of human body, such as respiratory, urogenital and gastrointestinal. Clinical laboratories must be able to accurately differentiate S. pneumoniae from other VGS commonly found in clinical samples to facilitate appropriate antimicrobial therapy [2] but it is important to identify the various species of VGS in some clinical settings, such as sepsis, endocarditis and in immunocompromised and transplant patients [1,3,4]. The taxonomy within this group of bacteria is not yet well defined and it is continuously updated by the results of the molecular typing methods [4,5,6,7]
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