Abstract
Despite the benefits of MALDI-TOF MS technology (Matrix-Assisted Laser Desorption-Ionization Time-Of-Flight Mass Spectrometry) reported worldwide and the continuous improving of the available databases, discrimination between Streptococcus pneumoniae and closely related species within the Streptococcus mitis group (SMG) using this methodology has been suboptimal. However, the accurate identification at the species level of this group of microorganisms is important for the appropriate management of infected patients. In this study, 216 SMG isolates -101 S. pneumoniae and 115 corresponding to 7 non-pneumococcal species within this group- were analyzed. All the isolates had been previously identified by conventional methods (optochin and bile solubility tests) and non-pneumococcal isolates were confirmed by sequence analysis (sodA and plys genes) when required. The isolates were also identified with the MALDI Biotyper 3.1 (Bruker Daltonics, Bremen, Germany) using an updated library containing 6,903 Main Spectra Profiles (MSPs). All the analyzed S. pneumoniae were correctly identified with MALDI-TOF MS at species level using the most updated database and all the non-pneumococcal SMG isolates were also identified at the group level. Several peaks (4,964.32, 6,888.90, and 9,516.46 m/z) have been found to be specific of S. pneumoniae, whilst a different set of peaks have proved to be present only in S. mitis (6,839.07 m/z) and S. oralis (5,297.61, 5822.53, and 6,839.07 m/z). Peak analysis allowed correct species assignment of 101/101 S. pneumoniae isolates (100%) and 102/105 S. mitis/oralis isolates (97.1%). Thus, the implementation of MALDI-TOF MS plus peak analysis for the identification of this group of microorganisms may provide precise species-level information that will allow the early implementation of directed antibiotic therapy.
Highlights
The wide implementation of MALDI-TOF MS (Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry) has led to its use for species assignment within the viridans group streptococci (VGS) and its differentiation from Streptococcus pneumoniae
216 Streptococcus mitis group (SMG) isolates were identified with MALDI-TOF MS as part of the clinical microbiology laboratory routine
All nonpneumococcal isolates were correctly identified at group level, at the species level 14 S. mitis/oralis isolates were misidentified according to superoxide dismutase (sodA) sequencing analysis results (Table 1)
Summary
The wide implementation of MALDI-TOF MS (Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry) has led to its use for species assignment within the viridans group streptococci (VGS) and its differentiation from Streptococcus pneumoniae. Efforts to get an accurate identification of VGS species are directed to determine the clinical significance of the organism and to promptly initiate directed antibiotic treatment. Conventional tests such as optochin sensitivity and bile solubility are still applied in laboratory routine and represent the reference method for the identification of S. pneumoniae isolates (Yahiaoui et al, 2016). Sequencing of housekeeping genes such as 16S rRNA does not allow enough discrimination of these species (Hoshino et al, 2005) and different targets are required. Efforts have been directed to the amino acid sequencing of the protein encoded by the gyrB gene in order to reliably identify VGS species (Galloway-Peña et al, 2014)
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