Evaluation of protein spectra cluster analysis for Streptococcus spp. identification from various swine clinical samples

Traditional microbiological methods enable genus-level identification of Streptococcus spp. isolates. However, as the species of this genus show broad phenotypic variation, species-level identification or even differentiation within the genus is difficult. Herein we report the evaluation of protein spectra cluster analysis for the identification of Streptococcus species associated with disease in swine by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). A total of 250 S. suis-like isolates obtained from pigs with clinical signs of encephalitis, arthritis, pneumonia, metritis, and urinary or septicemic infection were studied. The isolates came from pigs in different Brazilian states from 2001 to 2014. The MALDI-TOF MS analysis identified 86% (215 of 250) as S. suis and 14% (35 of 250) as S. alactolyticus, S. dysgalactiae, S. gallinaceus, S. gallolyticus, S. gordonii, S. henryi, S. hyointestinalis, S. hyovaginalis, S. mitis, S. oralis, S. pluranimalium, and S. sanguinis. The MALDI-TOF MS identification was confirmed in 99.2% of the isolates by 16S rDNA sequencing, with MALDI-TOF MS misidentifying 2 S. pluranimalium as S. hyovaginalis. Isolates were also tested by a biochemical automated system that correctly identified all isolates of 8 of the 10 species in the database. Neither the isolates of the 3 species not in the database (S. gallinaceus, S. henryi, and S. hyovaginalis) nor the isolates of 2 species that were in the database (S. oralis and S. pluranimalium) could be identified. The topology of the protein spectra cluster analysis appears to sustain the species phylogenetic similarities, further supporting identification by MALDI-TOF MS examination as a rapid and accurate alternative to 16S rDNA sequencing.