Abstract
ABSTRACTStreptococcus tigurinus is a new member of the Mitis group and is associated with infective endocarditis. Low and high virulent variants have been described. A search was made in the national reference collection of endocarditis isolates for S. tigurinus–like strains by sequencing housekeeping genes (16S rRNA-gene, gdh, groEL, sodA). The strains were further profiled by polymerase chain reaction (PCR) targeting a choice of virulence genes (rib-like, cshA-like, gtfR, int, pitA, hylA). To study the prevalence and abundance of S. tigurinus in the saliva and on the mucosal membranes of 35 healthy adults, PCRs detecting two variants of the 16S operon and virulence genes were applied. Among the endocarditis isolates, eight strains (all gtfR-negative and former S. oralis) holding the specific S. tigurinus 16S motif were found, but the pattern of genes related to high virulence found in the S. tigurinus type strain could not be detected in any of these strains. A close phylogenetic proximity between S. tigurinus and S. oralis was observed, with intersectional hybrid strains formed. This was supported by concatenated housekeeping sequences, in silico DNA–DNA hybridization, pathogenomic profiling, and multidimensional scaling. In the oral samples, S. tigurinus could be detected frequently, especially in the most common operon variant, but none of the type strain–related virulence factors were found. Low virulent S. tigurinus–like strains can be found frequently and in high prevalence (66%) and abundance (12.5%) in the oral cavity of healthy adults. In strain collections, they are among the formerly known gtfR-negative S. oralis. Highly virulent strains seem to be uncommon. Though closely related, S. oralis and S. tigurinus can be separated by the presence or absence of gtfR and dextran production. Hybrids of both species can be found. The variable arsenal of virulence genes found in this study emphasizes the genetic plasticity of Mitis group streptococci.
Highlights
The precise description and classification of prokaryotic microorganisms has never been an easy task [1]
Strains and phylogenetic analysis DNA was extracted from S. oralis SN 16495, SN 17127, SN 28194, SN 31376, SN 37569, SN 37737, SN 39325, SN 40525, SN 45448, SN 48861, SN 50746, SN 51446, SN 54788, SN 58364, SN 58746, SN 59433, SN 60579, SN 63707, S. infantis SN 54787, SN 57625, SN 17128, SN 19640, ‘S. tigurinus’ SN 62386, and from four S. tigurinus reference strains, including type strain AZ_3a, as well as small colony variants (2425, 2426) of parental strain 1366 [19] kindly provided by A
RRNA gene-based tree with 73 Mitis group strains and S. oligofermentans strain AS 1.3089T as an outgroup
Summary
The precise description and classification of prokaryotic microorganisms has never been an easy task [1]. One reason for the difficulties might be that the zoological definition of a species as ‘groups of interbreeding or potentially interbreeding natural populations that are reproductively isolated from other such groups’ cannot be applied to prokaryotes [2]. A kind of (man-made) rule of thumb has been established, stating that strains of Bacteria or Archaea possessing 16S rRNA-gene sequences with >97% identity belong to the same species, but this needs to be checked by DNA–DNA hybridization. This concept has been updated, stating that the cutoff of 97% was too low and can be raised to 98.7% [3]. One problem here is that – especially in some taxa such as oral fusobacteria [4]
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