Abstract
Streptococcus pneumoniae is a member of the Mitis group of streptococci which, according to 16S rRNA-sequence based phylogenetic reconstruction, includes 12 species. While other species of this group are considered prototypes of commensal bacteria, S. pneumoniae is among the most frequent microbial killers worldwide. Population genetic analysis of 118 strains, supported by demonstration of a distinct cell wall carbohydrate structure and competence pheromone sequence signature, shows that S. pneumoniae is one of several hundred evolutionary lineages forming a cluster separate from Streptococcus oralis and Streptococcus infantis. The remaining lineages of this distinct cluster are commensals previously collectively referred to as Streptococcus mitis and each represent separate species by traditional taxonomic standard. Virulence genes including the operon for capsule polysaccharide synthesis and genes encoding IgA1 protease, pneumolysin, and autolysin were randomly distributed among S. mitis lineages. Estimates of the evolutionary age of the lineages, the identical location of remnants of virulence genes in the genomes of commensal strains, the pattern of genome reductions, and the proportion of unique genes and their origin support the model that the entire cluster of S. pneumoniae, S. pseudopneumoniae, and S. mitis lineages evolved from pneumococcus-like bacteria presumably pathogenic to the common immediate ancestor of hominoids. During their adaptation to a commensal life style, most of the lineages gradually lost the majority of genes determining virulence and became genetically distinct due to sexual isolation in their respective hosts.
Highlights
Phylogenetic analysis based on 16S rRNA sequences of type strains of the more than 50 Streptococcus species reveal a clustering pattern that reflect their pathogenic potential and ecological preferences [1,2]
S. pneumoniae, S. mitis, S. oralis, and S. infantis plus eight sets of sequences extracted from S. pneumoniae genomes revealed a remarkable sequence polymorphism resulting in a total of 92 ddl alleles, 99 gdh alleles, 95 rpoB alleles, and 94 sodA alleles
In spite of close genetic relationships, the pneumococcus and other members of the Mitis group of streptococci have strikingly different pathogenic potential. This difference is striking for S. pneumoniae and S. mitis, which are genetically very closely related as evidenced by Figure 1
Summary
Phylogenetic analysis based on 16S rRNA sequences of type strains of the more than 50 Streptococcus species reveal a clustering pattern that reflect their pathogenic potential and ecological preferences [1,2]. Expression by occasional isolates of the commensal species of proteins that are known to contribute to virulence in S. pneumoniae has been taken as evidence of horizontal gene transfer [3,18,19,20]. Such data led Hakenbeck and coworkers [18] to suggest ‘‘a smooth transition’’ between these species. To what extend transitional forms are involved in occasionally reported cases of meningitis, septicemia, toxic-shock-like syndrome, and eye infections attributed to the otherwise commensal species is not known
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
