Background
Streptococcus pneumoniae is an important human pathogen representing a major cause of morbidity and mortality worldwide. We sequenced the genome of a serotype 11A, ST62 S. pneumoniae invasive isolate (AP200), that was erythromycin-resistant due to the presence of the erm(TR) determinant, and carried out analysis of the genome organization and comparison with other pneumococcal genomes.
Conclusions
The sequence of S. pneumoniae AP200 chromosome revealed a dynamic genome, characterized by chromosomal rearrangements and horizontal gene transfers. The overall diversity of AP200 is driven mainly by the presence of the exogenous elements Tn1806 and φSpn_200 that show large gene exchanges with other genetic elements of different bacterial species. These genetic elements likely provide AP200 with additional genes, such as those conferring antibiotic-resistance, promoting its adaptation to the environment.
Results
The genome sequence of S. pneumoniae AP200 is 2,130,580 base pair in length. The genome carries 2216 coding sequences (CDS), 56 tRNA, and 12 rRNA genes. Of the CDSs, 72.9% have a predicted biological known function. AP200 contains the pilus islet 2 and, although its phenotype corresponds to serotype 11A, it contains an 11D capsular locus. Chromosomal rearrangements resulting from a large inversion across the replication axis, and horizontal gene transfer events were observed. The chromosomal inversion is likely implicated in the rebalance of the chromosomal architecture affected by the insertions of two large exogenous elements, the erm(TR)-carrying Tn1806 and a functional prophage designated φSpn_200. Tn1806 is 52,457 bp in size and comprises 49 ORFs. Comparative analysis of Tn1806 revealed the presence of a similar genetic element or part of it in related species such as Streptococcus pyogenes and also in the anaerobic species Finegoldia magna, Anaerococcus prevotii and Clostridium difficile. The genome of φSpn_200 is 35,989 bp in size and is organized in 47 ORFs grouped into five functional modules. Prophages similar to φSpn_200 were found in pneumococci and in other streptococcal species, showing a high degree of exchange of functional modules. φSpn_200 viral particles have morphologic characteristics typical of the Siphoviridae family and are capable of infecting a pneumococcal recipient strain. Conclusions: The sequence of S. pneumoniae AP200 chromosome revealed a dynamic genome, characterized by chromosomal rearrangements and horizontal gene transfers. The overall diversity of AP200 is driven mainly by the presence of the exogenous elements Tn1806 and φSpn_200 that show large gene exchanges with other genetic elements of different bacterial species. These genetic elements likely provide AP200 with additional genes, such as those conferring antibiotic-resistance, promoting its adaptation to the environment.