Comparative genomics has revealed an extensive genomic diversity within and among closely related bacterial species. Our objective is to combine evolutionary and population studies to identify specific genomic traits related to host adaptation and to pathogenicity mechanisms. We focus our studies on the genus Streptococcus, which encompasses major pathogens for humans and animals. We also address the molecular mechanisms leading to this diversity, involving lateral gene transfers, transmission of mobile genetic elements and mutational events.

Population genomics of Streptococcus agalactiae: S. agalactiae is the first cause of neonatal infections; it is also responsible for bovine mastitis and invasive infection in fish. We have reconstructed the evolutionary history of human strains and shown that they derived from a common ancestor and have subsequently exchanged long DNA regions with unrelated strains. We have also been able to reproduce these DNA transfers under laboratory conditions. We are currently comparing the S. agalactiaelineages to identify loci related to virulence, in particular by focusing on the hypervirulent ST17 clone. We have shown by genomic analysis that S. agalactiaestrains pathogenic for fish have undergone a massive genome reduction involving multiple deletions and gene inactivation. We are using genome comparisons with other fish isolates from different parts of the world to propose a scenario for the emergence of this lineage and the underlying mechanism of gene loss.

Transcriptional regulation in Streptococcus agalactiae: In their host, GBS strains encounter different environments. The cell envelope is the primary site of interaction for sensing and to respond to changes in the environment. We have analyzed in collaboration with P. Trieu-Cuot (IP) the regulation of the synthesis and modification of the cell envelope. We have characterized the response of the bacteria to the absence of D-alanylation of lipoteichoic acid, and identified a control loop involving two two-components regulatory systems that allows the fine-tuning of D-alanylation, which determine the net charge at the cell surface and the sensitivity to antimicrobial peptides.

Host specificity within the Streptococcus gallolyticus group: S. gallolyticusis a second Streptococcusspecies that colonizes both human and bovines. The genome analysis of an isolate responsible for an endocarditis associated with a colon cancer revealed putative virulence factors together with specific metabolic and biosynthetic features linked to its adaptation to the bovine rumen. In contrast, comparison with the closely related food grade subspecies S. macedonicusshowed that this subspecies had lost most of the virulence factors. This represents another particularly interesting model to study gene gain and gene loss within the genus Streptococcus.

Molecular mechanisms involved in genome dynamics. We have shown that the multiple mobile genetic elements inserted into the genome of S. agalactiaeplayed a central role in the transfer of large DNA regions by conjugation. We are currently studying the mechanisms involved in these transfers. In particular we study in depth a new family of conjugative transposons we have discovered, which share unique properties. First we want to unravel the expression program allowing the stability and the transfer of these elements, which associate DDE transposition, conjugation and replication. Second, we combine genetic and biochemical approaches to define the molecular mechanisms responsible for the insertion specificity of these transposable elements that specifically target the promoter regions of sigma A dependent promoters.

Keywords: streptococci; population genomics; lateral gene transfers; conjugative transposons; regulation; host specificity

1. Rusniok, C., Couve E., Da Cunha V., El Gana R., Zidane N., Bouchier C., Poyart C., Leclercq R., Trieu-Cuot P., and Glaser P.. 2010. Genome sequence of Streptococcus gallolyticus: insights into its adaptation to the bovine rumen and its ability to cause endocarditis. J. Bacteriol. 192 :2266-2276

2. Brochet M, Couvé E, Bercion R, Sire JM, Glaser P. (2009) Population structure of human isolates of Streptococcus agalactiae from Dakar and Bangui. J. Clin. Microbiol. 47(3):800-803

3. Brochet, M., V. Da Cunha, E. Couve, C. Rusniok, P. Trieu-Cuot, and P. Glaser. 2009. Atypical association of DDE transposition with conjugation specifies a new family of mobile elements. Mol. Microbiol. 71:948-959.

4. Brochet, M., C. Rusniok, E. Couve, S. Dramsi, C. Poyart, P. Trieu-Cuot, F. Kunst, and P. Glaser. 2008. Shaping a bacterial genome by large chromosomal replacements, the evolutionary history of Streptococcus agalactiae. Proc. Natl. Acad. Sci. USA 105:15961-6.

5. Brochet, M., E. Couve, M. Zouine, C. Poyart, and P. Glaser. 2008. A naturally occurring gene amplification leading to sulfonamide and trimethoprim resistance in Streptococcus agalactiae. J.Bacteriol. 190:672-80.