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Biosynthesis, role(s) and regulation of the PI-2b pilus in the hypervirulent ST-17 clone of Streptococcus agalactiae

Abstract : Streptococcus agalactiae (also known as Group B Streptococcus, GBS) is an opportunistic Gram-positive pathogen responsible for severe invasive infections, especially in neonates. GBS strains belonging to the ST-17 sequence type are responsible for 80% of late-onset neonatal meningitis. Genomic comparison of ST-17 strains to non-ST-17 GBS isolates revealed a few surface proteins that are characteristic of ST-17 clone, such as HvgA and Srr2, which contribute to colonization and dissemination. Similarly, the PI-2b type pilus is conserved in ST-17 strains and the main goal of this PhD project was to decipher the role of this pilus in the physiopathology of ST-17 strains. In the first part of this work, we compared the expression of the PI-2b pilus in our ST-17 representative strain BM110, and a non-ST-17 human clinical isolate, A909. We showed that PI-2b expression, although variable, was lower in ST-17 isolates as compared to non ST17 isolates. In the representative strain BM110, we demonstrated that the lower expression was be due to the presence of a 43-base pair (bp) hairpin-like structure in the upstream region of PI-2b, preventing read-through transcription from upstream antigen B (AgB) operon. Furthermore, gene reporter assays to characterize the Ppi-2b promoter region revealed the requirement of an extended 5’ region and of GBS-specific regulatory factors to drive PI-2b transcription. PI-2b transcription was shown to be maximal at 37 °C. Collectively our results suggest a complex regulation of PI-2b expression in ST-17 clinical isolates, that may confer a selective advantage in the human host either by reducing host immune responses and/or increasing their dissemination potential.In the second part of this work, we sought to investigate the role of the putative adhesin AP1-2b, and the two accessory genes lep and orf in the biosynthesis of PI-2b pilus. We showed that both orf and lep are important for PI-2b expression. Our results suggest that Lep is a functional signal peptidase involved in the optimal processing of the major PI-2b pilin. The role of orf remains to be uncovered
Keywords : PI-2b ST-17 CC-17 CovR
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Noémi Réka Szili. Biosynthesis, role(s) and regulation of the PI-2b pilus in the hypervirulent ST-17 clone of Streptococcus agalactiae. Microbiology and Parasitology. Université Sorbonne Paris Cité, 2017. English. ⟨NNT : 2017USPCC062⟩. ⟨tel-01932468⟩

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