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PLoS One. 2017 Jan 20;12(1):e0169840. doi: 10.1371/journal.pone.0169840. eCollection 2017.

Regulation of PI-2b Pilus Expression in Hypervirulent Streptococcus agalactiae ST-17 BM110.

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Institut Pasteur, Biology of Gram-positive Pathogens Unit, Paris, France.
Centre National de la Recherche Scientifique (CNRS) ERL3526, Paris, France.
Institut Pasteur, Unité Ecologie et Evolution de la Résistance aux Antibiotiques, Paris, France.
Équipe Barrières et Pathogènes, Institut Cochin, Inserm 1016, CNRS UMR, Paris France.
Assistance Publique-Hôpitaux de Paris, Hôpitaux Universitaires Paris Centre-Site Cochin, France.
CNR Streptocoques, Hôpitaux Universitaires Paris Centre, site Cochin, AP-HP.
DHU Risques et Grossesse, AP-HP, Paris, France.


The widely spread Streptococcus agalactiae (also known as Group B Streptococcus, GBS) "hypervirulent" ST17 clone is strongly associated with neonatal meningitis. The PI-2b locus is mainly found in ST17 strains but is also present in a few non ST17 human isolates such as the ST-7 prototype strain A909. Here, we analysed the expression of the PI-2b pilus in the ST17 strain BM110 as compared to the non ST17 A909. Comparative genome analyses revealed the presence of a 43-base pair (bp) hairpin-like structure in the upstream region of PI-2b operon in all 26 ST17 genomes, which was absent in the 8 non-ST17 strains carrying the PI-2b locus. Deletion of this 43-bp sequence in strain BM110 resulted in a 3- to 5-fold increased transcription of PI-2b. Characterization of PI-2b promoter region in A909 and BM110 strains was carried out by RNAseq, primer extension, qRT-PCR and transcriptional fusions with gfp as reporter gene. Our results indicate the presence of a single promoter (Ppi2b) with a transcriptional start site (TSS) mapped 37 bases upstream of the start codon of the first PI-2b gene. The large operon of 16 genes located upstream of PI-2b codes for the group B carbohydrate (also known as antigen B), a major constituent of the bacterial cell wall. We showed that the hairpin sequence located between antigen B and PI-2b operons is a transcriptional terminator. In A909, increased expression of PI-2b probably results from read-through transcription from antigen B operon. In addition, we showed that an extended 5' promoter region is required for maximal transcription of gfp as a reporter gene in S. agalactiae from Ppi2b promoter. Gene reporter assays performed in Lactococcus lactis strain NZ9000, a related non-pathogenic Gram-positive species, revealed that GBS-specific regulatory factors are required to drive PI-2b transcription. PI-2b expression is up-regulated in the BM110ΔcovR mutant as compared to the parental BM110 strain, but this effect is probably indirect. Collectively, our results indicate that PI-2b expression is regulated in GBS ST17 strains, which may confer a selective advantage in the human host either by reducing host immune responses and/or increasing their dissemination potential.

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