Abstract
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.
Highlights
Group B Streptococcus (GBS; known as Streptococcus agalactiae) is the leading cause of severe invasive neonatal infections worldwide
Surface distribution of the major PI-2b pilin Spb1 in strains A909 and BM110 was analysed by immunofluorescence
Spb1 level on the bacterial surface was higher in A909 than in BM110
Summary
Group B Streptococcus (GBS; known as Streptococcus agalactiae) is the leading cause of severe invasive neonatal infections worldwide. For EOD, transmission to newborns probably originates from colonized mothers, by inhalation of GBS-contaminated amniotic or vaginal fluid during delivery. For LOD, the mode of transmission and the infection route remain poorly defined. Several epidemiological studies have pinpointed a remarkable association of serotype III sequence type (ST) 17 GBS strains with meningitis, during LOD [1,2,3]. These strains belonging to the clonal complex 17 (CC17) have historically been designated as “hypervirulent”. Deciphering the molecular bases of their higher pathogenicity constitute an important focus of our research
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