Natural Mutations in Streptococcus agalactiae Resulting in Abrogation of β Antigen Production

Abstract

Streptococcus agalactiae genome encodes 21 two-component systems (TCS) and a variety of regulatory proteins in order to control gene expression. One of the TCS, BgrRS, comprising the BgrR DNA-binding regulatory protein and BgrS sensor histidine kinase, was discovered within a putative virulence island. BgrRS influences cell metabolism and positively control the expression of bac gene, coding for β antigen at transcriptional level. Inactivation of bgrR abrogated bac gene expression and increased virulence properties of S. agalactiae. In this study, a total of 140 strains were screened for the presence of bac gene, and the TCS bgrR and bgrS genes. A total of 53 strains carried the bac, bgrR and bgrS genes. Most of them (48 strains) expressed β antigen, while five strains did not express β antigen. Three strains, in which bac gene sequence was intact, while bgrR and/or bgrS genes had mutations, and expression of β antigen was absent, were complemented with a constructed plasmid pBgrRS(P) encoding functionally active bgrR and bgrS gene alleles. This procedure restored expression of β antigen indicating the crucial regulatory role of TCS BgrRS. The complemented strain A49V/BgrRS demonstrated attenuated virulence in intraperitoneal mice model of S. agalactiae infection compared to parental strain A49V. In conclusion we showed that disruption of β antigen expression is associated with: i) insertion of ISSa4 upstream the bac gene just after the ribosomal binding site; ii) point mutation G342A resulting a stop codon TGA within the bac gene and a truncated form of β antigen; iii) single deletion (G) in position 439 of the bgrR gene resulting in a frameshift and the loss of DNA-binding domain of the BgrR protein, and iv) single base substitutions in bgrR and bgrS genes causing single amino acid substitutions in BgrR (Arg187Lys) and BgrS (Arg252Gln). The fact that BgrRS negatively controls virulent properties of S. agalactiae gives a novel clue for understanding of S. agalactiae adaptation to the human.

Immunogenic Proteins of Group B Streptococcus—Potential Antigens in Immunodiagnostic Assay for GBS Detection

“…Although both proteins are encoded by genes located nearby on the chromosomes, they do not demonstrate a close relationship [15,17,30]. However, β indicates homology to the Alp3 protein, and it consists of 1159 residues [28,31]. The distinguishing feature of this protein, compared to most of the surface proteins representative of Gram-positive cocci, is lack of long repetitive tandem sequences [32].…”

Two-Component Signal Transduction Systems in the Human Pathogen Streptococcus agalactiae

“…In the absence of BgrS, BgrR is still able to upregulate bac gene transcription and ␤-antigen expression, indicating potential cross talk between BgrR and other sensor kinases (52). The BgrRS TCS is thought to be found in approximately 80% to 90% of GBS strains, but some strains include naturally occurring mutations in both bgrR and bgrS that impair their ability to upregulate bac (53).…”