Genetic Screen To Define The Regulation Of Beta-Hemolysin Toxin Expression In Streptococcus Agalactiae

Group B Streptococcus (GBS) is a normal constituent of the intestinal and vaginal flora in 15–30% of healthy adults. The bacterium can, however, cause infections in elderly and immunocompromised patients, pregnant women, and is the main case of fatal invasive disease in newborns. For the development of sepsis and meningitis, GBS colonizes the colon and the small intestine, translocates across the intestinal epithelium, spreads in the bloodstream, evades immune mechanisms preventing GBS clearance from the blood stream and finally disrupts the blood-brain barrier entering the central nervous system. A key virulence factor, a GBS pore-forming toxin β-hemolysin (also known as the β-hemolytic lipid pigment or granadaene) facilitates translocation across the epithelial and endothelial walls, including the blood–brain barrier. The genes responsible for the hemolytic pigment expression are encoded in the cyl gene cluster which is negatively regulated by the major two-component system CovS/R and the redox-sensing regulator Rex. Because many haemolytic strains of GBS are not pigmented on blood agar, detection of GBS pigment is generally carried out on the starch-serum based Granada-type media in which the production of the toxin is stimulated by a folate pathway inhibitor. We developed a new chemically defined medium (CDM) which triggers the pigment expression. Using this medium, we demonstrated that the unknown molecule secreted by GBS strains strongly inhibits the production of β-hemolysin. Here we propose to use the new CDM to screen the transposon library of GBS CJB111 to identify the mechanisms of regulation of β- hemolysin expression. Using RNA-seq approach, we additionally identify GBS genes dysregulated by the inhibitor molecule. Isogenic mutants in the candidate genes defined by transposon and RNA-seq approaches will be constructed and analyzed to determine if gene deletions affect β-hemolysin expression and regulation of the cyl gene cluster. The results of this exploratory work have a potential to lead to development of a novel drug which inhibits the invasion state of GBS.