S. flexneri is subtyped into various serotypes based on the combination(s) of antigenic determinants present in the O-antigen. Structural studies have shown that all S. flexneri O-antigens, except for serotype 6, are polymers of the basic tetrasaccharide repeating unit, which is found in S. flexneri serotype Y strains. Addition of glucosyl and/or O-acetyl residues to the basic O-antigen results in serotype conversion (O-antigen modification), which is mediated by temperate bacteriophages.
The factors involved in O-antigen modification have been identified and characterised from these bacteriophages. We are interested in studying fundamental processes related to the O-antigen modification. My group has shown that three integral membrane proteins (glucosyltransferases, Gtrs) are involved in this process. We have recently determined the topological structures of all three proteins encoded by the O-antigen modification gene cluster of phage SfV providing the basis for the identification of putative functional domains and essential residues.
We are now investigating how these proteins function in the bacterial membrane and determine serotype specificity in S. flexneri. This information is being utilised to design a multivalent vaccine strain which would provide protection against shigellosis caused by different serotypes of S. flexneri.