Abstract 

Nineteen serotypes of Shigella flexneri have been recognized, all of which cause bacillary dysentery. Serotype diversity has posed additional challenges to the vaccine development against S. flexneri. Temperate bacteriophages play a crucial role in serotype diversity in S. flexneri by encoding genes to modify the O-antigen (distal part of lipopolysaccharide). Moreover, bacteriophage-encoded factors have also been found to play an important role(s) in host pathogenesis. Sf101 is one of the serotype-converting bacteriophages isolated from serotype 1c strains, encodes O-acetyltransferase B (oacB) gene, which mediates O-antigen modification. Previously, it was found that the oacB gene was present at two different locations on the S. flexneri chromosome which raises questions about the distribution of oacB in serotype 1c strains.

In my project, I explored how oacB gene was acquired in these strains and the mechanism by which OacB modifies O-antigen. Using bioinformatics tools, I investigated the distribution of oacB gene in whole-genome sequences of over 80 clinical isolates of S. flexneri. The mechanism behind oacB mediated O-antigen modification is yet unknown. In my Ph.D., I studied the topological features of OacB and identified critical residues which were important for OacB function. I also investigated the potential role of oacB and three other novel orfs (orf17, orf41, and orf56) encoded by Sf101, in the virulence of S. flexneri using C. elegans as an in vivo model and HeLa cells as an in vitro model. The results showed the presence of these orfs (including oacB) did not affect the virulence phenotype of the host. This study has provided a detailed characterization of oacB and three other novel orfs of serotype converting Sf101 phage of S. flexneri and opened avenues for the upcoming research to understand the serotype conversion in S. flexneri.