Shigellosis have continued to be a major cause of childhood death under the age of five, mostly in the developing countries. Among four species of Shigella, S. flexneri is the primary cause of Shigellosis in the developing countries. S. flexneri serotype 1c is a novel serotype first isolated in Bangladesh in 1980s and since then it has been isolated and identified in different parts of the world. S. flexneri serotype 1c possess highly complex O-antigen structure modified by three different bacteriophages. Several bacteriophages encoded virulence factors have been identified that play roles in different stages of the bacterial pathogenesis including toxin production, host epithelial cell invasion, adhesion, intracellular survival and antigenicity. In this study, we plan to investigate the role of O-antigen modifying genes-gtrI and gtrIC genes in Shigella virulence. We knocked out these two genes using lambda red mediated recombinase approach. The mutants and the wild type strain were used to examine the role of these genes in virulence using in vitro assays such as plaque and invasion assays using HeLa cells and in vivo assays such as accumulation assays and killing assays using Caenorhabditis elegans. The virulence assays for these two mutants are under progress and the results will be presented in BacPath 15 conference. The clear understanding of these bacteriophage genes in the survival of S. flexneri in the human host can pave the way to the identification of potential attenuation targets and vaccine candidate antigens.