Invasion of Salmonella into intestinal epithelial cells is believed to be essential for the pathogenesis of Salmonella infections. Invasion is mediated by genes located on the Salmonella pathogenicity Island I (SPI-1), which are needed for assembling a type three secretion system, that mediates injection of bacterial proteins into the cytosol of epithelial cells, resulting in cytoskeletal rearrangements and as a consequence invasion. HilA is the key regulator of the Salmonella Pathogenicity Island I. To assess the role of hilA in colonization of gut and internal organs in poultry, animals were infected with 10(8) CFU of a delta hilA mutant of S. Enteritidis and its parent strain at day of hatch. Very low numbers of delta hilA mutant strain were able to colonize the internal organs shortly after infection, but they were not eliminated from internal organs at 4 weeks post-infection. At that time, the colonization level of the wild type bacteria in internal organs was decreased to the same low level compared with delta hilA mutant strain bacteria. Shedding of the delta hilA mutant strain and colonization of the caeca was seriously decreased relative to the parent strain starting from Day 5 post-infection. At 4 weeks post-infection, the delta hilA mutant strain was more or less eliminated from the chicken gut, while the parent strain was still shed to a high level and colonized the caeca to a high extent (more than 10(7) CFU/g). It is concluded that hilA is involved in long-term shedding and colonization of S. Enteritidis in the chicken caeca