Campylobacter jejuni is the leading cause of bacterial gastroenteritis in developed countries and a major cause of bacterial diarrhea throughout the world. The gastrointestinal tracts of many birds serve as a natural reservoir for the bacterium and humans most often come into contact with the organism by consuming contaminated chicken meats. Despite the prevalence of C. jejuni in human disease, our understanding of pathogenic mechanisms and virulence factors of the bacterium is sparse compared to other enteric pathogens. This lack of knowledge is mainly due to the relative scarcity of available genetic tools to study the organism. We have developed a system of random transposon mutagenesis for C. jejuni and used the derived isogenic mutants to identify genes required for flagellar motility, including carB and carC (encoding proteins with homology to bacterial chemotaxis proteins and the aerotaxis receptor of E. coli, respectively) and rpoN (encoding sigma 54). The aims in this proposal are to understand the roles of the Car proteins and sigma 54 in C. jejuni motility, a phenotype required for efficient colonization of chickens. In addition, we will modify the transposon and perform signature-tagged mutagenesis in C. jejuni to identify genes required for chicken colonization. This last aim will allow us to begin to understand requirements of C. jejuni for colonization of a natural host.