Colibacillosis is a common disease in poultry and includes both local and systemic infections. Increased knowledge is necessary for development of effective methods to prevent disease, such as improved management routines, modification of environment, and vaccine development. The first study (WP1) has an epidemiological focus with risk analysis by in-depth studies of farms with healthy birds and poultry diagnosed with colibacillosis. Samples will be collected (WP2) for isolation of pathogenic (APEC) and commensal E. coli. Whole genome sequencing (WP3) will enable comparison of strains, identification of specific virulence factors and detection of antibiotic resistance genes. Antimicrobial susceptibility testing will be performed to examine possible links between APEC and resistance (WP4). The results will contribute to new knowledge regarding risk factors for outbreaks, control methods, and the industry's ability to prevent and treat outbreaks of colibacillosis in a sustainable way.