Darryl C. Wilkie. The study about the non-antibiotic approaches to control pathogens in the gastrointestinal tract of the broiler chicken. The purpose of this work was to examine the effectiveness of several replacements for antibiotics in broiler chickens using bacterial challenge models. For this work, pathogen challenge models were developed using three model pathogens; two human pathogens (Salmonella enteritidis and Campylobacter jejuni), and one poultry pathogen (Clostridium perfringens). The first set of experiments involved the selection and use of 2 model probiotics; Bifidobacterium animalis and Lactobacillus fermentum. Oral administration of either probiotic did not significantly reduce (P < 0.05) the level of intestinal colonization by either S. enteritidis or C. jejuni in experimentally infected broiler chickens. The next set of experiments examined the effectiveness of orally administered