Engineering a Hyper-Adherent E. coli Nissle Probiotic Strain that Reduces Intestinal Carriage of SalmonellaPathogens in Poultry
Salmonellosis continues to be one of the most important causes of food-borne illness in the U.S. An additional concern with this bacterial pathogen is that infections with multiple-antibiotic-resistant Salmonella strains are becoming untreatable infectious diseases. Poultry meat and eggs are major sources of Salmonella food-borne illness, due to carriage of these bacterial pathogens in the intestinal microbiome of chickens. A food safety priority, as stated by the USDA, is a significant reduction in carriage of pathogenic Salmonella species in poultry which would significantly improve food safety and reduce cases of human salmonellosis contracted from consumption of contaminated poultry. While this goal has been a priority for many years, basic research and animal management efforts have not achieved significant control of Salmonella carriage. This study represents an alternative approach to reduce or eliminate carriage of Salmonella in poultry flocks. We characterized a type 1 fimbrial allele of Salmonella that confers high levels of adherence to various host cells. We then engineered an E. coli Nissle 1917 probiotic strain that expresses this Salmonella adherence factor at high levels. The E. coli Nissle 1917 is used as the scaffold strain for this work since this E. coli strain has received the FDA designation of Generally Regarded As Safe (GRAS) and has been used for many years as a probiotic to treat human intestinal disorders. Our E. coli Nissle strain was engineered to use an in vivo selection system for a plasmid carrying the cloned Salmonella type 1 fimbrial genes, so that the strain can be used as a probiotic without any antibiotic resistance-encoding genes requiring antibiotic selection for maintenance of the desired phenotype. Our probiotic strain displays high levels of adherence to host cells, in fact higher levels of adherence than a Salmonella strain carrying the same type 1 fimbrial genes. We demonstrate that the probiotic strain significantly outcompetes pathogenic Salmonella strains for adherence to tissue culture cells and in vivo experimental challenges revealed that the probiotic strain mediates a significant exclusion of Salmonella from the intestines of broilers, layers, and turkeys.