Commensal Enterobacteriaceae protect against Salmonella colonization by competing for oxygen

Neonatal chicks are highly susceptible to colonization with Salmonella enterica serovar (S.) Enteritidis, but the underlying mechanism is not fully resolved. We show that neonatal colonization with S. Enteritidis required a virulence factor-dependent increase in epithelial oxygenation, which drove expansion of the pathogen in the lumen of the chick intestine by aerobic respiration. Co-infection experiments with an Escherichia coli strain carrying an oxygen-sensitive reporter suggested that S. Enteritidis competes with commensal Enterobacteriaceae for oxygen. Association with either spore-forming bacteria or commensal Enterobacteriaceae from adult chicken microbiota did not protect germ-free mice from S. Enteritidis colonization. However, a combination of Enterobacteriaceae and spore-forming bacteria conferred colonization resistance against S. Enteritidis at levels similar to those observed in germ-free mice associated with adult chicken microbiota. Combining spore-forming bacteria with a single avian E. coli isolate protected germ-free mice from pathogen colonization, but protection was lost when the ability to respire oxygen under microaerophilic conditions was genetically ablated in E. coli. These results show that commensal Enterobacteriaceae contribute to colonization resistance by competing with S. Enteritidis for oxygen, a resource critical for pathogen expansion.