The immune response modulated by inoculation of commensal bacteria at birth impacts the intestinal microbiota and prevents Salmonella colonization

Super- and low-shedding phenomena have been observed in genetically homogeneous hosts infected by a single bacterial strain. To decipher the mechanisms underlying these phenotypes, we examined the impact of four commensal bacteria (Mix4), inoculated at hatching, on chicken systemic immune response and intestinal microbiota composition and functions, before and after Salmonella infection. These phenotypes were not linked to changes in cell invasion capacity of bacteria during infection. Mix4 inoculation had both short- and long-term effects on immune response and microbiota and promoted the low-shedder phenotype. Kinetic analysis revealed that Mix4 activated immune response from day 4, which modified the microbiota on day 6. This change promotes a more fermentative microbiota, which inhibited Salmonella colonization by day 11 and beyond. In contrast, control animals exhibited a delayed immune response and developed an anaerobically respiring microbiota due to local inflammation, facilitating Salmonella growth. This strategy offers promising opportunities to strengthen the barrier effect against pathogens.