Inflammatory responses induced by the monophasic variant of Salmonella typhimurium in pigs play a role in the high shedder phenotype and fecal microbiota composition

Pigs infected with Salmonella may excrete large amounts of Salmonella, increasing the risk of spread of this pathogen in the food chain. Identifying Salmonella high shedder pigs is therefore required to mitigate this risk. We analyzed immune-associated markers and composition of the gut microbiota in specific-pathogen-free pigs presenting different shedding levels after an oral infection with Salmonella. Immune response was studied through total blood cell counts, production of anti-Salmonella antibodies and cytokines, and gene expression quantification. Total Salmonella shedding for each pig was estimated and hierarchical clustering was used to cluster pigs into high, intermediate, and low shedders. Gut microbiota compositions were assessed using 16S rRNA microbial community profiling. Comparisons were made between control and inoculated pigs, then between high and low shedders pigs. Prior to infection, high shedders had similar immunological profiles compared to low shedders. As soon as 1 day postinoculation (dpi), significant differences in the cytokine production level and on the expression level of several host genes related to a proinflammatory response were observed between high and low shedders. Infection with Salmonella induced an early and profound remodeling of the immune response in all pigs, but the intensity of the response was stronger in high shedders. In contrast, low shedders seroconverted earlier than high shedders. Just after induction of the proinflammatory response (at 2 dpi), some taxa of the fecal microbiota were specific to the shedding phenotypes. This was related to the enrichment of several functional pathways related to anaerobic respiration in high shedders. In conclusion, our data show that the immune response to Salmonella modifies the fecal microbiota and subsequently could be responsible for shedding phenotypes. Influencing the gut microbiota and reducing intestinal inflammation could be a strategy for preventing Salmonella high shedding in livestock.

Methods
We analyzed immune-associated markers and composition of the gut microbiota (faeces, ileum, caecum) in pigs presenting different shedding levels after an oral infection with 109 CFU of Salmonella of 40 specific-pathogen-free (SPF) piglets. Blood and faecal samples were collected for three weeks post-infection. Immune response was studied through total blood cell counts, production of anti-Salmonella antibody and TNF-a, IL-1b, IL-6 and IFN-g cytokines using ELISAs and gene expression. Total Salmonella shedding for each pig was calculated using the area under the log curve (AULC) over three weeks. Pigs were then clustered into high-, intermediate-, and low-shedders after hierarchical clustering. Microbiota compositions were assessed using 16S rRNA microbial community profiling. Comparisons were made between control and inoculated pigs, then between high- and low-shedders pigs.