Atypical Salmonella Infantis variants display different shedding and colonization behaviour in broiler chickens and bear the ability to revert to motility

In recent years, Salmonella Infantis became the most significant serovar in broilers. Additionally, the emergence of atypical phenotypes of S. Infantis is reported, which evade conventional Salmonella detection. This study evaluated the infection dynamics of four atypical S. Infantis variants in broilers. Their phenotypic growth was characterized by aberrant motility and/or production of H2S. All four S. Infantis variants were biofilm producers. The shedding behaviour, colonization in caecum, liver, and spleen, and the humoral immune response were assessed. The study also focused on comparing two enrichment methods in regard to their detection capability. While birds infected with variant 2 were mainly categorized as high shedders, birds infected with variants 1, 3, and 4 proved less shedding. Also, the highest colonization and bacterial load in the caecum, liver, and spleen were found in birds infected with variant 2. Variant 1 did not spread to inner organs and proved to be stable non-motile. In contrast, re-isolates from birds infected with variants 3 and 4, which were initially non-motile, regained motility after host passage, coinciding not only with an increased shedding behaviour but also with colonization of the liver and spleen. In contrast to variants 1 and 2, variants 3 and 4 induced a significant (P < 0.05) rise in antibody levels, which can be seen in connection with the presence of the pESI plasmid. These findings emphasize that S. Infantis variants differ in the infection dynamics in in vivo testing and might undergo genetic or phenotypic alteration, following in vivo passages, to adapt to environmental conditions.RESEARCH HIGHLIGHTS Four atypical S. Infantis variants differ in the infection dynamics in broilers.Non-motile S. Infantis variants regain motility following host passage.