Probiotic Lacticaseibacillus rhamnosus P118 attenuates Salmonella pullorum infection in chicken embryo and chick models: Mechanisms of colonization inhibition and barrier enhancement

Salmonella enterica serovar Gallinarum biovar Pullorum (S. Pullorum) is a host-restricted pathogen that causes high mortality in young poultry, with mortality rates of up to 100 % within the first 2-3 weeks post-hatching. This pathogen poses substantial economic challenges to the global poultry industry through vertical and horizontal transmission, rapid systemic invasion, and organ colonization. This study evaluated the efficacy of the probiotic Lacticaseibacillus rhamnosus P118 in attenuating S. Pullorum virulence via in vitro assays and in vivo chicken embryo and post-hatch chick infection models. In vitro, L. rhamnosus P118 demonstrated antimicrobial activity, inhibiting the growth and biofilm formation of S. Pullorum. In the chick model, prophylactic administration of L. rhamnosus P118 reduced cumulative mortality from 55 % to 35 %, suppressed pathogen colonization in the cecum, liver, and spleen, and decreased fecal shedding. Histopathological assessments confirmed mitigation of tissue damage, accompanied by upregulated expression of tight junction proteins (ZO-1, Occludin, and Claudin-1), thereby reinforcing intestinal barrier integrity. In the embryo model, L. rhamnosus P118 delayed mortality and promoted S. Pullorum clearance from tissues, though live bacteria showed limited efficacy, possibly due to competitive inhibition by the pathogen. These results highlight L. rhamnosus P118's capacity to counteract S. Pullorum colonization and virulence, suggesting its potential for application as a probiotic-based, antibiotic-alternative strategy for controlling Pullorum disease and further enhancing poultry health sustainability.