Functional mechanism of SptP from Salmonella Typhimurium in modulating environmental adaptability, intracellular proliferation and intestinal inflammation in Wenchang chickens.

Salmonella enterica subsp. enterica serovar Typhimurium (S.Typhimurium) is a critical foodborne zoonotic pathogen that impairs poultry production and threatens public health. The virulence effector SptP encoded by Salmonella Pathogenicity Island 1 (SPI-1) exerts unclear pathogenic functions in indigenous Wenchang chickens, a high-susceptible local broiler breed in China. Here, wild-type (WT) S. Typhimurium and an isogenic ΔSptP knockout mutant were combined with in vitro stress assays, HD11 chicken macrophage infection, in vivo oral challenge of Wenchang chicks, and ileal RNA sequencing (RNA-seq) to systematically dissect the roles of SptP in bacterial environmental fitness, intracellular replication, and intestinal inflammatory injury. In vitro tests revealed that SptP significantly boosted bacterial tolerance to acid, oxidative and hyperosmotic stresses, and accelerated biofilm formation. Within HD11 macrophages, SptP promoted intracellular bacterial proliferation and upregulated pro-inflammatory mediators IL-1β, IL-6 and inducible nitric oxide synthase (INOS). In vivo trials demonstrated that WT-infected chicks exhibited more severe growth retardation, higher diarrhoea incidence, elevated bacterial loads in visceral and intestinal tissues, and aggravated macroscopic lesions compared with the ΔSptP group. SptP markedly suppressed the transcription of intestinal tight junction genes Occludin, Claudin-1 and ZO-1. A total of 126 differentially expressed genes (DEGs) were identified from ileal tissues, which were mainly enriched in lipid metabolism, immune response and the peroxisome proliferator-activated receptor (PPAR) signalling pathway. Collectively, SptP acts as a core virulence determinant that facilitates intestinal colonization, triggers inflammatory cascades and disrupts epithelial barrier integrity. Deletion of sptP drastically attenuates bacterial virulence. This study provides theoretical support for developing live attenuated vaccine candidates and antibiotic-free green control strategies against avian salmonellosis.