Engineered Lactobacillus paracasei 3×pBD1/ΔHLJ-27 as a novel antibiotic alternative for livestock: Dual efficacy in growth promotion and protection against Salmonella infection via integrated host-microbiota-immune regulation

To address the antimicrobial resistance (AMR) crisis driven by livestock agriculture, effective antibiotic alternatives are urgently needed. However, current probiotic and antimicrobial peptide strategies are critically hindered by limited efficacy, poor stability, and a lack of systemic protection. This study employed recombinant Lactobacillus paracasei 3 × pBD1/ΔHLJ-27, which can secrete and express multicopy porcine β-defensin-1 (pBD1), a cationic antimicrobial peptide with broad-spectrum activity. The study demonstrate that this engineered strain confers systemic protection against Salmonella Typhimurium (S. Typhimurium) infection by coordinately enhancing intestinal barrier integrity and modulating host immune responses. Specifically, 3 × pBD1/ΔHLJ-27 improved intestinal morphology and upregulated tight junction proteins, while promoting balanced immune activation characterized by increased CD4 ⁺ T cells and elevated SIgA and IgG levels. In infection models, the strain significantly reduced mortality, limited bacterial dissemination, and alleviated intestinal pathology. Mechanistically, these protective effects are associated with suppression of excessive inflammatory responses and reprogramming of host-microbiota interactions, as evidenced by reduced pro-inflammatory signaling and enrichment of beneficial microbial taxa. Collectively, these findings identify 3 × pBD1/ΔHLJ-27 as a multifunctional engineered probiotic that integrates antimicrobial activity with immune regulation, offering a promising and scalable alternative to antibiotics in animal production.