Molecular and functional insights into probiotic interference with Salmonella typhimurium invA virulence gene expression

Background Salmonella enterica serovar Typhimurium ( S. typhimurium ) is a major foodborne pathogen increasingly complicated by antimicrobial resistance (AMR). The invA gene is a conserved virulence determinant essential for host epithelial invasion and widely used as a molecular marker. This study investigated the impact of Lactobacillus acidophilus and Bifidobacterium longum supplementation on S. typhimurium colonization and invA gene expression in a murine model. Methods Eighty- four poultry-derived samples (faeces, eggs, chicken meat) were screened for S. typhimurium isolation using selective enrichment and biochemical asssays. Forty female albino mice were assigned to five groups: negative control (administered phosphate-buffered saline only), positive control ( S. typhimurium only), and three probiotic- treatment groups at low (250 mg/kg), medium (500 mg/kg), and high (750 mg/kg) doses. Probiotics were administered for 14 days; infection occurred on Day 7. Clinical signs were monitored, and liver and kidney tissues were analysed. Quantitative PCR (qPCR) targeting invA , normalized to GapA , evaluated virulence gene expression using the 2 ^−ΔΔCt method. Results Fifteen isolates of S. typhimurium (17.86%) were recovered, with highest prevalence in fecal samples (53.33%). Probiotic-treated mice maintained baseline activity and appearance, in contrast to the infected untreated mice showing lethargy, rough coats and heavy fecal discharge. Probiotic supplementation reduced bacterial load in fecal and intestinal samples, with the highest reduction at 750 mg/kg. qPCR revealed dose-dependent suppression of invA gene expression, with ΔCt ranging from – 8.05 (positive controls) to + 7.95 (highest- dose group). Conclusion Probiotic supplementation attenuates S. typhimurium virulence by suppressing invA expression in vivo, highlighting its potential application as a prophylactic strategy against Salmonellosis . This findings emphasize probiotic-based interventions as a promising adjunct approaches in AMR mitigation.