Role of the PhoP/PhoQ Two-Component Regulatory System in Biofilm Formation in Acid-Adapted Salmonella typhimurium

<i>Salmonella typhimurium</i> is a global foodborne pathogen, and controlling its persistence is critical for public health. This study investigated the regulatory role of the PhoP/PhoQ two-component system (TCS) in biofilm formation under the acid adaptation condition. A <i>phoP</i> deletion strain (Δ<i>phoP</i>) was constructed and compared with the wild type (WT) after acid induction (pH 5.4). Without acid adaptation, Δ<i>phoP</i> and WT showed similar acid tolerance and biofilm formation. However, after acid induction, Δ<i>phoP</i> exhibited markedly reduced biofilm formation, swimming ability, metabolic activity, and extracellular polymer production. RNA-seq analysis further revealed defects in Δ<i>phoP</i> under acid-induced conditions: (i) first leads to downregulation of lipopolysaccharide biosynthesis, peptidoglycan synthesis, and cationic antimicrobial peptide resistance pathways, thereby weakening the bacteria's envelope modification capacity and structural stability; (ii) it also disrupts signal regulations in acidic environments, further impairing energy metabolism, flagellar function, and chemotaxis, thereby affecting bacterial adhesion capacity and environmental adaptability. These results demonstrate that under acid adaptation, the PhoP/PhoQ TCS is critical for coordinating cell envelope remodelling, energy metabolism, and motility to support biofilm formation in <i>S</i>. <i>typhimurium</i>. Understanding the contribution of this system to biofilm formation is essential for addressing the stress resistance and persistence of <i>Salmonella</i> in the food industry.