Understanding Bacillus response to salt stress: Growth inhibition, enhanced EPS secretion, and molecular adaptation mechanisms

Abstract

This study investigates the secretion pattern of extracellular polymeric substances (EPS) by Bacillus sp. under varying salt concentrations and elucidates the molecular mechanisms governing EPS synthesis and secretion. Salt stress inhibited cell proliferation, while optimal salt stimulation promoted EPS secretion, resulting in increased viscosity of the culture medium and the formation of bacterial clusters. Fourier infrared spectrum analysis revealed functional groups such as C-O-C and N-H within the EPS. Soluble-EPS (S-EPS) contained sulfur and phosphorus groups associated with heavy metal ions adsorption. The study also identified a novel polysaccharide formed through bonding EPS (B-EPS). High salt concentrations correlated with elevated levels of tryptophan protein and its derivatives, increased tyrosine polysaccharide derivatives, and decreased aromatic polysaccharides. B-EPS exhibited higher levels of aromatic polysaccharides, with Na⁺ promoting detachment of B-EPS from the cell surface. Transcriptome sequencing (RNA-seq) analysis under salt stress revealed significant expression of spore kinase (KinD) and response regulatory protein Spo0A in the phosphoric acid relay system. Key transcriptional regulatory factors, including OmpR and exopolysaccharide biosynthesis, were closely associated with EPS synthesis and secretion. This study establishes a theoretical foundation for the industrial production and practical application of EPS by elucidating the molecular mechanisms underlying Bacillus' response to salt stress.