Stress, Phages, and EPS: Factors That Enhance Bradyrhizobium Biofilm Formation

Abstract

Biofilms and their exopolysaccharide (EPS) matrix help soil rhizobia ride out stress, colonize roots, and ultimately succeed in symbiosis. We asked whether bacteriophages and common abiotic stressors modulate EPS-linked biofilm traits in Bradyrhizobium S3.9.2GM. Using a validated crystal violet (CV) microtiter assay for biomass and Congo Red (CR) colony phenotyping for matrix-linked traits, we found that phage and sublethal stressors (osmotic, oxidative, temperature shock) generally increased biofilm biomass, with the largest effects under combined phage + stress conditions. One-way ANOVA showed a strong treatment effect on OD570 (biofilm biomass; p < 0.001), while mucoid score and CR halo intensity trended upward in combined treatments but were not significant at α = 0.05. These patterns are consistent with literature showing stress-induced EPS in rhizobia and context-dependent phage–biofilm interactions, and they suggest that simultaneous biotic and abiotic pressures can amplify EPS-associated phenotypes in the rhizosphere (Carezzano et al., 2023; Sutherland, 2001; Hansen et al., 2019).