Bradyrhizobium diazoefficiens cultures display phenotypic heterogeneity

Abstract

Bacteria growing in liquid culture are assumed to be homogenous in phenotype. Characterization of individual cells shows that some clonal cultures contain more than one phenotype. Bacteria appear to employ bet hedging where various phenotypes help the species survive in diverse niches in soil and rhizosphere environments. We asked whether the agriculturally significant bacterium Bradyrhizobium diazoefficiens USDA 110, which fixes nitrogen with soybean plants, displays phenotypic heterogeneity when grown under laboratory conditions. We observed differential binding of sugar-specific lectins in isogenic populations, revealing differential surface properties. We employed Percoll™ density gradient centrifugation to separate clonal populations of exponential and stationary phase B. diazoefficiens into four fractions and characterized their phenotype by proteomics. Specific phenotypes were then characterized in detail. Fractions varied by cell size, polyhydroxyalkanoate content, lectin binding profile, growth rate, cellular adenosine triphosphate, chemotaxis, and respiration activity. Phenotypes were not heritable because the specific buoyant densities of fractions equilibrated within 10 generations. We propose that heterogeneity helps slow growing B. diazoefficiens proliferate and maintain populations in the different environments in soil and the rhizosphere.