Isolation and characterization of rhizobiophages specific for Bradyrhizobium japonicum USDA 117

Abstract

The isolation and characterization of two phages specific for Bradyrhizobium japonicum USDA 117 are reported. The original phage was isolated from a Chester silt loam soil cropped to soybeans. A mutant of this isolate was obtained during long-term incubation in soil of the original isolate. Both phages were specific for B. japonicum USDA 117. The primary distinction between the two phages was the plaque size produced on the host. The original isolate produced plaques with a maximum size of 2.2 mm2. The mutant phage produced plaques with a maximum size of 11.4 mm2. Both phages exhibited similar morphologies. The head was hexagonal in shape with a diameter of 60 nm. An adsorption rate experiment revealed that the mutant phage was adsorbed faster to the host than the original isolate. Maximum adsorption of the original isolate to the host occurred after 10 min, while the mutant phage required 7 min. Characterization of the original isolate in a one-step growth experiment revealed that the burst size, rise period, and generation time were 100 plaque-forming units/cell, 12 min, and 80 min, respectively. A similar experiment for the mutant phage demonstrated a burst size, rise period, and generation time of 210 plaque-forming units/cell, 6 min, and 70 min, respectively. The ecological competitiveness of the mutant phage appeared to have been altered. In a greenhouse soil incubation experiment, the original isolate reduced nodule number, nodule weight, shoot weight, and acetylene reduction activity to a significantly greater extent than the mutant phage. These results indicate that, while the overall growth rate of the mutant phage was faster, it was less efficient in its parasitism of its host.