Bacterial culture history affects the attachment of Bradyrhizobium japonicum to host Glycine max roots

Abstract

To understand the physiological control of attachment of Bradyrhizobium japonicum, bacterial growth and preincubation conditions were varied before exposing the bacteria (104 cfu mL−1) to excised soybean root segments. The attachment rate of strain USDA 110 was constant through a 7-h exposure (0.9% of the population attached per hour). Maximal attachment of strain USDA 110 occurred during log phase and for strains USDA 76 and USDA 31, during early stationary phase. The ability of log-phase cultures to attach to roots was significantly correlated with their competitive ability to nodulate soybean. Cells of strains USDA 110 attached to roots were isolated and reexposed to roots 15 times over a 6-month period. No significant increase in attachment ability of these serially exposed bacteria was observed (a mean of 1.4% of the population attached at each 2-h exposure). Attachment was stimulated 2.4- to 7-fold when cells of strains USDA 110 or USDA 31 were preincubated in a minus-nitrogen plant nutrient solution and on new cell growth. The stimulation of attachment could not be mimicked by preconditioning cells in solutions of NaCl, CaCl2, or CaSO4 of the same normality as the nutrient solution. The ability of B. japonicum strains to attach to host roots is a dynamic property affected by growth medium, culture age, and cell pretreatment conditions. The progeny of attached cells adhered to roots in the same low proportions as cells not previously exposed to roots. Key words: rhizobia, adhesion, competition, bacterial pretreatment.