Procedures to enhance heat resistance of Rhizobium

Means were sought to enhance the heat resistance of a strain of Rhizobium leguminosarum biovar phaseoli that died rapidly in soil at 43 degrees C. Bacteria were more resistant to high temperatures in soil than broth or phosphate buffer. Starving the cells in phosphate buffer enhanced their heat resistance. Heat-resistant variants were obtained that grew at 45 degrees C. Resistance of the parent culture to inactivation at 43 degrees C was enhanced by immobilizing the cells in Ca alginate, amending the soil with clay or immobilization of the cells in clays. The bacteria survived better at 40 degrees C in soil receiving montmorillonite-immobilized cells than in montmorillonite-amended soil. These results suggest that the tolerance of rhizobia to high temperatures in soils may be enhanced.