Expression of the suppressive effect of bacillus subtilis on phytopathogens in inoculated composts

The survivability of Bacillus subtilis NB22-1(a streptomycin-resistant mutant), suppressive to various phytopathogens, and the expression of its suppressiveness, were investigated by inoculating the bacterium into compost products or during the composting process. When cells of the bacterium were mixed with compost products made of various organic wastes at a concentration of 10⁷-10⁸ cells/g wet compost and incubated at 30°C, the cell number of the bacterium was reduced to a concentration in the other of 10⁵, but a suppressive effect in vitro was observed. With a larger inoculation of 10⁹, the cell number remained constant for 35 d incubation, reflecting the larger degree of suppressiveness. When gamma-irradiated compost products were mixed with the bacterium at an initial concentration of 10⁸, no reduction in cell number was observed. When 1.3 × 10⁶ cells/g dry compost were inoculated into raw sewage sludge and a thermophilic composting reaction effected, the concentration of B. subtilis NB22-1 remained almost constant during the thermophilic composting reaction. The compost product thus produced exhibited clear suppressiveness to 6 kinds of phytopathogenic microorganisms. When the initial concentration of the bacterium was raised to 8 × 10⁸-1 × 10⁹ in bark compost, the dominant bacterium during the composting reaction became B. subtilis NB22-1, suggesting that B. subtilis NB22-1 was primarily responsible for the degradation of organic matter in the thermophilic composting.