Survival of Bacillus subtilis NB22 and its transformant in soil

Abstract

Bacillus subtilis NB22 is an anti fungal-antibiotic (iturin) producer that expresses broad suppressibility of phytopathogenic fungi and bacteriaand is considered as a potential biological pesticide. The survival of B. subtilis NB22-1, which is a spontaneous streptomycin-resistant mutant of NB22 and its transformants with plasmid DNA, was investigated in soil, The number of vegetative cells of the host strain NB22-1 declined initially in soil at 15°C, but the population was stabilized at the level of spores which were formed during incubation in soil. Neither nutritional amendment nor the raising of temperature were effective for the higher induction of spore formation in soil, However, simultaneous treatment of increasing the temperature and nutritional amendment was effective for the rapid induction of spores in the soil, which led to stabilization of the population at the level of 10 7 spores g −1 dry soil of the initially added cell number. Vegetative cells of the transformants NB22-1 (pC 194) and NB22-1(pUB1 10) also sporulated efficiently in soil under these conditions and were stable for 50 days, but died out within 10 days without such treatment. The stability of plasmid pUB110 was high (around 90%), but pC194 was quickly cured and its stability declined to about 50%. The stability of the two plasmids was similar to that in liquid culture using sporulation medium. When spores of NB22-1 or its transformants were introduced into the soil, they persisted at high level and showed no loss in their viability until the end of the experiment at 50 days.