SURVIVAL OF BACILLUS-SUBTILIS NB22 AND ITS TRANSFORMANT IN SOIL

Bacillus subtilis NB22 is an antifungal-antibiotic (iturin) producer t hat expresses broad suppressibility of phytopathogenic fungi and bacte ria and is considered as a potential biological pesticide. The surviva l of B. subtilis NB22-1, which is a spontaneous streptomycin-resistant mutant of NB22 and its transformants with plasmid DNA, was investigat ed in soil. The number of vegetative cells of the host strain NB22-1 d eclined initially in soil at 15 degrees C, but the population was stab ilized at the level of spores which were formed during incubation in s oil. Neither nutritional amendment nor the raising of temperature were effective for the higher induction of spore formation in soil. Howeve r, simultaneous treatment of increasing the temperature and nutritiona l amendment was effective for the rapid induction of spores in the soi l, 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 c ells of the transformants NB22-1(pC194) and NB22-1(pUB110) also sporul ated efficiently in soil under these conditions and were stable for 50 days, but died out within 10 days without such treatment. The stabili ty of plasmid pUB110 was high (around 90%), but pC 194 was quickly cur ed and its stability declined to about 50%. The stability of the two p lasmids 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 viabili ty until the end of the experiment at 50 days.