PERSISTENCE OF BACILLUS-SUBTILIS RB14 AND ITS DERIVATIVE STRAINS IN SOIL WITH RESPECT TO THE LPA-14 GENE

Bacillus subtilis RB14 produces the antibiotics iturin A and surfactin , and expresses broad suppressibility against several phytopathogenic microorganisms. The gene lpa-14, cloned from RB14, is responsible for the production of both antibiotics. In this study, the contribution of lpa-14 to the persistence of strain RB14 or its derivative strains in soil was investigated. Strain RB14-C is a spontaneous streptomycin-re sistant mutant of RB14. When its survival in soil was examined at 30 d egrees C, immediately after the inoculation of vegetative cells, rapid sporulation occurred until the cell population was stabilized as spor es at a level of over 10(7) cfu/g-dry soil. At the same temperature, b oth R Delta 1, a mutant in which lpa-14 was deleted, and R Delta 1(pC1 15), a transformant in which antibiotic productivity was restored by t ransforming Rbl with a plasmid carrying lpa-14, showed similar soil po pulation dynamics to those of RB14-C. At 15 degrees C, although the ra te of decline in the total cell number and the sporulation rate of RB1 4-C were significantly reduced, the numbers of cells in the soil was f inally stabilized at the same level as at 30 degrees C. However, R Del ta 1 cells were stabilized at a much lower level at 15 degrees C, main ly due to a higher death rate of vegetative cells and a slower sporula tion rate. This was especially significant in non-sterile soil, presum ably due to a reduced ability to compete with indigenous soil microflo ra caused by the loss of iturin A and surfactin production. In the cas e of R Delta 1(pC115), the survival rate of vegetative cells and sporu lation ability at 15 degrees C were not completely restored to the lev els of those of RB14-C. The results showed that lpa-14 was associated not only with the production of lipopeptides but also with survival of vegetative cells and the decline of the sporulation rate at 15 degree s C.