抗真菌リポペプチドを分泌する新規土壌細菌
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概要
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Most plant deseases are caused by filamentous fungi. Rice blast caused by infection of an ascomycetes Magnaporthe grisea is a well-known example in Japan. We isolated from a commertial compost named 369(MIROKU)a soil bacterium named HK-c which inhibited the growth of a representative pathogenic fungus Rosellinia necatrix. Nucleotide sequence analysis of 16S rRNA gene and biochemical experiments suggested that HK-c is a novel strain belonging to the genus Bacillus. HK-c inhibited the growth of a wide variety of filamentous fungi tested as well as R. necatrix. HK-c grew at temperatures between 27℃and 55℃. The optimal temperature for growth was 37℃. The antifungal compounds were supposed to be secreted immediately after they were synthesized, because the almost all of antifungal activity was detected in the culture filtrate and the cell fraction showed only slight activity. The antifungal compounds were surprisingly heat stable so that no activity was lost after being autoclaved.The antifungal compounds were soluble in n-buthanol and at least three active fractions were obtained after purification through silica gel column chromatography and Sephadex LH-20 column chromatography. LC-MS (negative mode)analysis suggested that two fractions of them are small cyclic lipopeptides, Bacillopeptin A(C46H72N10O16)and Bacillopeptin B(C47H74N10O16), which belong to iturin group and have been described only in Bacillus subtilis FR-2 isolated from the rhizosphere of garlic. These results show that Bacillopeptin A and Bacillopeptin B are produced by a novel soil bacterium isolated from the commercial compost 369 as well as Bacillus subtilis FR-2. Growth of a filamentous fungus Neurospora crassa was more severely inhibited by the antifungal compounds in the medium containing sorbose than in the medium without sorbose. Sorbose has been described to inhibit cell wall sythesis. The result suggests that the antifungal avtivity of the compounds is involved in cell wall synthesis.