Predominant Culturable Bacillus Species in Japanese Arable Soils and Their Potential as Biocontrol Agents

Abstract

To reveal predominant culturable Bacillus species in Japanese arable soils, aerobic spore-forming bacteria were isolated from different soils in Japan and their phylogenic positions were estimated from the 16S rDNA sequences. Bacillus species were also isolated from tomato roots grown in soils for 4 weeks to estimate the difference in phylogenic positions between soil and root isolates. The number of culturable Bacillus species ranged from 6.4 to 7.4 log cfu g-1 soil and 4.7 to 6.7 log cfu g-1 root in soils and tomato roots, respectively, and the proportion of Bacillus species among total culturable bacteria ranged from 3.8% to 17.9% and 0.1% to 6.7% in soils and roots, respectively. The differences in the number and percentage of culturable Bacillus species between the soils and roots were highly significant (P=0.0000 and 0.0023, respectively), indicating that Bacillus species were more predominant in soil than in root. A total of 108 isolates were characterized using the restriction fragment length polymorphism (RFLP) method and partial 16S rDNA sequences. They affiliated to 22 species including the genus Bacillus, Brevibacillus and Paenibacillus. No nitrogen fixers were isolated. The predominant species was Bacillus megaterium to which 42 isolates (39%) belonged. Some of the species, such as Bacillus luciferensis and Brevibacillus agri, were isolated only from the tomato roots and considered to be more adapted to the rhizosphere than soil. Among 108 isolates, only one strain, N2S6, belonging to Bacillus megaterium, showed consistent antagonistic activity against the bacterial wilt of tomato caused by Ralstonia solanacearum.