Abstract：Recently, the use of biological control agents has received great attention because of their ability of suppressing different plant diseases. Some species of the genus Bacillus are the most promising candidates for bacterial biocontrol agents, as the species are well known as safe micro-organisms in the environment, versatility to protect plants effectively against plant pathogens. Bacillus species also have an outstanding ability to sporulation, which assures their prevalence in the environment. Bacillus strains often produce a range of antimicrobial cyclic lipopeptides, including iturins, fengycins and surfactines antibiotics in the family iturin, especially iturin A, is most famous for biocontrol activity. Iturin A is heptapeptide with a β-amino fatty acid that exhibits strong antifungal activity, and shows a strong antibiotic activity with a broad antifungal spectrum. A Bacillus strain FJAT-28592 kept in Microbiological Culture Collection Center of Fujian Academy of Agriculture Sciences was selected to search for naturally occurring biological control agents. The Bacillus strain FJAT-28592 identified as Bacillus siamensis by 16S rRNA analysis had antifungal activity against fungal pathogens Fusarium oxysporum. FJAT-28592 growed at 25~50 ℃, pH 5.0~9.0 and in the presence of 0~6% (W/V) NaCl. The optimal culture conditions was 35 ℃, pH 6.0~8.0 and salt concentration of 2%. Colonies grown on LB agar for 2 d were white, dry and flat. Cell morphology was observed with transmission electron microscopy, B. siamensis FJAT-28592 was motile rods, 0.76~0.94 μm in diameter and 1.76~2.94 μm in length after 2 d culture on LB agar. By blood agar plate and oil spreading test，the strain was confirmed to produce biosurfactant. The crude extract was gained by acid precipitation. Menthol extraction was compared with standard iturin A by high performance liquid chromatography (HPLC). Iturin A synthesis related malonyl-CoA-transacylase gene (ituD) and 4 '- phosphate generic acyl cysteamine transferase gene (lpa-14) were cloned by PCR. The chemical and molecular genetic evidence thus indicated that the biosurfactant from FJAT-28592 was a kind of iturin A. The fermentation crude extract containing iturin A possessed significantly inhibitory activities against F. oxysporum. B. siamensis FJAT-28592 was an ideal potential biological control agent with the aim of reducing the use of chemical pesticides in agriculture. This study provides basic data on the developing biocontrol Bacillus and exploring bacterial biocontrol substances.
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