Isolation of Bacillus subtilis strain SEM-2 from silkworm excrement and characterisation of its antagonistic effect against Fusarium spp.

Fusarium wilt is a devastating soil-borne disease mainly caused by highly host-specific formae speciales of Fusarium spp. Antagonistic microorganisms play a very important role in Fusarium wilt control. Isolation of potential biocontrol strains has become increasingly important. Bacterial strain SEM-2 was isolated from the high-temperature stage of silkworm excrement composting. SEM-2 exhibited a considerable antagonistic effect against Fusarium graminearum mycelial growth and spore germination. The results of pot experiments suggested that SEM-2 has a better inhibitory effect on the early stage of disease occurrence. The green fluorescent protein labelled SEM-2 coated on the surface of tomato seeds colonised the roots of tomato plants in 15 days. Genome sequencing identified SEM-2 as a new strain of Bacillus subtilis, and genome annotation and analysis determined gene clusters related to the biosynthesis of antimicrobials, such as bacillaene, fengycin, bacillibactin, subtilosin A, surfactin, and bacilysin. Interestingly, liquid chromatography – quadrupole time-of-flight mass spectrometry revealed that metabolites in pathways associated with the synthesis of secondary metabolites and antibiotics were highly differentially expressed. These findings may help to explain the mode of action of B. subtilis SEM-2 against Fusarium spp.