Investigating interkingdom interactions of Bacillus silvicola against insect pests and agricultural phytopathogens

Bacillus silvicola IBL-03679 T is a crystalliferous bacterial strain that was recently classified as a novel species belonging to the Bacillus cereus group. Information concerning the bacterium and its interactions with other organisms was unknown, until this study. Here, we used genomics and an array of bioassays to determine the antagonistic activity of B. silvicola IBL-03679 against insects and fungi in conjunction with well-studied Bacillus thuringiensis (Bt) subspecies. Biosynthetic gene clusters (BGCs) identified within B. silvicola IBL-03679 and Bt subsp. kurstaki IBL-01447 (syn. HD-1) strains revealed both species contain homologs for the siderophores bacillibactin and petrobactin, and the lasso peptide paeninodin. Conversely, only Bt IBL-01447 contains BGCs for the antibiotic zwittermicin A and bacteriocin thurincin H. Direct plating of B. silvicola IBL-03679 and Bt subsp. kurstaki IBL-01447 against eight different phytopathogenic fungi showed growth inhibition of all fungal isolates examined. However, additional tests to determine non-contact (volatile) activity and filtered broth from the Bacillus spp. had limited or no fungal growth inhibition. Further analysis of strain IBL-03679 found 23 putative insecticidal genes, including predicted Cry gene homologues with undefined functions amongst the crystalliferous Bacillus species. However, insect bioassays using spore and crystal suspensions of B. silvicola IBL-03679 with Lymantria dispar dispar (Lepidoptera; European spongy moth), Helicoverpa zea (Lepidoptera; corn earworm), Leptinotarsa decemlineata (Coleoptera; Colorado potato beetle), and Aedes aegypti (Diptera; yellow fever mosquito) did not yield insecticidal activity under the tested conditions. Finally, apple co-inoculations of the Bacillus spp. and four fungal phytopathogens did not impact disease development under these conditions, nor did co-inoculations of both Bacillus spp. and two fungal phytopathogens using detached coffee leaves. This study demonstrates previously unknown interactions of a new Bacillus sp., B. silvicola , and lays groundwork for expanding the potential target applications for commercialized Bt IBL-01447 as a biocontrol against agricultural pests and pathogens through exploitation of Bacillus antifungal production.