Biocontrol of Soft Rot Dickeya and Pectobacterium Pathogens by Broad-Spectrum Antagonistic Bacteria within Paenibacillus polymyxa Complex

Polymyxin-producing bacteria within the complex have broad-spectrum activities against fungi and bacteria. Their antibacterial activities against soft rot and phytopathogens containing multiple polymyxin-resistant genes were not clear. Here, we selected nine strains within the complex having broad-spectrum antagonistic activities against phytopathogenic fungi and a polymyxin-resistant strain causing stem and root rot disease of sweet potato and did antagonistic assays on nutrient agar and sweet potato tuber slices. These strains within the complex showed clear antagonistic activities against in vitro and in vivo. The most effective antagonistic strain ShX301 showed broad-spectrum antagonistic activities against all the test and strains, completely eliminated from sweet potato seed tubers, and promoted the growth of sweet potato seedlings. Cell-free culture filtrate of ShX301 inhibited growth, swimming motility, and biofilm formation and disrupted plasma membranes, releasing nucleic acids and proteins. Multiple lipopeptides produced by ShX301 may play a major role in the bactericidal and bacteriostatic actions. This study clarifies that the antimicrobial spectrum of polymyxin-producing bacteria within the complex includes the polymyxin-resistant and phytopathogens and strengthens the fact that bacteria within the complex have high probability of being effective biocontrol agents and plant growth promoters.