Complex biochemical synergistic interactions between two rhizobacteria grown in consortium, Bacillus subtilis SL-44 and Enterobacter hormaechei Wu-15

Plant growth-promoting rhizobacteria (PGPR) are widely used in consortia to improve crop yield and coping with stress. However, the interaction mechanism between two PGPRs, i.e., Bacillus subtilis and Enterobacter hormaechei, remains unclear. Two strains Gram-positive SL-44 and Gram-negative Wu-15 with good plant growth-promoting properties previous isolated from cotton by our research group were chosen to research their synergistic interactions. The plate confrontation and liquid medium mixed culture results showed that Wu-15 had a remarkable advantage in the early growth stage, whereas SL-44 could grow better in the later period. The extracellular polymeric substances (EPS) produced by SL-44 contained more carbohydrate, fulvic acid, and humic acid than those produced by Wu-15. While the EPS produced by Wu-15 contained more tyrosine, tryptophan, and soluble microbial metabolites than that of SL-44. The mixed culture results showed balanced EPS composition. In addition, the production of indole acetic acid (IAA) and siderophore of the mixed bacteria was higher than that of either SL-44 or Wu-15 alone. In electrochemical detection, Wu-15 had higher reduction ability to ferric ions than SL-44. The production of siderophores was further increased, and the reduction ability of ferric ion was also significantly enhanced in ΔWu-15Hsp. EPS played significant roles in the interaction and growth promoting metabolism produce process.