Effect of substrate characteristics on bacterial growth and sporulation of two biocontrol microorganisms during solid state cultivation

Soybean is the leading oilseed crop in the United States, accounting for 90% of the total oilseed production. However, soybean rust disease, caused by the fungus Phakopsora pachyrizi, has become one of the most serious threats to the soybean cultivation, especially in North America. The disease can lead to severe yield loss; however, the prevailing technique to combat soybean rust is using fungicides, which are synthetic agrochemicals that can harm the environment and ecosystem. Biocontrol agents are a group of naturally occurring organisms capable of interrupting the lifespan and suppressing the propagation of disease organisms. The use of biocontrol agents offers an environment-friendly and sustainable solution to the synthetic agrochemicals. In this study, we investigated parboiled rice and millets as substrates for spore production of two model biocontrol microorganisms (Bacillus pumilus and Streptomyces griseus) under solid state cultivation (SSC) conditions. The effects of cultivation parameters such as initial moisture content, water activity, and cultivation time on spore production were studied. Furthermore, texture profile analysis (TPA) was performed to test the stress & strain curve and the hardness and stickiness of the substrate. The greatest spore production occurred at 50% moisture content with millets as a substrate, yielding a count of 1.34×10⁸ spores/g-wet-substrate enumerated with plate count analysis and 1.70×10⁸ events/g-wet-substrate using flow cytometry analysis. Substrate texture profile was shown highly correlative to the moisture content and substrate type and proved to be an essential process variable in controlling the bacterial growth and sporulation during SSC processes.