Optimizing the batch fermentation process (agitation and aeration) of the biocontrol agent, <i>Bacillus velezensis</i> strain KSAM1, and its influence on soilborne fungus, <i>Macrophomina phaseolina</i>

Macrophomina phaseolina is a soilborne fungus responsible for developing root-rot and charcoal-rot diseases in various plants. Seventeen Bacillus strains were isolated and assessed as potential biocontrol agents to determine their capability to suppress M. phaseolina growth. The KSAM1 isolate demonstrated the highest efficacy in suppressing fungal mycelial growth, achieving an inhibition rate of 38.6%. The 16S rRNA gene sequencing, BLAST analysis, and phylogenetic tree construction demonstrate that KSAM1 is Bacillus velezensis . It is registered in GenBank as B. velezensis strain KSAM1 (Acc# PQ288980). Subsequently, two batch fermentation processes were implemented in an agitated tank bioreactor to optimize agitation and aeration to achieve the highest possible level of culture biomass and secondary metabolite production. The maximum achievable level of the culture biomass was 3.92 g L -1 , which was achieved at 10.5 hours through the utilization of batch fermentation No. 2. This process involved stirring at rates between 200 and 600 rpm, along with an aeration rate of 1 VVM. This achievement was realized while upholding a steady specific growth rate (µ) of 0.08 h -1 . The observations indicated that the biomass yield coefficient was established at 0.7 g cells/g glucose. The analysis of bacterial filtrate extract using gas chromatography-mass spectrometry indicated that diisooctyl phthalate was the most bioactive secondary metabolite compound in the chromatogram, accounting for 36.07 percent of the total area. Overall, B. velezensis strain KSAM1 may serve as a biocontrol agent for M. phaseolina , as indicated by the results of the present investigation.