Identification of antagonistic bacteria isolated from Thai fermented soybean (Thua Nao) for biocontrol of Lasiodiplodia theobromae

Abstract

In this study, bacterial biocontrol agents against the phytopathogen Lasiodiplodia theobromae were screened from Thua Nao, a Thai fermented soybean product. A total of 170 bacterial strains were isolated and initially screened for their antagonistic activity by dual culture test. Of these, 39 isolates were able to inhibit the fungal growth showing the percentage of inhibition ranging from 25.0–67.5. Among them, the isolate TN79 was selected as a potential antagonistic strain for further study. For this, the bacterial strain TN79 was cultured on nutrient agar for 2 weeks and its crude extracts were prepared using phosphate buffÍer pH 7.0. The bacterial crude extracts prepared were active and could inhibit all four fungal strains of L. theobromae. The optimum pH for antifungal activity of the extracts was 7. In addition, the extracts were also active when exposed to the UV light (254 nm) up to 1 hour and to proteinase K treatment (1 mg/ml). The bacterial strain TN79 was then characterized in terms of their phenotypic and genotypic properties including morphology, biochemical profiles, and rRNA gene sequence. Based on this analysis, the bacterium TN79 was closely related to Bacillus velezensis.