Chiang Mai Journal of Science

     Bacterial leaf blight disease of rice caused by Xanthomonas oryzae pv. oryzae (Xoo) impairs grain yield and is difficult to control. Therefore, we investigated the antibacterial activity of 100 bacterial isolates, obtained from plant tissues and agricultural soils, against Xoo strain E. Bacteria were grown in modified Wickerhams Antibiotic Test Medium (WATM), cell-free supernatants were collected, and their activity was evaluated using the agar well diffusion assay. The most inhibitory strain was TBRC 16324, isolated from Fragaria × ananassa roots, and its antimicrobial activity was further investigated. Ethyl acetate crude extracts were subjected to Sephadex LH-20 column chromatography followed by TCL-bioautographic assays. Liquid Chromatography-Electrospray Ionization-Mass Spectrometry (LC-ESI-MS) analysis of combined fraction 1 (with biological activity) showed that the pseudomolecular ion (m/z 1036.6919) closely matched the pseudomolecular ion of surfactin (m/z 1036.6928 [M + H]⁺). As the ethyl acetate crude extract was non-toxic to mammals using the L-929 mouse fibroblast assay, an in vivo experiment was undertaken. TBRC 16324 live cells and crude extract slightly reduced Xoo disease severity in Phitsanulok 2 rice seedlings in a screen-house. These findings suggest that TBRC 16324 may be a suitable candidate for further evaluation in field trials.