Antibiofilm activity screening against plant pathogenic bacteria from insect-pathogenic fungi

The aim of this study was to select insect-pathogenic fungi and screen them for their antibacterial and antibiofilm activities against plant pathogenic bacteria that cause an 80% decrease in rice yield. The selected insect pathogenic fungi belong to a group that exhibits complex characteristics that can potentially lead to misidentification. Therefore, a combination of morphological study and phylogenetic analysis was used to accurately identify these fungi. A total of 166 specimens were selected from three genera, including Ascopolyporus, Beauveria, and Metarhizium. They were screened for activities using disc diffusion and broth microdilution techniques. The effectiveness of the extracts was determined by using disc diffusion at 500, 1000 and 20000 micrograms per milliliter. It was found that all the extracts at the concentration of 500 and 1000 micrograms per milliliter did not inhibit the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo). Eight percent of fungal extracts, at the concentration 20000 micrograms per milliliter, showed inhibition against Xoo. The maximum zone of inhibition was obtained from the fungus Metarhzium ellipsoideum (P-MY04008) with diameters of clear zone 17.33±1.15 mm, followed by Beauveria cf. asiatica (Y-NHJ06455BE, 15.33±1.15) and Metarhizium ovoidosporum (P-MY02604BE, 15.00±1.00). Twenty-one crude extracts were then tested for their minimal inhibitory concentrations (MICs), which ranged from 312.5 to 20000 micrograms per milliliter. Most exhibited antibacterial activity, with an MIC value of 20000 micrograms per milliliter. However, extracts from Metarhizium nornnoides (P-MY02088BE) and Ascopolyporus purpuratus (P-MY00619BE, Y-MY12019BE) showed the best antimicrobial activity, with an MIC value of 10,000 µg/ml. About sixtynine percents of the isolates showed antibiofilm activity against Xoo at 1024 micrograms per milliliter. Among them, 7.38% exhibited good biofilm inhibition, 63.62% had less than 50% prevention, and 31.58% showed poor results owing to increased biofilm formation. The highest percentage of inhibition was obtained from the fungal extract of Ascopolyporus purpuratus (Y-MY00619CE) with 64.74% inhibition against Xoo. Based on the morphological and molecular study conducted through multiple gene analyses, Ascopolyporus can be identified as five new species: A. khaoyaiensis, A. purpuratus, A. galloides, A. albus, and A. griseoperitheciatus. Additionally, Beauveria can be identified as a new species, named B. mimosiformis. Furthermore, a teleomorph link with B. asiatica was discovered.