Isolation and evaluation of antimicrobial activity of endophytic actinobacteria on May Chang tree (Litsea cubeba) against pathogenic bacteria causing diseases on common carp and tilapia

Abstract

Tilapia and common carp are two main cultured species with high production annually in freshwater aquaculture in northern Vietnam, however, there are serious problems caused by bacterial infection. The use of antibiotics is not sufficient to mitigate the outbreaks due to antibiotic resistance rate are increasing. Therefore, to overcome the challenges of antibiotic resistance, antimicrobial compounds with a new mechanistic approach should be urgently sought. This study aimed isolate and evaluate antimicrobial activity of endophytic
actinobacteria from May Chang tree (Litsea cubeba) against three pathogenic bacterial species Aeromonas hydrophila GL14, Aeromonas caviae HD60 and S. agalactiae HY10. The results showed that 9/32 (28.20%) endophytic actinobacteria isolates could inhibit at least one target pathogenic bacteria. Three isolates MTR711, MTR622 and MTL121 showed the highest antibacterial response with minimum inhibitory concentration (MIC)
ranged from 93.30 to 300 µl mL–1, amongst these the lowest value is for MTR711 and MTR622 without significant difference. When combining three individual actinobacteria mentioned above for fractional inhibitory concentration (FIC) test, the synergistic effect was found for the pair of MTR711-MTR622 against three tested pathogenic bacteria
chosen with ∑FIC≤0.50. The combination of two actinobacteria MTR711 and MTR622 improved bacterial inhibitory effect at least 4 times compared to individual treatment. The results are motivating enough to conduct further studies on use of endophytic actinobacteria for treating pathogenic bacteria in aquatic animals