Selection of Herb Extracts to Inhibit Pathogenic Bacterial Infection in Nile Tilapia (Oreochromis niloticus)

The objective of this study was to investigate the effect of herbal extracts with inhibitory activity against two pathogenic bacteria in Nile tilapia; Streptococcus agalactiae and Aeromonas hydrophila. The inhibitory activity against pathogenic bacteria was determined by disc diffusion and broth dilution method. The 29 herb (32 herb extracts) species including kaempfer, allium, holy basil, galanga, ginger, white curcuma, turmeric, celery, wildbetal leafbush, climbing wattle, Lao chili wood, lemon grass, green shallot, asiatic pennywort, coriander, culantro, dill, Vietnamese coriander, pepper, kariyat, kaffir lime, leech lime, mullilam, bitter gourd, horse radish tree, tree basil, kitchen mint, Siamese neem tree, shallot, and brown onion were extracted with 95% ethanol (ratio of herbs to solvent equal to 1:6 (w/v)). Galanga showed the highest inhibition of two pathogenic bacteria (A. hydrophila and S. agalactiae) with inhibition zone of 2.80±0.17 and 2.36±0.05 cm., respectively. In this study, the minimum inhibitory concentration (MIC) could not be observed due to the color of the herbs being very dark, obscuring the turbidity of the pathogenic bacteria growth in the test tubes. The result of minimum bactericidal concentration (MBC) against S. agalactiae showed that the extracts of tumeric yielded the best result at 9.7 mg/ml. As for A. hydrophilia, it was found that kaempfer extracts yielded the best MBC value at 156.2 mg/ml. The results of antibacterial testing suggested that turmeric, kaempfer, and white curcuma extracts could be applied in the treatment of bacterial infection in tilapia.