##plugins.themes.huaf_theme.article.main##
Abstract
The curent study was conducted to investigate the antagonism effect towards Streptococcus agalactiae serotype III and S. agalactiae serotype Ib in tilapia (Oreochromis sp.) by herbal extracts, namely perilla leaf (Perilla frutescens), Vietnamese balm (Elsholtzia ciliata), angelica (Angelica dahurica), king of bitter (Andrographis paniculata), white eclipta (Eclipta prostrata), bermuda grass (Cynodon dactylon), wedelia grass (Wedelia chinensis), bitter melon (Momordica charantia). The extracts of these herbs were prepared in distilled water, ethanol 96% or methanol 99.8%, which were subsequently subjected to heat treatment and vacuum evaporation to remove the solvents. The antagonist results of these extract showed that, the extract of perilla leaf, Vietnamese balm, king of bitter and white eclipta in either distilled water exhibited the strongest growth-inhibiting effect towards Streptococcus agalactiae ST283 serotype III and S. agalactiae ST1395 serotype Ib strains. The MBC (minimal bactericidal concentration) and MIC (minimal inhibitory concentration) values of the extract of perilla leaf, Vietnamese balm, king of bitter and white eclipta in distilled water for both serotypes of S. agalactiae strains were from 312 mg/L to 2500 mg/L. The results of the toxicity experiment showed that all these four herbal extracts at the dose of MBC, 2x MBC and 10x MBC were safe for fish. The in vivo investigations on the resistance to S. agalactiae of these four herbal extracts by injection method in tilapia showed the high protection suggesting the potential herbal plants for aquaculture use, in the prevention and treatment of S. agalactiae infection in fish.