In Vitro Antibacterial Activity of Tropical Plant Extracts Against Fish Pathogens in Vietnam

Crude methanol extracts of 43 Vietnamese plants were screened in vitro for their antibacterial activity against three common freshwater fish pathogens, including Aeromonas hydrophila, Edwardsiella ictaluri and Streptococcus agalactiae. The agar disc diffusion method was used to evaluate the antibacterial activity, followed by minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were determined using the broth dilution method. Among the 43 plants screened, eight plant extracts (Bouea oppositifolia, Wedelia chinensis, Terminalia catappa, Punica granatum, Sonneratia caseolaris, Sonneratia ovata, Ludwigia hyssopifolia and Phyllanthus urinaria) exhibited wide-spectrum antibacterial activity to all three common freshwater fish pathogens. Six plant extracts (Ageratum conyzoides, Alpinia galanga, Borassus flabellifer, Abutilon indicum, Eupatorium odotatum, and Scoparia dulcis) might be good candidates for the prevention of co-infection of E. ictaluri and S. agalactiae in tilapia whereas Muntingia calabura and Camellia sinensis could be applied to striped catfish to combat E. ictaluri and A. hydrophila. Through MIC and MBC determination, L. hyssopifolia, A. galanga, Ageratum conyzoides extracts showed a bactericidal activity to A. hydrophila, E. ictuluri and S. agalactiae, respectively, while the other extracts could prevent the growth of tested bacteria. The screening results suggested the potential application of plant extracts as alternative therapeutic agents against bacterial infections in aquaculture.