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Compounds from Terminalli brownii extracts with toxicity against the fish pathogenic bacterium Flavobacterium columnare

Kevin K. Schrader, Charles L. Cantrell, Jacob O. Midiwo, Ilias Muhammad
Natural Product Communications 2016 v.11 no.11 pp. 1679-1682
Flavobacterium columnare, Ictalurus punctatus, Terminalia brownii, antibiotic resistance, antibiotics, bacteria, bark, bioassays, catfish, columnaris disease, ellagic acid, ethanol, fish industry, fractionation, high performance liquid chromatography, medicated feeds, minimum inhibitory concentration, plant extracts, pond culture, toxicity, United States
The pond-raised channel catfish (Ictalurus punctatus) industry in the United States of America can incur losses of over a $100 million annually due to bacterial diseases including columnaris disease caused by Flavobacterium columnare. One management approach available to catfish producers is the use of medicated-feed containing antibiotics. However, the negative attributes of antibiotic use in agriculture include public concerns and the potential development of antibiotic-resistant bacteria. Therefore, the discovery of environmentally-safe natural compounds for use as therapeutants would greatly benefit the catfish industry. In this study, a rapid bioassay was used to evaluate crude plant extracts as the first step in the discovery of natural therapeutants. Plant extracts from Terminalia brownii were found to be inhibitory towards F. columnare. The minimum inhibitory concentration (MIC) of the 5% water-methanol extract of T. brownii (stem bark) was 10 µg/mL and the 24 h 50% inhibition concentration (IC(50)) was 40 µg/mL. Subsequent bioassay-guided fractionation of the T. brownii ethanol extract using reverse phase C-4 chromatography revealed the highest level of activity in the aqueous:methanol (50:50) fraction. HPLC analysis and subsequent purification of this fraction provided two compounds identified as ellagic acid (1) and 4-O-(3'',4''-di-O-galloyl-α-L-rhamnopyranosyl)ellagic acid (2). Compound 2 was the most active isolated compound, with a minimum inhibitory concentration (MIC) of 10±0 µg/mL and 24 h IC(50) of 31±1 µg/mL. Although 1 was more active according to a MIC of 6±5 µg/mL, its 24-h IC(50) was >100 µg/mL, and, therefore, it was less active overall between the two most active isolated compounds.