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

Abstract

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 medicatedfeed 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 therapeutic agents 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 (IC50) 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 IC50 of 31±1 μg/mL. Although 1 was more active according to a MIC of 6±5 μg/mL, its 24 h IC50 was >100 μg/mL, and, therefore, it was less active overall of the two most active isolated compounds.