Isolation and Identification of Flavobacterium columnare and Streptococcus iniae Antibacterial Compounds from the Terrestrial Plant Atraphaxis laetevirens

Columnaris disease, enteric septicemia of catfish, and streptococcosis are common bacterial diseases of certain freshwater fish and are caused by Flavobacterium columnare, Edwardsiella ictaluri, and Streptococcus iniae, respectively. During the process of evaluating several species of plants to isolate and identify compounds with toxicity against these bacteria, a promising extract from the aerial parts of the terrestrial plant Atraphaxis laetevirens (Ledeb.) Jaub. et Spach (Polygonaceae Juss.) was selected for bioassay-guided fractionation using a rapid microplate bioassay. The active dichloromethane extract was subjected to liquid liquid partitioning, and active fractions were further separated by normal-phase column chromatography and normal-phase high-performance liquid chromatography (HPLC). Nepodin (3) and emodin (4) were isolated from two fractions with strong toxicities against S. iniae. A chloroform fraction was further separated by normal-phase column chromatography to yield two active fractions against F. columnare, and these fractions contained chrysophanol (1), physcion (2), and nepodin (3). Compound 1 had strong activity, and compound 3 had moderate activity against F. columnare, while compounds 2 and 4 were not toxic at the concentrations tested.