Submitted to: Aquaculture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2006
Publication Date: 6/22/2006
Citation: Schrader, K., Harries, M.D. 2006. A rapid bioassay for bactericides against the catfish pathogens edwardsiella ictaluri and flavobacterium columnare. Aquaculture Research. 37(9):928-937. Interpretive Summary: A rapid bioassay was developed to help discover natural compounds for use in managing the two most common bacterial diseases in channel catfish aquaculture. This bioassay will also help identify promising compounds as alternatives to currently used antibiotics for managing these types of infections in farm-raised catfish.
Technical Abstract: The most common bacterial diseases in pond-raised channel catfish (Ictalurus punctatus) are enteric septicemia of catfish (ESC) and columnaris, which are caused by Edwardsiella ictaluri and Flavobacterium columnare, respectively. Medicated feed containing antibiotics (e.g., oxytetracycline, sulfadimethoxine in combination with ormetoprim) is one management approach that catfish producers use in the treatment of bacterial diseases. Another antibiotic, florfenicol, was approved in 2005 for use in catfish aquaculture to manage ESC. However, concerns about the development of antibiotic resistant strains of E. ictaluri and F. columnare from the use of antibiotics and public concerns about the environmental impact from antibiotic-laden feeds in agriculture make the future use of medicated feed in catfish aquaculture uncertain. We developed a rapid 96-well microplate bioassay utilizing E. ictaluri and F. columnare to evaluate natural compounds and extracts to help discover effective alternatives to the use of antibiotics in catfish aquaculture. In this bioassay, bacterial growth is determined by absorbance measurements (630 nm) of microplate wells after 24 hours incubation at 29 degrees Celsius and then confirmed by detecting cell viability (570 nm) after the addition of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) with additional incubation for 24 hours. Dehydrogenases in metabolically active cells reduce the MTT to form purple formazan crystals. The minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and 50% inhibition concentration (IC50) are determined by graphing the absorbance data. The 24-h IC50 results of test compounds are compared to the 24-h IC50 results of the drug controls oxytetracycline and florfenicol to help identify promising compounds. This bioassay is rapid, reproducible, and economical for evaluating a large number of test compounds and extracts.