A rapid bioassay for bactericides against the catfish pathogens Edwardsiella ictaluri and Flavobacterium columnare

The most common bacterial diseases in pond-raised channel cat¢sh Ictalurus punctatus (Ra¢nesque) are enteric septicemia of cat¢sh and columnaris, caused by Edwardsiella ictaluri and Flavobacterium columnare respectively. Medicated feed containing antibiotics is one management approach that cat¢sh producers use in the treatment of bacterial diseases. However, the future use of all types of medicated feed in cat¢sh aquaculture is uncertain. To discover e¡ective alternatives to antibiotics, a rapid 96-well microplate bioassay utilizing E. ictaluri and F. columnare to evaluate natural compounds and extracts was developed. In this bioassay, bacterial growth is determined by absorbance measurements of microplate wells after 24 h incubation and then con¢rmed by detecting cell viability after the addition of 3-(4,5-dimethylthiazol2-yl)-2,5-diphenyl tetrazolium bromide with additional incubation for 24 h. The minimum inhibitory concentration , minimum bactericidal concentration and 50% inhibition concentration (IC50) are determined by graphing the absorbance data. The 24 h IC50 results of test compounds are compared with the 24 h IC50 results of the drug controls oxytetracycline and £orfenicol. Among the antibiotics evaluated, doxycycline and tetracycline appear more e¡ective against E. ictaluri and F. columnare than either drug control. This bioassay is rapid, reproducible and economical for evaluating a large number of compounds and extracts.