Abstract
Three organic extracts (chloroform, ethyl acetate and n-butanol) of ten cyanobacterial species (Anabaena solitaria, Anabaena variabilis, Anabaena cylindrical, Anabaena spiroides, Anabaena circinalis, Oscillatoria ornate, Oscillatoria salins, Oscillatoria tenuis, Oscillatoria rubescens and Oscillatoria prolifica) were investigated for their antibacterial activities against 4 fish pathogenic bacterial species belonging to genus Aeromonas, namely, Aer. salmonicida, Aer. hydrophila, Aer. formicans and Aer. liquefaciens using oxytetracycline as reference antibiotic. Of all cyanobacterial samples, only ethyl acetate extracts of A. variabilis and A. circinalis were proved to be the most effective against all tested Aeromonas species. Bioautographic investigation for 13 organic solvent systems indicate that the spot with R-f = 0.79 for A. variabilis and that of R-f = 0.84 for A. circinalis were the most effective. The median lethal doses (LD50) for the purified antibacterial compounds against mice were 246.67 and 231.67 mg/kg for that extracted from A. variabilis and A. circinalis, respectively.