Antimicrobial activity of the cyanobacteria Anabaena wisconsinense and Oscillatoria curviceps against pathogens of fish in aquaculture

Cultures of the blue green algae (cyanobacteria) Anabaena wisconsinense and Oscillatoria curviceps were isolated from fish farms and their antimicrobial effects were studied. Solvent extracts of exponential phase algae were screened for antimicrobial activity against different species of Gram positive (Lactobacillus sp. and Bacillus firmus) and Gram negative (Aeromonas hydrophila, Pseudomonas fluorescens and Pseudomonas anguilliseptica) bacteria and the fungi (Aspergillus niger and Saprolegnia parasitica) which were all isolated from diseased fish. Maximum antimicrobial activity was observed with ethanolic extract of O. curviceps against Lactobacillus sp. and Aeromonas hydrophilia, whereas, methanol extracts of A. wisconsinense and O. curviceps had antibacterial effects against B. firmus, A. hydrophila, P. fluorescens, P. anguilliseptica, and the fungi A. niger and S. parasitica. It was also observed that solvent extracts of A. wisconsinense when injected into Oreochromis niloticus which was already injected intraperitoneally with 2 x 10(6) CFU/ml P. anguilliseptica significantly decreased fish mortality from 50 to 12.5%.