ANTIBACTERIAL ACTIVITY OF SOME SELECTED MARINE MACROALGAE AGAINST FISH PATHOGENIC BACTERIA

In the marine eco system, algae (seaweeds) are directly exposed and are susceptible to ambient microorganisms such as bacteria, fungi and viruses. There is an increasing need for new antibiotics and other bioactive compounds that control the spread of disease not only in human but also in other animals evolving resistance for microbes to existing antibiotics. They are known to synthesize bioactive secondary metabolites which have antimicrobial activities. The aim of the present study was to identify seaweeds, which possess antibacterial activities against fish pathogenic bacteria and could be an alternative to the commonly used antibiotics in aquaculture.

Crude extracts were prepared using the solvent of methanol from the six marine macro-algae viz, Ulva lactuca and Chaetomorpha aerea (Chlorophyta), Sargassum tenerrimum (Phaeophyta), Kappaphycus alvarezii, Gracilaria edulis and Gelidiella acerosa (Rhodophyta) species using methanolic extracts were evaluated for antibacterial activity by Well diffusion assay and Broth dilution assay against both Gram Positive bacteria (Staphylococcus aureus and B. subtilis) Gram Negative bacteria (Klebsiella pneumonia and E. coli) by using three different concentrations viz., 25, 50, 75 µg/mL. The standard drug as commercial antibiotic Ampicillin. The maximum inhibition of antibacterial activity was observed in Gelidiella acerosa (Plate: 1) followed by Chaetomorpha aerea, Ulva lactuca, Gracilaria edulis while Sargassum tenerrimum and Kappaphycus alvarezii showed comparatively low inhibition recorded of antibacterial activity. G. acerosa (75µg/mL) showed a maximum activity against B. subtilis (21 mm) and S. aurues (20 mm). Minimum inhibitory concentrations (MIC) for the selected six seaweeds were carried out the better performing solvent extracts in the broth dilution method using methanolic extract. MIC antibacterial activity against in B. subtilis (from well diffusion assay) was ranging from 25µg/mL to 100µg/mL. The commercial antibiotic of Ampicillin with
B. subtilis showed a maximum activity against 75µg/mL concentration (77.05%) and minimum activity against 25µg/mL (67.65%). Observation made
G. acerosa with B. subtilis showed a maximum activity against 50µg/mL concentration (80.56%) and minimum activity against 25µg/mL (75.26%) of the tested selected seaweed Gelidiella acerosa.