Antibacterial activity of methanol extract of Gracilaria edulis (Rhodophyseae) in Sri Lanka

Fisheries and Aquaculture

Seaweeds are important living organisms in the marine environment because they are recognized as a potential source of bioactive natural products. The methanol extract of Gracilaria edulis seaweed was prepared to screen its antibacterial activity against four common bacterial pathogens using the disc diffusion method. The tested pathogenic strains were Escherichia coli, an extended spectrum beta lactamases (ESBL) producing coliform, Pseudomonas aeruginosa and Staphylococcus aureus. The Minimum Inhibitory Concentrations (MIC) of the extract against the 4 organisms was determined using the broth macro dilution method. Methanol saturated sterile discs were used as the negative control and did not show any inhibitory zone against the tested 4 organisms. Four antibiotics, such as ceftazidime, cefotaxime, augmentin and ampicillin were used for comparison. The zone of inhibition (ZOI) produced by methanol extract of G.edulis was maximum against S.aureus (16±0.5 mm), medium against E.coli (12±0.8 mm) and ESBL producing coliforms (12±0.5 mm) but with minimum activity against P.aeruginosa (10±0.8 mm). In comparison, the ZOI produced by ceftazidime, cefotaxime and augmentin against E.coli were 26±0.8 mm, 32±0.8 mm and 18±0.8 mm respectively, against the ESBL producing coliform 25±0.5 mm, 32±1.4 mm, and 16±0.5 mm respectively and against S.aureus 9±0.5 mm, 21±1.9 mm and 30±0.5 mm respectively. A ZOI of 16±1.3 mm and 20±1.9 mm was exhibited only by ceftazidime and cefotaxime against P.aeruginosa. Ampicillin was relatively inactive against all test strains, giving a ZOI of 11.7±0.5 mm and 11.5±0.5 mm against E.coli and the ESBL producing coliform respectively. The MIC of the methanol extract was lowest for S.aureus at 1.25 μg/ml, 2.5 μg/ml for E.coli and ESBL producing coliform and 5 μg/ml against P.aeruginosa. The activity of the methanol extract of G.edulis against S.aureus, an ESBL producing coliform and E.coli is promising antibacterial and needs to be studied further.

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