Antibacterial activity of four Gracilaria species of red seaweeds collected from Mandapam Coast, Gulf of Mannar Marine Biosphere Reserve, India

Objective: To study the antibacterial activities of diethyl ether, toluene, ethanol and methanol extracts of red seaweeds such as Gracilaria crassa (G. crassa), Gracilaria folifera (G. folifera), Gracilaria debilis (G. debilis) and Gracilaria corticata. Methods: The crude extracts were tested against different types of Gram-positive and -negative bacterial strains and all the seaweed extracts were tested a broad spectrum of antibacterial activity. Antibacterial activity was made using paper disc diffusion method. Four organic solvents (diethyl ether, toluene, methanol and ethanol) were used separately in a Soxhlet apparatus for seven bacterial strains. Antibacterial activity of the known antibiotics such as chloramphenicol, streptomycin, kanamycin and ampicillin was determined by testing them against different test organisms. Results: The high antibacterial activity was noted in the extracts of G. crassa, G. folifera and G. debilis. However, G. crassa and G. debilis have good antibacterial activity. Pathogens like Bacillus subtilis and Escherichia coli were less susceptible to the methanol and diethyl ether extracts of G. folifera. The comparative study on the antibacterial activity was also made by using 200 μg concentration of solvent extracts (diethyl ether, ethanol, toluene and methanol)and different five antibiotics such as chloramphenicol, streptomycin, kanamycin, amoxicillin and ampicillin. The bacterial strains tested were more sensitive to chloramphenicol, streptomycin, kanamycin, and ampicillin when compared to algal extracts. Conclusions: The present study proved that the extracts of G. crassa, G. folifera and G. debilis have high antibacterial activity. Although G. crassa and G. debilis showed good antibacterial activity, many known antibiotics are active against a few organisms individually. Hence, the extracts of seaweeds were active against all test organisms used and the activities were comparable to that of antibiotics and the seaweeds offer a feasible alternative for the development of new antibiotics. The results also suggest the need for a more dynamic search for pharmaceutically interesting substances from Indian seaweeds