Phytochemical composition and antibacterial activity of brown seaweed, padina australis against human pathogenic bacteria

Abstract

Seaweeds have become targets for chemical screening in search of new compounds that have potential medical value, including antibacterial activity. The aim of this study was to evaluate the chemical composition and antibacterial activity of the extract and fractions of Padina australis. The chemical compounds in seaweed samples were extracted with methanol, then sequentially partitioned with hexane, dichloromethane, ethyl acetate (EA) and n-butanol. The EA fraction was further separated using silica gel column chromatography to yield eight subfractions (EA-1 to EA-8). The antibacterial activity was tested against seven pathogenic bacteria through the disc diffusion method and its minimum inhibitory concentration was tested using the standard broth dilution method. The highest activity against gram-positive bacteria was observed for the EA-1 subfraction with inhibition zones of 9.37±0.09 mm, 12.25±0.60 mm and 10.30±0.05 mm for Bacillus subtilis ATCC6633, Staphylococcus aureus ATCC25923 and methicillin-resistant S. aureus KL046, respectively. None of the extracts showed activity against gram-negative bacteria. Moreover, the antibacterial activity was positively correlated with the total phenolic content in the extracts against B. subtilis (r = 0.656), S. aureus (r = 0.800) and methicillin-resistant S. aureus (r = 0.880). The highest fucoxanthin content was also observed in the EA-1 subfraction. The active compounds of the EA-1 subfraction were identified using gas chromatography coupled with mass spectroscopy, and the major components were fucosterol, (3β, 24Z)-Stigmasta-5, 24(28)-dien-3-ol and phloroglucinol. Based on these results, the bioactive compounds responsible for the antibacterial activity might be the phenolic compounds fucoxanthin, fucosterol and fucosterol derivative.