Bioassay-guided isolation of antibiotics from selected marine fungi

The emergence of new life-threatening infectious diseases and the rapid spread of antibiotic-resistant bacteria in nature have raised the awareness of researchers towards screening for novel antibiotics. In response to the marine environment as a potential source of novel secondary metabolites, a study of antibacterial activity in marine fungi isolates from marine biofilm was carried out. Three fungal isolates designated as 4.1.2, P 2.2.1 and P 8.1.2 screened for antibiotic activity using agar overlay technique against one Gram-positive, Staphylococclls aurells and three Gram-negative, Escherichia coli. Salmonella typhi and Enterobacter aerogenes test bacteria. Result showed that only isolate 4.1.2 exerted strong antibacterial activity against Staphylococcus aureus and Enterobacter aerogenes. In contrast, only weak antibacterial activity was obtained against Escherichia coli and Salmonella typhi. Fungal isolates, 4.1.2, P 2.2.1 and P 8.1.2 were putatively identified using slide culture method up to genus level as Bipolaris sp., Endophragmia sp. and Penicillium sp., respectively. Hexane, chloroform, dichloromethane (OCM), ethyl acetate, water and methanol extracts from isolate 4.1.2 (Bipolaris sp.) were subjected to antibacterial screening against the four test bacteria using disc diffusion method. However, none of the extracts showed antibacterial activities. This result suggested that antimicrobial substances produced by Bipolaris sp. could be antimicrobial peptides, and thus failure in obtaining antimicrobial peptide activity could be due to the denaturation of protein-based antimicrobial. The result of thin layer chromatography demonstrated that solvent system OCM-ethyl acetate in ratio of 2: 1 as a better solvent system for fractionation of OCM extract as three spots were visualized under UV light, and with vanillin solution. Further optimization of solvent system could be done for better separation of OCM extract into distinct components. Further studies upon the antifungal activity of Bipolaris sp. can be taken into consideration as its antifungal property was detected against different fungal species