Antibacterial Activities of Lichen-associated Fungi in Mangrove Ecosystems in Sri Lanka as Potent Candidates for Novel Antibiotic Agents

Abstract

Antimicrobial resistance is a global threat to humans, prompting an increasing interest in exploring and developing novel antimicrobial substances derived from diverse sources. Together with the emergence of new diseases the search for novel drug leads has intensified. Less explored microbial habitats have become prime targets in mining for novel antimicrobial molecules. Secondary metabolites synthesized by lichen-associated fungi are good potential targets in this regard. Hence, this study was carried out to explore the antibacterial potential of lichenassociated fungi in mangrove ecosystems by taking National Aquatic Resources Research and Development Agency (NARA) Regional Research Centre, Kalpitiya, Puttalam District, Sri Lanka as the study site. Lichen-associated fungi were isolated from collected lichens by plating out surface sterilized lichen thalli pieces. Antibacterial activities of the isolates were tested using two gram-positive bacteria: Staphylococcus aureus and Bacillus cereus and two gram-negative bacteria: Pseudomonas aeruginosa and Escherichia coli. In this study, 72 putative fungal isolates were primarily screened for their antibacterial activity using agar plug diffusion assay and ethyl acetate crude fungal extracts of nine fungal isolates with marked activity were secondarily screened using the well diffusion assay in triplicate. Isolate LIF 0803 identified as Trichosporon faecale showed the most outstanding antibacterial activities as 2.58 ± 0.29, 3.43 ± 0.05, 4.2 ± 0, 4.5 ± 0.14 cm of zone diameter at 100 mg/mL and 1.95 ± 0.59, 3.08 ± 0.13, 3.7 ± 0.12, 4.3 ± 0.19 cm of zone diameter at 50 mg/mL against P. aeruginosa, S. aureus, B. cereus, and E. coli. All nine fungal isolates showed promising antimicrobial activity against both gram positive and negative bacteria. Therefore, this study showed that lichen-associated fungi in mangrove ecosystems have potent antibacterial activities. Hence, bioassay guided fractionation of active compounds from lichen-associated fungi and structure elucidation are warranted.