Antimicrobial and Antibiofilm Activity of an Endophytic Fungus, Daldinia sp. Isolated From Avicennia officinalis

Mangrove ecosystems are dynamic coastal habitats that support distinctive microbial communities capable of producing structurally diverse and biologically potent secondary metabolites. Among these, endophytic fungi residing asymptomatically within mangrove plants represent an underexplored resource with substantial potential for the discovery of novel antimicrobial agents. In this study, an endophytic fungus (Daldinia sp.) was isolated from the leaves of Avicennia officinalis collected from the mangrove forests of Goa. The ethyl acetate extract of the identified endophyte demonstrated significant antibacterial activity, with zones of inhibition ranging from 13.67 ± 0.57 to 17.33 ± 0.57 mm and minimum inhibitory concentrations ranging from 312.5 to 625 μg/mL against Escherichia coli and Shigella sp., following Clinical and Laboratory Standards Institute guidelines. Flow cytometry analysis of the extract showed 84.04% ± 5.89% and 62.15% ± 1.77% of cells stained with propidium iodide (PI) in E. coli and Shigella sp., respectively, indicating substantial membrane damage. The extract induced the generation of reactive oxygen species in E. coli and Shigella sp., leading to an increase in 2',7'-dichlorofluorescein (DCF) fluorescence intensity. Furthermore, the extract exhibited antibiofilm activity, inhibiting biofilm formation by 93.03% ± 1.67% in E. coli and 89.38% ± 0.91% in Shigella sp. Scanning electron microscopy also supports the antibiofilm and antibacterial potential of the extract, as evidenced by reduced biofilm density and visible physical damage to bacterial cells, respectively. The metabolite profiling indicates the presence of various metabolites with antimicrobial properties. Collectively, the findings underscore the potential of an isolated endophytic fungus as a valuable source of potent antibacterial and antibiofilm compounds.