Antibacterial potential of epiphytic fungi obtained from brown algae of Kenyan coastal waters

Abstract

The rapid rise of antimicrobial resistance, coupled with the limited development of new therapeutic agents, is a critical public health concern that needs urgent and innovative interventions. Marine fungi represent an important but still largely unexplored source of novel and diverse secondary metabolites with potential to combat multidrug-resistant pathogens. The objective of the present study was to investigate the antibacterial properties of fungal epiphytes associated with coastal brown algae against multidrug-resistant pathogens. Epiphytic fungi were isolated from brown algae of the Kenyan coast and identified using morphological and ITS-based molecular approaches. Phylogenetic analysis confirmed the taxonomic placement of the isolates in genera well known for bioactive metabolite production, in agreement with morphological and cultural features. Following the primary screening, nine potent and broadly active isolates were characterized and evaluated for antibacterial activity against multidrug-resistant ESKAPE microorganisms (Enterobacter cloacae, Acinetobacter baumannii, Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, and Pseudomonas aeruginosa). The best-performing extracts were subjected to SEM observation, followed by chemical profiling using GC-MS analysis. Both the methanolic and ethyl acetate extracts inhibited bacterial growth, with zones of inhibition ranging from 14 to 30.40 mm. MIC/MBC assays presented potent activity, with values as low as 0.039 mg/mL and 0.156 mg/mL, particularly in extracts from Alternaria spp. (Dib-4), Curvularia spp. (Dib-3), and Penicillium spp. (Sac-12). In contrast, some isolates displayed only moderate or selective inhibitory activity. Scanning electron microscopy (SEM) analysis demonstrated morphological alterations in bacterial cells, including membrane blebbing, collapse, and lysis, supporting a membrane-targeting mechanism of action. GC-MS analysis revealed the availability of various bioactive metabolites. These findings indicate that brown seaweed-surface associated fungi from the Kenyan coast represent an untapped reservoir of bioactive constituents with bactericidal activity against multidrug-resistant pathogens. The solvent-dependent recovery of such compounds and the strong activity of selected isolates highlight their promise as candidates for the discovery of novel antimicrobial agents in the future.