Molecular characterization and antibacterial activities of mangrove endophytic fungi from coastal Kenya

The advent of resistant pathogenic bacteria and fungi across the globe is threatening the efficacy of antibiotic drugs. Thus, microbial infections are becoming a threat to life. Endophytic fungi remain a viable source of secondary metabolites with unique spectra of biological activities. This study isolated and characterized endophytic fungi from selected mangrove species of coastal Kenya and further ascertained their activities. A total of 18 fungal endophytes selected from mangrove species were investigated for antimicrobial activity against gram-positive Staphylococcus aureus and gram-negative Escherichia coli. Potato dextrose agar and potato dextrose broth were used for isolation, purification, and fermentation at 28oC for 7–15 days. Extraction of fungal metabolites was achieved using ethyl acetate (1:1 v/v) and ethyl acetate in 10% methanol (9:1 v/v). Solvents were recovered in a fume hood and extracts were dissolved in 1 ml of dimethyl sulfoxide. Molecular characterization completely identified 9 species, namely: Aspergillus flavus, Aspergillus niger, Aspergillus tubingensis, Aspergillus oryzae, Rhizophora nomius, Aspergillus awamori, Aspergillus aculeatus, Aspergillus bravionivious, and Aspergillus welwitchiae. The minimum inhibitory concentration of ethyl acetate crude extracts of the most active fungal isolate, A. flavus (MT447532.1), was 0.91 ± 0.05 mg/ml and 0.82 ± 0.052 mg/ml against S. aureus and E. coli, respectively. Results showed that some crude extracts of mangrove fungal endophytes from coastal Kenya are effective against bacteria, hence a promising source of novel organic natural metabolites with a possible wide range of biological activities.