Isolation and Antibacterial Characterization of Endophytic Fungi from Rotheca myricoides (Hochst.) Steane & Mabb. and Their Secondary Metabolites

Endophytic fungi were isolated from healthy tissues of Rotheca myricoides. Antibacterial activity was screened using dual-culture and disc-diffusion assays against Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa and Escherichia coli. Active isolates were identified through morphological characterization and molecular sequence analysis. Six antibacterial-active isolates were selected from 62 culturable endophytic fungi and were assigned to genus, species complex, or species based on ITS or beta-tubulin sequences: Fusarium incarnatum agg., Fusarium solani agg., Fusarium equiseti agg., Alternaria arborescens agg., Purpureocillium sp., and Colletotrichum acutatum. C. acutatum produced the largest inhibition zones in dual culture, followed by A. arborescens agg. Bioassay-guided purification yielded five metabolites from C. acutatum, namely N-acetyltyramine (1), tyrosol (2), N-(2-phenylethyl)acetamide (3), N-acetyltryptamine (4) and indole-3-acetic acid (5). One compound, alternariol monomethyl ether (AME) (6) was obtained from A. arborescens agg. AME (6) exhibited the strongest antibacterial activity, with minimum inhibitory concentration (MIC) values of 31.25 µg/mL against S. aureus (ATCC 29203) and 62.5 µg/mL against K. pneumoniae (DCM 132), P. aeruginosa (ATCC 10145), and E. coli (ATCC 25922). Although the isolated compounds do not represent novel antibiotic classes and generally exhibited only moderate antibacterial activity, they expand current knowledge of the chemical diversity of Kenyan fungal endophytes and provide a basis for future studies aimed at elucidating their mechanisms of action, structure–activity relationships, and pharmacological relevance.