Antibacterial potential of endophytic fungi of the Amazon medicinal plant mulateiro Calycophyllum spruceanum (Benth.) Hook. f. ex K. Schum.

Calycophyllum spruceanum, known as mulateiro, is an Amazonian medicinal plant traditionally used to treat skin infections, scars, and gastrointestinal and uterine conditions. Some of its therapeutic properties are attributed to bioactive compounds produced in association with endophytic fungi. This study aimed to evaluate the antibacterial potential of endophytic fungi isolated from C. spruceanum. Leaves and stems underwent surface sterilization and were cultured on four types of media: PDA, PDA supplemented with plant extract, SDA, and SDA supplemented with plant extract, incubated at 18 °C and 28 °C. The isolated fungi were classified into morphospecies using classical taxonomy. Crude extracts of fungal metabolites were tested for antibacterial activity against Gram-positive and Gram-negative bacteria through the disk diffusion assay. In total, 650 fungal isolates were obtained, 486 filamentous fungi and 164 yeasts, grouped into 248 morphospecies. The most frequently isolated genus was Phomopsis (30.9%), followed by Colletotrichum (16.7%) and Guignardia (5.1%). A higher number of fungi were recovered from leaves (60.3%) compared to stems (39.7%). Among the isolates, 23 demonstrated antibacterial activity against Escherichia coli, and one isolate exhibited inhibitory activity against Klebsiella pneumoniae, both Gram-negative bacteria of clinical relevance. This is the first report describing the endophytic fungal community associated with C. spruceanum, highlighting its potential as a source of novel antibacterial compounds, particularly effective against Gram-negative pathogens. These findings contribute to the bioprospecting of Amazonian biodiversity and open perspectives for the development of natural products with pharmaceutical applications.