Antimicrobial activities of endophytic fungi isolated from Thai medical plants

Abstract

This study aimed to search endophytic fungus isolates for antimicrobial activities against resistant and/or low susceptible microbial strains. Gradient plate technique was used to select for the low susceptible mutants from the standard strains including Staphylococcus aureus ATCC 29213, Escherichia coli ATCC 25922, Candida albicans ATCC 10231 and Saccharomyces cerevisiae ATCC 9763. The mutant that resisted to the antibiotic in concentration of at least 4 MIC was selected. Susceptibility test by disk diffusion method showed that all of low susceptible strains of S. aureus and E. coli were resistant strains except for S. aureus strain with low susceptibility to tetracycline antibiotic. A total of 58 endopytic fungus isolates could be collected from surface-sterilezed samples of 6 Thai medicinal plants, i.e. Canangium odoratum Bali., Hesperethusa crenulata Roem., Lawsonia inermis Linn., Murraya Paniculala (L.) Jack., Piper sarmentosum Roxb. and Sauropus androgynus Merr. By dual-culture agar diffusion assay, most of them (74.14%) exhibited antimicrobial activities against the standard strains. Effect of culture media, used in cultivation of endophytic fungus isolates, on expression of antimicrobial activities was observed. Higher number of active endophytic fungus isolated grown on media containing malt extract expressed their activities against all test organisms except Pseudomonas aeruginosa ATC 27953 and Trichophyton mentagrophytes (clinical isolate). Yeast extract sucrose was found to promote the expression of activities against these two organisms. Actimicrobial activities of these active isolates and active isolates preserved in stock culture were found to be unstable. A total of 29 (29.00%), 4 (23.53%), 4 (7.14%) and 1 (3.70%) isolates of the active fungal endophyte were still active against S. aureus, E. coli, C. albicans and S. cerevisiae respectively. It was found that only 5 isolates (17.24%) exhibited activities against the resistant and/or low susceptible S. aureus. Four isolates were found to be mycelia sterilia and one isolate was identified as Alternaria sp.