Open access publications in the SLU publication database

Abstract

This thesis presents isolation and structure elucidation of twenty-five antifungal antibiotics. The compounds were isolated from four different microorganisms, which had all been found to suppress fungal pathogens in bioassays. Eight cyclic depsipeptides, enniatins, were isolated from the fungus Fusarium sp. F31. Four of them have not previously been isolated from natural sources. They inhibited Botrytis cinerea spore germination at levels down to 100 μg/ml. From the bacterium Pseudomonas sp. MF381-IODS, two linear polyene polyketides were isolated, pseudotrienic acids A and B, together with the known antifungals didehydro-deepoxirhizoxin, and pyrrolnitrin. The pseudotrienic acids inhibited Gram-positive bacteria at levels down to 70 μg/ml. The Pseudomonas sp. Ki19 strain produced four different dialkylresorcinols, of which two were new analogues. All four were active against fungi at levels down to 100 μg/ml, and also against Gram-positive bacteria down to 5 μg/ml. The actinomycete Kutzneria sp. 744 was found to produce a spectrum of nine di- and trichlorinated cyclic depsipeptides, containing several unusual amino acids. The trichlorinated compounds were significantly more active and inhibited fungi down to 60 μg/ml and Gram-positive bacteria down to 5 μg/ml A robust sample work-up method was developed for isolation of antifungal metabolites from both fungi and bacteria. It consisted of bioassay-guided isolation by solid phase extraction (SPE) and HPLC. The bioassay was based on inhibition of fungal spores or bacterial cells and was able to detect compounds with a range of antifungal activities. Such a multi-target functional bioassay was used in order to isolate compounds with several types of antifungal mechanisms. Bacteria were included in the bioassays in order to gain a broader knowledge of the antibiotic spectrum. The structural elucidation of the compounds relied on NMR and MS techniques together with chemical degradation and derivatization methods, and GCMS analysis for stereochemical investigation. The absolute configuration was determined for all previously unpublished compounds.