RESUMEN
Didymellanosine (1), the first analogue of the decahydrofluorene-class of natural products bearing a 13-membered macrocyclic alkaloid conjugated with adenosine, and a new benzolactone derivative, ascolactone C (4) along with eight known compounds (2, 3, 5-10), were isolated from a solid rice fermentation of the endophytic fungus Didymella sp. IEA-3B.1 derived from the host plant Terminalia catappa. In addition, ascochitamine (11) was obtained when (NH4)2SO4 was added to rice medium and is reported here for the first time as a natural product. Didymellanosine (1) displayed strong activity against the murine lymphoma cell line L5178Y, Burkitt's lymphoma B cells (Ramos) and adult lymphoblastic leukemia T cells (Jurkat J16), with IC50 values of 2.0, 3.3 and 4.4 µM, respectively. When subjected to a NFκB inhibition assay, didymellanosine (1) moderately blocked NFκB activation in the triple-negative breast cancer cell line MDA-MB 231. In an antimicrobial assay, ascomylactam C (3) was the most active compound when tested against a panel of Gram-positive bacteria including drug-resistant strains with MICs of 3.1-6.3 µM, while 1 revealed weaker activity. Interestingly, both compounds were also found active against Gram-negative Acinetobacter baumannii with MICs of 3.1 µM, in the presence of a sublethal concentration (0.1 µM) of colistin.
RESUMEN
The need for new antibiotic compounds is rising and antimicrobial peptides are excellent candidates to fulfill this object. The bacteriocin subgroup lantibiotics, for example, are active in the nanomolar range and target the membranes of mainly Gram-positive bacteria. They bind to lipid II, inhibit cell growth and in some cases form pores within the bacterial membrane, inducing rapid cell death. Pharmaceutical usage of lantibiotics is however hampered by the presence of gene clusters in human pathogenic strains which, when expressed, confer resistance. The human pathogen Streptococcus agalactiae COH1, expresses several lantibiotic resistance proteins resulting in resistance against for example nisin. This study presents a highly potent, pore forming nisin variant as an alternative lantibiotic which bypasses the SaNSR protein. It is shown that this nisin derivate nisinC28P keeps its nanomolar antibacterial activity against L. lactis NZ9000 cells but is not recognized by the nisin resistance protein SaNSR. NisinC28P is cleaved by SaNSR in vitro with a highly decreased efficiency, as shown by an cleavage assay. Furthermore, we show that nisinC28P is still able to form pores in the membranes of L. lactis and is three times more efficient against SaNSR-expressing L. lactis cells than wildtype nisin.
