A potent feed preservative candidate produced by Calcarisporium sp., an endophyte residing in stargrass (Cynodon dactylon)

Natural Products from Marine Fungi--Still an Underrepresented Resource

Marine fungi represent a huge potential for new natural products and an increased number of new metabolites have become known over the past years, while much of the hidden potential still needs to be uncovered. Representative examples of biodiversity studies of marine fungi and of natural products from a diverse selection of marine fungi from the author's lab are highlighting important aspects of this research. If one considers the huge phylogenetic diversity of marine fungi and their almost ubiquitous distribution, and realizes that most of the published work on secondary metabolites of marine fungi has focused on just a few genera, strictly speaking Penicillium, Aspergillus and maybe also Fusarium and Cladosporium, the diversity of marine fungi is not adequately represented in investigations on their secondary metabolites and the less studied species deserve special attention. In addition to results on recently discovered new secondary metabolites of Penicillium species, the diversity of fungi in selected marine habitats is highlighted and examples of groups of secondary metabolites produced by representatives of a variety of different genera and their bioactivities are presented. Special focus is given to the production of groups of derivatives of metabolites by the fungi and to significant differences in biological activities due to small structural changes.

Production of Calcaride A by Calcarisporium sp. in Shaken Flasks and Stirred Bioreactors

Increased interest in marine resources has led to increased screening of marine fungi for novel bioactive compounds and considerable effort is being invested in discovering these metabolites. For compound discovery, small-scale cultures are adequate, but agitated bioreactors are desirable for larger-scale production. Calcarisporium sp. KF525 has recently been described to produce calcaride A, a cyclic polyester with antibiotic activity, in agitated flasks. Here, we describe improvements in the production of calcaride A in both flasks (13-fold improvement) and stirred bioreactors (200-fold improvement). Production of calcaride A in bioreactors was initially substantially lower than in shaken flasks. The cultivation pH (reduced from 6.8 to <5.4), carbon source (sucrose replacing glucose), C/N ratio and nature of mycelial growth (pellets or filaments) were important in improving calcaride A production. Up to 4.5 mg·g⁻¹ biomass (85 mg·L⁻¹) calcaride A were produced in the bioreactor, which was only slightly less than in shaken flasks (14 mg·g⁻¹, 100 mg·L⁻¹). The results demonstrate that a scalable process for calcaride A production could be developed using an iterative approach with flasks and bioreactors.

Calcarides A-E, antibacterial macrocyclic and linear polyesters from a Calcarisporium strain

Bioactive compounds were detected in crude extracts of the fungus, Calcarisporium sp. KF525, which was isolated from German Wadden Sea water samples. Purification of the metabolites from the extracts yielded the five known polyesters, 15G256α, α-2, β, β-2 and π (1–5), and five new derivatives thereof, named calcarides A–E (6–10). The chemical structures of the isolated compounds were elucidated on the basis of one- and two-dimensional NMR spectroscopy supported by UV and HRESIMS data. The compounds exhibited inhibitory activities against Staphylococcus epidermidis, Xanthomonas campestris and Propionibacterium acnes. As the antibacterial activities were highly specific with regard to compound and test strain, a tight structure-activity relationship is assumed.

Calcaripeptides A-C, cyclodepsipeptides from a Calcarisporium strain

The isolation and structure elucidation of the novel calcaripeptides A (1), B (2), and C (3) and studies on their biosynthetic origin are described. The calcaripeptides were identified from Calcarisporium sp. strain KF525, which was isolated from the German Wadden Sea. Compounds 1-3 are macrocyclic structures composed of a proline and a phenylalanine residue as well as a nonpeptidic substructure. Structure elucidation was achieved by applying one- and two-dimensional NMR spectroscopy supported by high-resolution mass spectrometry. X-ray crystallography was performed to determine the relative configuration of 1. The absolute configuration of 1 was assigned by HPLC of the amino acids after hydrolysis of the molecule and derivatization with chiral agents. Studies on the biosynthesis by feeding ¹³C-labeled substrates revealed that the nonpeptidic part of 1 originates from acetate and l-methionine. The involvement of a hybrid between a polyketide synthase and a nonribosomal peptide synthetase in the biosynthesis of the calcaripeptides is discussed.

Characterization of Graphislactone A as the Antioxidant and Free Radical-Scavenging Substance from the Culture of Cephalosporium sp. IFB-E001, an Endophytic Fungus in Trachelospermum jasminoides

The extracts derived from cultures of 1626 endophytic strains harbored in Trachelospermum jasminoides were assayed for more potent antioxidant and/or free radical-scavenging agents. The free radical-scavenging assessment was carried out using l,l-diphenyl-2-picrylhydrazyl (DPPH) and hydroxyl radical assays, and the antioxidant actions on linoleic acid and human low-density lipoprotein (LDL) models. After extensive spectroscopic analyses, graphislactone A was characterized as the most bioactive secondary metabolite of endophytic Cephalosporium sp. IFB-E001 with its free radical-scavenging (in a dose-dependent manner) and antioxidant activities ascertained in vitro to be stronger than those of butylated hydroxytoluene (BHT) and ascorbic acid, the two positive references coassayed in the study. From the demonstrated efficacy of graphislactone A in preventing and protecting against oxidative injury, it can be predicted that this metabolite could be a potential agent in the management of oxidative damage-initiated diseases.