Calamene-Type Sesqui-, Mero-, and Bis-sesquiterpenoids from Cultures of Heimiomyces sp., a Basidiomycete Collected in Africa

Tailored culture strategies to promote antimicrobial secondary metabolite production in Diaporthe caliensis: a metabolomic approach

BACKGROUND

In the search for new antimicrobial secondary metabolites of fungi, optimizing culture conditions remains a critical challenge, as standard laboratory approaches often result in low yields. While non-selective methods, such as modifying culture media, have been effective in expanding the chemical diversity of fungal metabolites, they have not yet established a direct link to key process parameters crucial for further optimization. This study investigates the capacity of Diaporthe caliensis as a biofactory for biologically active secondary metabolites, employing tailored culture media to explore the relationship between chemical diversity and critical process variables.

RESULTS

The metabolomic profiles, antibacterial activities, and production yields of the extracts were analyzed to progressively adjust the culture conditions. This study was conducted in five steps, evaluating carbon and nitrogen source concentration, nitrogen source type, salt supplementation, and pH adjustment. Altering the rice starch concentration affected biomass yield per unit of oxygen consumed, while modifications to the nitrogen source concentration influenced both the bioactivity and chemical space by Diaporthe caliensis. Despite changes at the metabolome level, the extracts consistently exhibited potent antibacterial activities, influenced by the nitrogen source, added salts and pH adjustments. For instance, when using corn steep liquor and rice starch, supplemented with micronutrients, different metabolites were produced depending on whether buffer or water was used, though both conditions showed similar antibacterial activities (IC50 ≈ 0.10 mg mL- 1 against Staphylococcus aureus and ≈ 0.14 mg mL- 1 against Escherichia coli). In the treatment where buffer was used to stabilize pH change, there was an increase in the production of phomol-like compounds which are associated with known antibiotic properties. In contrast, in the treatments using water, the drop in pH stimulated the production of previously unidentified metabolites with potential antimicrobial activity.

CONCLUSIONS

This study proposes a strategic methodology for the tailored formulation of culture media aiming to promote the biosynthesis of diverse secondary metabolites. This approach revealed the critical role of nutrient limitation and pH regulation in stimulating the production of polyketide-lactone derivatives, including the antibiotic phomol. Ultimately, the systematic, custom-designed culture conditions developed in this work offer a promising strategy for expanding the chemical diversity of Diaporthe caliensis, while providing valuable insights into the key parameters needed for optimizing this fungal biofactory.

Diversity of African fungi, chemical constituents and biological activities

Besides plants and animals, the fungal kingdom consists of several species characterized by various forms and applications. Fungi are amazing producers of bioactive natural products with applications in medicine and agriculture. Though this kingdom has been extensively investigated worldwide, it remains relatively underexplored in Africa. To address the knowledge gaps, encourage research interest, and suggest opportunities for the discovery of more bioactive substances from African fungi, we considered it appropriate to extensively review the research work carried out on African fungi since 1988. This review summarizes the diversity and distribution of fungi throughout Africa, the secondary metabolites yet reported from studied fungi, their biological activities and, the countries where they were collected. The studied fungi originated from eleven African countries and were mainly endophytic fungi and higher fungi (macrofungi). Their investigation led to the isolation of five hundred and three (503) compounds with polyketides representing the main class of secondary metabolites. The compounds exhibited varied biological activities with antibacterial and antiproliferative properties being the most prominent.

New bioactive secondary metabolites from fungi: 2023

Fungi have been identified as a prolific source of structurally unique secondary metabolites, many of which display promising biological and pharmacological properties. This review provides an overview of the structures of new natural products derived from fungi and their biological activities along with the research strategies, which focuses on literature published in the representative journals in 2023. In this review, a total of 553 natural products including 219 polyketides, 145 terpenoids, 35 steroids, 106 alkaloids, and 48 peptides are presented. By summarising the latest findings, this review aims to provide a guide and inspire further innovation in the fields of the discovery of fungal natural products and pharmaceutical development.

Depicting the Chemical Diversity of Bioactive Meroterpenoids Produced by the Largest Organism on Earth

In this investigation, we explored the diversity of melleolide-type meroterpenoids produced by Armillaria ostoyae, one of the largest and oldest organisms on Earth, using extracts from liquid and solid fermentation media. The study unveiled three unprecedented dimeric bismelleolides and three novel fatty-acid-substituted congeners, along with 11 new and 21 known derivatives. The structures were elucidated by 1D and 2D NMR spectroscopy and HRESI-MS, and ROESY spectral analysis for relative configurations. Absolute configurations were determined from crystal structures and through ECD spectra comparison. A compound library of melleolide-type meroterpenoids facilitated metabolomics-wide associations, revealing production patterns under different culture conditions. The library enabled assessments of antimicrobial and cytotoxic activities, revealing that the Δ2,4 double bond is not crucial for antifungal activity. Cytotoxicity was linked to the presence of an aldehyde at C1, but lost with hydroxylation at C13. Chemoinformatic analyses demonstrated the intricate interplay of chemical modifications on biological properties. This study marks the first systematic exploration of Armillaria spp. meroterpenoid diversity by MS-based untargeted metabolomics, offering insight into structure-activity relationships through innovative chemoinformatics.

Heimionones A-E, New Sesquiterpenoids Produced by Heimiomyces sp., a Basidiomycete Collected in Africa

With heimionones A-E (1-5), five new terpenoids were isolated from submerged cultures of Heimiomyces sp. in addition to the previously described compounds hispidin, hypholomin B, and heimiomycins A and B. Planar structures of the metabolites were elucidated by 1D and 2D NMR in addition to HRESIMS data. While ROESY data assigned relative configurations, absolute configurations were determined by the synthesis of MTPA esters of 1, 3, and 5. The [6.3.0] undecane core structure of compounds 3-5 is of the asteriscane-type, however, the scaffold of 1 and 2 with their bicyclo [5.3.0] decane core and germinal methyl substitution is, to our knowledge, unprecedented. Together with several new compounds that were previously isolated from solid cultures of this strain, Heimiomyces sp. showed an exceptionally high chemical diversity of its secondary metabolite profile.