Light Signaling Regulates Aspergillus niger Biofilm Formation by Affecting Melanin and Extracellular Polysaccharide Biosynthesis

Reduction of oxidative rancidification of fungal melanin-coated films in pork lard preservation in trading

Storage of meat has always been challenging due to its deterioration caused by oxidative rancidity and microbial activity, especially in trading. The melanin-coated film acts as a potent antioxidant, prevents the oxidation of fatty acids, and neutralizes the reactive oxygen species (ROS) helping to withstand or perpetuate the oxidative stress of meat. This study emphasizes the production of fungal melanin extracted from Curvularia lunata and the preparation of two different melanin film combinations of gelatin/melanin and agar/melanin at 0.1% and 0.5% formulation for rancidity stability of coated pork lard. Interpretations revealed the delayed rancidity in both peroxide and acid values with 5.76% in 0.5% agar-coated melanin up to the 11th day which was supported by arithmetical analysis showing p < 0.05 are statistically significant. Further, upon testing the brine shrimp assay for melanin toxicity, 7% were in a mortal state at 1000 µg/mL concentration, considered zero lethality. This result implies that modified coatings, particularly when trading meats, that include fungal melanin can effectively prevent the oxidation of pork lard.

Host immune response against fungal biofilms

Fungal biofilms are a multilayered community of cells attached to mucosal or abiotic surfaces enclosed in a coating of self-produced extracellular polymeric matrix. The sheer density of cells protected by a polymeric shield not only makes the biofilm impermeable to antimicrobials or immune cells but also hidden from host recognition. Biofilms also serve as a reservoir of drug-resistant persister cells and dispersal cells armored with virulence factors adept at evading the immune system. Here, we summarize the latest knowledge on the immunomodulatory properties of biofilms formed by Candida species and by other biofilm-forming fungal pathogens such as Aspergillus and Cryptococcus. Finally, we deliberate on promising strategies to help activate the immune system for combating fungal biofilms.

Synthesis and structural characteristics analysis of melanin pigments induced by blue light in Morchella sextelata

Morchella sextelata, a highly sought-after edible mushroom worldwide, is evaluated based on its cap color as an essential commercial property indicator. In the present study, the effects of blue light on cap pigmentation in M. sextelata, as well as the synthesis and structural characteristics of melanin pigments within the cap were examined. The results showed that an increase in the proportion of blue light within the lighting environment promoted melanin synthesis and melanization of the cap. Transmission and scanning electron microscopy revealed the localization of melanin within the mycelium and its ultrastructural characteristics. The UV-visible analysis demonstrated that melanin exhibited a maximum absorption peak at 220 nm and possessed high alkaline solubility as well as acid precipitability. The structural characteristics of melanin were analyzed using FTIR, NMR, HPLC, and elemental analysis, which confirmed the presence of eumelanin, pheomelanin, and allomelanin in both brown and black caps. Furthermore, blue light can stimulate the synthesis of both eumelanin and pheomelanin. The obtained results can serve as the foundation for comprehending the mechanism by which light regulates color formation in mushrooms.

Fungi's Swiss Army Knife: Pleiotropic Effect of Melanin in Fungal Pathogenesis during Cattle Mycosis

Fungal threats to public health, food security, and biodiversity have escalated, with a significant rise in mycosis cases globally. Around 300 million people suffer from severe fungal diseases annually, while one-third of food crops are decimated by fungi. Vertebrate, including livestock, are also affected. Our limited understanding of fungal virulence mechanisms hampers our ability to prevent and treat cattle mycoses. Here we aim to bridge knowledge gaps in fungal virulence factors and the role of melanin in evading bovine immune responses. We investigate mycosis in bovines employing a PRISMA-based methodology, bioinformatics, and data mining techniques. Our analysis identified 107 fungal species causing mycoses, primarily within the Ascomycota division. Candida, Aspergillus, Malassezia, and Trichophyton were the most prevalent genera. Of these pathogens, 25% produce melanin. Further research is required to explore the involvement of melanin and develop intervention strategies. While the literature on melanin-mediated fungal evasion mechanisms in cattle is lacking, we successfully evaluated the transferability of immunological mechanisms from other model mammals through homology. Bioinformatics enables knowledge transfer and enhances our understanding of mycosis in cattle. This synthesis fills critical information gaps and paves the way for proposing biotechnological strategies to mitigate the impact of mycoses in cattle.

Characterization and Biological Activities of Melanin from the Medicinal Fungi Ophiocordyceps sinensis

Melanin is a complex natural pigment that is widely present in fungi. The mushroom Ophiocordyceps sinensis has a variety of pharmacological effects. The active substances of O. sinensis have been extensively studied, but few studies have focused on the O. sinensis melanin. In this study, the production of melanin was increased by adding light or oxidative stress, namely, reactive oxygen species (ROS) or reactive nitrogen species (RNS), during liquid fermentation. Subsequently, the structure of the purified melanin was characterized using elemental analysis, ultraviolet-visible absorption spectrum, Fourier transform infrared (FTIR), electron paramagnetic resonance (EPR), and pyrolysis gas chromatography and mass spectrometry (Py-GCMS). Studies have shown that O. sinensis melanin is composed of C (50.59), H (6.18), O (33.90), N (8.19), and S (1.20), with maximum absorbance at 237 nm and typical melanin structures such as benzene, indole, and pyrrole. Additionally, the various biological activities of O. sinensis melanin have been discovered; it can chelate heavy metals and shows a strong ultraviolet-blocking ability. Moreover, O. sinensis melanin can reduce the levels of intracellular reactive oxygen species and counteract the oxidative damage of H₂O₂ to cells. These results can help us to develop applications of O. sinensis melanin in radiation resistance, heavy metal pollution remediation, and antioxidant use.

Ras2 Is Responsible for the Environmental Responses, Melanin Metabolism, and Virulence of Botrytis cinerea

Ras proteins are monomeric G proteins that are ubiquitous in fungal cells and play important roles in fungal growth, virulence, and environmental responses. Botrytis cinerea is a phytopathogenic fungus that infects various crops. However, under specific environmental conditions, the overripe grapes infected by B. cinerea can be used to brew valuable noble rot wine. As a Ras protein, the role of Bcras2 in the environmental responses of B. cinerea is poorly understood. In this study, we deleted the Bcras2 gene using homologous recombination and examined its functions. Downstream genes regulated by Bcras2 were explored using RNA sequencing transcriptomics. It was found that ΔBcras2 deletion mutants showed significantly reduced growth rate, increased sclerotia production, decreased resistance to oxidative stress, and enhanced resistance to cell wall stress. Additionally, Bcras2 deletion promoted the expression of melanin-related genes in sclerotia and decreased the expression of melanin-related genes in conidia. The above results indicate that Bcras2 positively regulates growth, oxidative stress resistance, and conidial melanin-related genes expression, and negatively regulates sclerotia production, cell wall stress resistance and sclerotial melanin-related genes expression. These results revealed previously unknown functions of Bcras2 in environmental responses and melanin metabolism in B. cinerea.

Blue Light Signaling Regulates Escherichia coli W1688 Biofilm Formation and l-Threonine Production

Escherichia coli biofilm may form naturally on biotic and abiotic surfaces; this represents a promising approach for efficient biochemical production in industrial fermentation. Recently, industrial exploitation of the advantages of optogenetics, such as simple operation, high spatiotemporal control, and programmability, for regulation of biofilm formation has garnered considerable attention. In this study, we used the blue light signaling-induced optogenetic system Magnet in an E. coli biofilm-based immobilized fermentation system to produce l-threonine in sufficient quantity. Blue light signaling significantly affected the phenotype of E. coli W1688. A series of biofilm-related experiments confirmed the inhibitory effect of blue light signaling on E. coli W1688 biofilm. Subsequently, a strain lacking a blue light-sensing protein (YcgF) was constructed via genetic engineering, which substantially reduced the inhibitory effect of blue light signaling on biofilm. A high-efficiency biofilm-forming system, Magnet, was constructed, which enhanced bacterial aggregation and biofilm formation. Furthermore, l-threonine production was increased from 10.12 to 16.57 g/L during immobilized fermentation, and the fermentation period was shortened by 6 h. IMPORTANCE We confirmed the mechanism underlying the inhibitory effects of blue light signaling on E. coli biofilm formation and constructed a strain lacking a blue light-sensing protein; this mitigated the aforementioned effects of blue light signaling and ensured normal fermentation performance. Furthermore, this study elucidated that the blue light signaling-induced optogenetic system Magnet effectively regulates E. coli biofilm formation and contributes to l-threonine production. This study not only enriches the mechanism of blue light signaling to regulate E. coli biofilm formation but also provides a theoretical basis and feasibility reference for the application of optogenetics technology in biofilm-based immobilized fermentation systems.

Filamentous fungal biofilms: Conserved and unique aspects of extracellular matrix composition, mechanisms of drug resistance and regulatory networks in Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is an ubiquitous mold that can cause invasive pulmonary infections in immunocompromised patients. Within the lung, A. fumigatus forms biofilms that can enhance resistance to antifungals and immune defenses, highlighting the importance of defining the mechanisms underlying biofilm development and associated emergent properties. A. fumigatus biofilms display a morphology and architecture that is distinct from bacterial and yeast biofilms. Moreover, A. fumigatus biofilms display unique characteristics in the composition of their extracellular matrix (ECM) and the regulatory networks governing biofilm formation. This review will discuss our current understanding of the form and function of A. fumigatus biofilms, including the unique components of ECM matrix, potential drug resistance mechanisms, the regulatory networks governing A. fumigatus biofilm formation, and potential therapeutics targeting these structures.

Colony growth and biofilm formation of Aspergillus niger under simulated microgravity

The biotechnology- and medicine-relevant fungus Aspergillus niger is a common colonizer of indoor habitats such as the International Space Station (ISS). Being able to colonize and biodegrade a wide range of surfaces, A. niger can ultimately impact human health and habitat safety. Surface contamination relies on two key-features of the fungal colony: the fungal spores, and the vegetative mycelium, also known as biofilm. Aboard the ISS, microorganisms and astronauts are shielded from extreme temperatures and radiation, but are inevitably affected by spaceflight microgravity. Knowing how microgravity affects A. niger colony growth, in particular regarding the vegetative mycelium (biofilm) and spore production, will help prevent and control fungal contaminations in indoor habitats on Earth and in space. Because fungal colonies grown on agar can be considered analogs for surface contamination, we investigated A. niger colony growth on agar in normal gravity (Ground) and simulated microgravity (SMG) conditions by fast-clinorotation. Three strains were included: a wild-type strain, a pigmentation mutant (ΔfwnA), and a hyperbranching mutant (ΔracA). Our study presents never before seen scanning electron microscopy (SEM) images of A. niger colonies that reveal a complex ultrastructure and biofilm architecture, and provide insights into fungal colony development, both on ground and in simulated microgravity. Results show that simulated microgravity affects colony growth in a strain-dependent manner, leading to thicker biofilms (vegetative mycelium) and increased spore production. We suggest that the Rho GTPase RacA might play a role in A. niger’s adaptation to simulated microgravity, as deletion of ΔracA leads to changes in biofilm thickness, spore production and total biomass. We also propose that FwnA-mediated melanin production plays a role in A. niger’s microgravity response, as ΔfwnA mutant colonies grown under SMG conditions showed increased colony area and spore production. Taken together, our study shows that simulated microgravity does not inhibit A. niger growth, but rather indicates a potential increase in surface-colonization. Further studies addressing fungal growth and surface contaminations in spaceflight should be conducted, not only to reduce the risk of negatively impacting human health and spacecraft material safety, but also to positively utilize fungal-based biotechnology to acquire needed resources in situ.

A Simple and Low-Cost Strategy to Improve Conidial Yield and Stress Resistance of Trichoderma guizhouense through Optimizing Illumination Conditions

Light is perceived by photoreceptors in fungi and further integrated into the stress-activated MAPK HOG pathway, and thereby potentially activates the expression of genes for stress responses. This indicates that the precise control of light conditions can likely improve the conidial yield and stress resistance to guarantee the low cost and long shelf life of Trichoderma-based biocontrol agents and biofertilizers. In this study, effects of wavelengths and intensities of light on conidial yield and stress tolerance to osmotic, oxidative and pH stresses in Trichoderma guizhouense were investigated. We found that 2 μmol photons/(m² × s) of blue light increased the conidial yield more than 1000 folds as compared to dark condition and simultaneously enhanced conidial stress resistance. The enhanced conidial stress resistance is probably due to the upregulated stress-related genes in blue light, which is under the control of the blue light receptor BLR1 and the MAP kinase HOG1.