Secondary metabolism and microbial physiology

Paraphoma chrysanthemicola Affects the Carbohydrate and Lobetyolin Metabolism Regulated by Salicylic Acid in the Soilless Cultivation of Codonopsis pilosula

Paraphoma chrysanthemicola, an endophytic fungus isolated from the roots of Codonopsis pilosula, influences salicylic acid (SA) levels. The interaction mechanism between SA and P. chrysanthemicola within C. pilosula remains elusive. To elucidate this, an experiment was conducted with four treatments: sterile water (CK), P. chrysanthemicola (FG), SA, and a combination of P. chrysanthemicola with salicylic acid (FG+SA). Results indicated that P. chrysanthemicola enhanced plant growth and counteracted the growth inhibition caused by exogenous SA. Physiological analysis showed that P. chrysanthemicola reduced carbohydrate content and enzymatic activity in C. pilosula without affecting total chlorophyll concentration and attenuated the increase in these parameters induced by exogenous SA. Secondary metabolite profiling showed a decrease in soluble proteins and lobetyolin levels in the FG group, whereas SA treatment led to an increase. Both P. chrysanthemicola and SA treatments decreased antioxidase-like activity. Notably, the FG group exhibited higher nitric oxide (NO) levels, and the SA group exhibited higher hydrogen peroxide (H2O2) levels in the stems. This study elucidated the intricate context of the symbiotic dynamics between the plant species P. chrysanthemicola and C. pilosula, where an antagonistic interaction involving salicylic acid was prominently observed. This antagonism was observed in the equilibrium between carbohydrate metabolism and secondary metabolism. This equilibrium had the potential to engage reactive oxygen species (ROS) and nitric oxide (NO).

Astaxanthin and Docosahexaenoic Acid Reverse the Toxicity of the Maxi-K (BK) Channel Antagonist Mycotoxin Penitrem A

Penitrem A (PA) is a food mycotoxin produced by several terrestrial and few marine Penicillium species. PA is a potent tremorgen through selective antagonism of the calcium-dependent potassium BK (Maxi-K) channels. Discovery of natural products that can prevent the toxic effects of PA is important for food safety. Astaxanthin (AST) is a marine natural xanthophyll carotenoid with documented antioxidant activity. Unlike other common antioxidants, AST can cross blood brain barriers (BBBs), inducing neuroprotective effects. Docosahexaenoic acid (DHA) is polyunsaturated ω-3 fatty acid naturally occurring in fish and algae. DHA is essential for normal neurological and cellular development. This study evaluated the protective activity of AST and DHA against PA-induced toxicity, in vitro on Schwann cells CRL-2765 and in vivo in the worm Caenorhbitidis elegans and Sprague Dawley rat models. PA inhibited the viability of Schwann cells, with an IC₅₀ of 22.6 μM. Dose-dependent treatments with 10–100 μM DHA significantly reversed the PA toxicity at its IC₅₀ dose, and improved the survival of Schwann cells to 70.5%–98.8%. Similarly, dose-dependent treatments with 10–20 μM AST reversed the PA toxicity at its IC₅₀ dose and raised these cells’ survival to 61.7%–70.5%. BK channel inhibition in the nematode C. elegans is associated with abnormal reversal locomotion. DHA and AST counteracted the in vivo PA BK channel antagonistic activity in the C. elegans model. Rats fed a PA-contaminated diet showed high levels of glutamate (GLU), aspartate (ASP), and gamma amino butyric acid (GABA), with observed necrosis or absence of Purkinjie neurons, typical of PA-induced neurotoxicity. Dopamine (DA), serotonin (5-HT), and norepinephrine (NE) levels were abnormal, Nitric Oxide (NO) and Malondialdehyde (MDA) levels were significantly increased, and total antioxidant capacity (TAC) level in serum and brain homogenates was significantly decreased in PA-treated rats. DHA and AST treatments effectively counteracted the toxic effects of PA and normalized most biochemical parameters in rats. DHA and AST can be useful food additives to prevent and reverse PA food-induced toxicity.

Antibiotics and evolution: food for thought

The role of secondary metabolites in effecting and modulating reactions during early biochemical evolution has been largely unappreciated. It is possible that low molecular weight effectors were gradually replaced by polypeptides as polymerizing reactions became more complex, but retained some ability to interact with original receptor sites. Indeed, by reviewing the era of antibiotics in this light we can begin to reconcile the ancient and contemporary activities of these molecules. The corollary being that secondary metabolites participate in a vast array of interactions in nature and investigating their intended receptors will be revealing in both pharmacological and evolutionary terms.

Hormesis and a Chemical Raison D'être for Secondary Plant Metabolites

In plants, accumulation in specific compartments and huge structural diversity of secondary metabolites is one trait that is not understood yet. By exploring the diverse abiotic and biotic interactions of plants above- and belowground, we provide examples that are characterized by nonlinear effects of the secondary metabolites. We propose that redox chemistry, specifically the reduction of reactive oxygen species (ROS) and, in their absence, reduction of molecular oxygen by the identical secondary metabolite, is an important component of the hormetic effects caused by these compounds. This is illustrated for selected phenols, terpenoids, and alkaloids. The redox reactions are modulated by the variable availability of transition metals that serve as donors of electrons in a Fenton reaction mode. Low levels of ROS stimulate growth, cell differentiation, and stress resistance; high levels induce programmed cell death. We propose that provision of molecules that can participate in this redox chemistry is the raison d'être for secondary metabolites. In this context, the presence or absence of functional groups in the molecule is more essential than the whole structure. Accordingly, there exist no constraints that limit structural diversity. Redox chemistry is ubiquitous, from the atmosphere to the soil.

The effect of substrate on mycotoxin production of selected Penicillium strains

Analytical methods are presented for detecting simultaneously 11 fungal metabolites (aflatoxins B1, B2, G1 and G2, citrinin, cyclopiazonic acid, mycophenolic acid, ochratoxin A, penicillic acid, penitrem A and roquefortine C) on different matrices. The methods were applied to determine the mycotoxins produced by different Penicillium crustosum, Penicillium nordicum and Penicillium verrucosum strains on yeast extract sucrose (YES) agar and cheese and bread analogues and are based on high-performance liquid chromatography (HPLC) and photodiode array detection (PDA). The growth substrate had a distinctive effect on the mycotoxin production ability of the fungi examined. The P. crustosum strains produced roquefortine C on all the substrates, with the highest amounts being detected on the cheese analogue. Penitrem A was synthesised on the cheese analogue only. The strains of P. verrucosum produced exclusively citrinin on YES, but both ochratoxin A and citrinin were detected in considerable amounts on the bread analogue. On the bread, toxin profiles varied significantly between the individual P. verrucosum strains. The cheese analogue was not favourable for the mycotoxin production of this species. The growth substrate had the least effect on the toxin production of the P. nordicum strains, which synthesised ochratoxin A in moderate amounts on all three media.

The natural functions of secondary metabolites

Secondary metabolites, including antibiotics, are produced in nature and serve survival functions for the organisms producing them. The antibiotics are a heterogeneous group, the functions of some being related to and others being unrelated to their antimicrobial activities. Secondary metabolites serve: (i) as competitive weapons used against other bacteria, fungi, amoebae, plants, insects, and large animals; (ii) as metal transporting agents; (iii) as agents of symbiosis between microbes and plants, nematodes, insects, and higher animals; (iv) as sexual hormones; and (v) as differentiation effectors. Although antibiotics are not obligatory for sporulation, some secondary metabolites (including antibiotics) stimulate spore formation and inhibit or stimulate germination. Formation of secondary metabolites and spores are regulated by similar factors. This similarity could insure secondary metabolite production during sporulation. Thus the secondary metabolite can: (i) slow down germination of spores until a less competitive environment and more favorable conditions for growth exist; (ii) protect the dormant or initiated spore from consumption by amoebae; or (iii) cleanse the immediate environment of competing microorganisms during germination.

A temporal signal, independent of agr, is required for hla but not spa transcription in Staphylococcus aureus

Staphylococcus aureus exoprotein expression is controlled by a global regulon known as agr. This system activates transcription of some target genes and represses transcription of others. Target genes expressed postexponentially such as alpha-hemolysin (hla) are activated by agr; target genes expressed during exponential phase such as protein A (spa) are repressed by agr. A unique feature of the agr system is that this transcriptional regulation is mediated by a 517-nucleotide transcript, RNAIII. While it is clear that agr differentially regulates the expression of exponential and postexponential exoproteins, the precise role of agr in the temporal control of these events has not yet been explored. In this report, we examine the effects of expressing RNAIII, the agr regulator, under the control of the inducible beta-lactamase (bla) promoter at different times in the growth cycle. We confirm previous results showing that agr is required for postexponential-phase expression of hla and further show that a separate postexponential-phase signal independent of agr function is also needed for activation of hla transcription. We also show that in an agr mutant transcription of spa occurs throughout the growth cycle, is inhibited immediately upon induction of RNAIII, and is thus indifferent to the postexponential signal required for hla activation.

Peptide antibiotics, beta-lactams, and related compounds

In the field of natural peptides, beta-lactams, and related compounds, recent exciting developments are discussed. The increasing interest in this class of bioactive amino-acid derived structures has been attributed to the use of new directed screens (enzyme inhibition assays, beta-lactam detection, immunomodulator studies), new and improved applications (antibiotic, transplantation, and cancer chemotherapy), and advances in functional studies (DNA binding peptides, nucleotide complexones, cell wall and protein processing inhibitors). Peptides offer unique access to modifications and analog production by in vivo (directed biosynthesis) and in vitro procedures (enzymatic synthesis) due to their general linear precursors permitting point replacements. Of special interest are recent developments in the genetics of these compounds (cyclic peptides and beta-lactams), which will find applications in production methods in the near future.

Standardized high-performance liquid chromatography of 182 mycotoxins and other fungal metabolites based on alkylphenone retention indices and UV-VIS spectra (diode array detection)

A general standardized method for the analysis of mycotoxins and other fungal secondary metabolites has been developed, based on high-performance liquid chromatography (HPLC) with an alkylphenone retention index and photodiode-array detection combined with thin-layer chromatography (TLC) in two different eluents. Each fungal secondary metabolite is characterized by its bracketed alkylphenone retention time index, its UV-VIS absorption maxima and its retardation factors relative to griseofulvin in two TLC eluents. This system is effective for the comparison of chemotaxonomic data in different laboratories and for a precise identification of fungi based on organic solvent extracts of fungal cultures. All important groups of mycotoxins and other fungal secondary metabolites could be detected in the HPLC system described and data are listed for 182 metabolites. The fungal secondary metabolites separated and characterized include aflatoxin B1, B2, G1 and G2, ochratoxin A, citrinin, penicillin acid, viomellein, penitrem A, patulin, sterigmatocystin, alternariol, tenuazonic acid, trichothecenes, roquefortines, fusarin C, zearalenone, PR-toxin, citreoviridin, viridicatumtoxin, verruculogen, rugulosin, cyclopiazonic acid, penicillin G and many other alkaloids, polyketides and terpenes.