Functional Characterization of New Polyketide Synthase Genes Involved in Ochratoxin A Biosynthesis in Aspergillus Ochraceus fc-1

Deciphering Staphylococcus xylosus and Staphylococcus equorum mode of action against Penicillium nordicum in a dry-cured ham model system

Penicillium nordicum is one of the major producers of ochratoxin A (OTA) in dry-cured ham. Staphylococcus xylosus Sx8 and Staphylococcus equorum Se31 have been previously proposed as biocontrol agents (BCAs) to prevent the OTA contamination, although their antifungal mode of action has not been established yet. Thus, the aim of this work was to elucidate their mode of action against P. nordicum in a dry-cured ham model system. For this, the effect of live cells, dead cells, and cell-free broth; the nutritional utilisation pattern, niche overlap index (NOI), interactions by dual-culture assays, antifungal effect of volatile compounds, OTA detoxification, and effect on fungal proteome were determined. No fungal growth was observed after 14 days of co-culture with live cells of each staphylococcus at 15 or 20 °C. However, such inhibition was not observed with either dead cells or extracellular extracts. The number of carbon sources utilised by P. nordicum was higher than those used by both cocci at 20 °C, whilst the opposite occurred at 15 °C. According to NOI, nutritional dominance depends on temperature, at 20 °C P. nordicum dominated the niche, but at 15 °C the mould is dominated by the BCAs. The volatile pattern generated by each coccus did not show antifungal effect, and both staphylococci failed to degrade or adsorb OTA. However, in the interaction assay, S. xylosus and S. equorum were able to decrease the fungal growth and its OTA production. In addition, proteomic analyses showed changes in the abundance of proteins related to the cell wall integrity (CWI), carbohydrate metabolism and the biosynthesis of secondary metabolites such as OTA. In conclusion, overall, the antagonistic effects of the two studied cocci against P. nordicum are greater at 15 °C than at 20 °C, being linked to competition for space and nutrients, triggering alterations in CWI pathway, OTA biosynthesis, and carbohydrate metabolism.

The Secondary Metabolites and Biosynthetic Diversity From Aspergillus ochraceus

Aspergillus ochraceus, generally known as a food spoilage fungus, is the representative species in Aspergillus section Circumdati. A. ochraceus strains are widely distributed in nature, and usually isolated from cereal, coffee, fruit, and beverage. Increasing cases suggest A. ochraceus acts as human and animal pathogens due to producing the mycotoxins. However, in terms of benefits to mankind, A. ochraceus is the potential source of industrial enzymes, and has excellent capability to produce diverse structural products, including polyketides, nonribosomal peptides, diketopiperazine alkaloids, benzodiazepine alkaloids, pyrazines, bis-indolyl benzenoids, nitrobenzoyl sesquiterpenoids, and steroids. This review outlines recent discovery, chemical structure, biosynthetic pathway, and bio-activity of the natural compounds from A. ochraceus.

Requirement of LaeA, VeA, and VelB on Asexual Development, Ochratoxin A Biosynthesis, and Fungal Virulence in Aspergillus ochraceus

Aspergillus ochraceus is reported to be the major contributor of ochratoxin A (OTA), classified as one of the possible human carcinogen (group 2B) by the International Agency for Research on Cancer. The heterotrimeric velvet complex proteins, LaeA/VeA/VelB, have been most studied in fungi to clarify the relation between light-dependent morphology and secondary metabolism. To explore possible genetic targets to control OTA contamination, we have identified laeA, veA, and velB in A. ochraceus. The loss of laeA, veA, and velB yielded mutants with differences in vegetative growth and conidial production. Especially, ΔlaeA almost lost the ability to generate conidiaphore under dark condition. The deletion of laeA, veA, and velB drastically reduced the production of OTA. The wild-type A. ochraceus produced about 1 and 7 μg/cm² OTA under light and dark conditions on media, whereas the three gene deletion mutants produced less than 20 ng/cm² OTA, which was correlated with a down regulation of OTA biosynthetic genes. Pathogenicity studies of ΔlaeA, ΔveA, and ΔvelB showed their reduction in disease severity in pears. Furthermore, 66.1% of the backbone genes in secondary metabolite gene cluster were significantly regulated, among which 81.6% were downregulated. Taking together, these results revealed that velvet complex proteins played crucial roles in asexual development, secondary metabolism, and fungal virulence in A. ochraceus.

Engineering of the Filamentous Fungus Penicillium chrysogenum as Cell Factory for Natural Products

Penicillium chrysogenum (renamed P. rubens) is the most studied member of a family of more than 350 Penicillium species that constitute the genus. Since the discovery of penicillin by Alexander Fleming, this filamentous fungus is used as a commercial β-lactam antibiotic producer. For several decades, P. chrysogenum was subjected to a classical strain improvement (CSI) program to increase penicillin titers. This resulted in a massive increase in the penicillin production capacity, paralleled by the silencing of several other biosynthetic gene clusters (BGCs), causing a reduction in the production of a broad range of BGC encoded natural products (NPs). Several approaches have been used to restore the ability of the penicillin production strains to synthetize the NPs lost during the CSI. Here, we summarize various re-activation mechanisms of BGCs, and how interference with regulation can be used as a strategy to activate or silence BGCs in filamentous fungi. To further emphasize the versatility of P. chrysogenum as a fungal production platform for NPs with potential commercial value, protein engineering of biosynthetic enzymes is discussed as a tool to develop de novo BGC pathways for new NPs.

Mycoviruses mediate mycotoxin regulation in Aspergillus ochraceus

To date, no demonstration of a direct correlation between the presence of mycoviruses and the quantitative or qualitative modulation of mycotoxins has been shown. In our study, we transfected a virus-free ochratoxin A (OTA)-producing isolate of Aspergillus ochraceus with purified mycoviruses from a different A. ochraceus isolate and from Penicillium aurantiogriseum. Among the mycoviruses tested, only Aspergillus ochraceus virus (AoV), a partitivirus widespread in A. ochraceus, caused a specific interaction that led to an overproduction of OTA, which is regulated by the European Commission and is the second most important contaminant of food and feed commodities. Gene expression analysis failed to reveal a specific viral upregulation of the mRNA of genes considered to play a role in the OTA biosynthetic pathway. Furthermore, AoOTApks1, a polyketide synthase gene considered essential for OTA production, is surprisingly absent in the genome of our OTA-producing isolate. The possible biological and evolutionary implications of the mycoviral regulation of mycotoxin production are discussed.

A Consensus Ochratoxin A Biosynthetic Pathway: Insights from the Genome Sequence of Aspergillus ochraceus and a Comparative Genomic Analysis

Ochratoxin A (OTA) is a toxic secondary metabolite produced by Aspergillus and Penicillium species that widely contaminates food and feed. We sequenced and assembled the complete ∼37-Mb genome of Aspergillusochraceus fc-1, a well-known producer of OTA. Key genes of the OTA biosynthetic pathway were identified by comparative genomic analyses with five other sequenced OTA-producing fungi: A. carbonarius, A. niger, A. steynii, A. westerdijkiae, and Penicillium nordicum OTA production was completely inhibited in the deletion mutants (ΔotaA, ΔotaB, ΔotaC, ΔotaD, and ΔotaR1), and OTA biosynthesis was restored by feeding a postblock substrate to the corresponding mutant. The OTA biosynthetic pathway was unblocked in the ΔotaD mutant by the addition of heterologously expressed halogenase. OTA biosynthesis begins with a polyketide synthase (PKS), OtaA, utilizing acetyl coenzyme A (acetyl-CoA) and malonyl-CoA to synthesize 7-methylmellein, which is oxidized to OTβ by cytochrome P450 monooxygenase (OtaC). OTβ and l-β-phenylalanine are combined by a nonribosomal peptide synthetase (NRPS), OtaB, to form an amide bond to synthesize OTB. Finally, OTB is chlorinated by a halogenase (OtaD) to OTA. The otaABCD genes were expressed at low levels in the ΔotaR1 mutant. A second regulator, otaR2, which is adjacent to the biosynthetic gene, could modulate only the expression of otaA, otaB, and otaD Thus, we have identified a consensus OTA biosynthetic pathway that can be used to prevent and control OTA synthesis and will help us understand the variation and production of the intermediate components in the biosynthetic pathway.IMPORTANCE Ochratoxin A (OTA) is a significant mycotoxin that contaminates cereal products, coffee, grapes, wine, cheese, and meat. OTA is nephrotoxic, carcinogenic, teratogenic, and immunotoxic. OTA contamination is a serious threat to food safety, endangers human health, and can cause huge economic losses. At present, >20 species of the genera Aspergillus and Penicillium are known to produce OTA. Here we demonstrate that a consensus OTA biosynthetic pathway exists in all OTA-producing fungi and is encoded by a gene cluster containing four highly conserved biosynthetic genes and a bZIP transcription factor.

Effect of Cinnamaldehyde on Morphological Alterations of Aspergillus ochraceus and Expression of Key Genes Involved in Ochratoxin A Biosynthesis

Ochratoxin A (OTA) is a potent nephrotoxic, hepatotoxic, and teratogenic compound which is a significant mycotoxin contaminates cereals during storage. Aspergillus ochraceus is the most common producer of OTA in cereals and cereal-derived products. Cinnamaldehyde is a natural substance derived from plant cinnamon playing an important role in the reduction of OTA contamination. In this study, the antifungal and antitoxigenic effect of cinnamaldehyde was investigated with its mechanisms of inhibition of fungal growth at the morphological and ultrastructural levels, and inhibition of OTA biosynthesis at the transcriptional level. Significant A. ochraceus growth was inhibited at 0.4⁻1.6 mmol/L with fumigation. A. ochraceus exposed to 0.4 mmol/L of cinnamaldehyde indicated irreversible harmful morphological and ultrastructural modifications such as the folding of the cell, the loss of integrity of the cell wall, the disruption of plasma membrane, the destruction of the mitochondria, and the absence of intracellular organelles. These alterations may be attributed to its inhibition of enzymatic reactions that regulate cell wall synthesis, thus disturbing the morphogenesis and growth of A. ochraceus. In the presence of cinnamaldehyde, the tested biosynthetic and regulatory genes like pks, nrps, veA, laeA and velB were highly downregulated. Moreover, the downregulation effect of cinnamaldehyde increased proportionally with the concentrations. These results suggest that the decrease of OTA production by cinnamaldehyde is attributed to the downregulation of the transcriptional levels of OTA biosynthetic and regulatory genes besides the inhibition of fungal growth. The study reveals the mechanisms of the antifungal and antitoxigenic activities of cinnamaldehyde against A. ochraceus, and further emphasizes that cinnamaldehyde could be a safe and effective natural agents against OTA contamination during cereals storage.

pH-Signaling Transcription Factor AopacC Regulates Ochratoxin A Biosynthesis in Aspergillus ochraceus

In Aspergillus and Penicillium species, an essential pH-response transcription factor pacC is involved in growth, pathogenicity, and toxigenicity. To investigate the connection between ochratoxin A (OTA) biosynthesis and ambient pH, the AopacC in Aspergillus ochraceus was functionally characterized using a loss-of-function mutant. The mycelium growth was inhibited under pH 4.5 and 10.0, while the sporulation increased under alkaline condition. A reduction of mycelium growth and an elevation of sporulation was observed in Δ AopacC mutant. Compared to neutral condition, OTA contents were respectively reduced by 71.6 and 79.8% under acidic and alkaline conditions. The expression of AopacC increased with the elevated pH, and deleting AopacC dramatically decreased OTA production and biosynthetic genes Aopks expression. Additionally, the Δ AopacC mutant exhibited attenuated infection ability toward pear fruits. These results suggest that AopacC is an alkaline-induced regulator responsible for growth and OTA biosynthesis in A. ochraceus and this regulatory mechanism might be pH-dependent.

Description of an orthologous cluster of ochratoxin A biosynthetic genes in Aspergillus and Penicillium species. A comparative analysis

Ochratoxin A (OTA) is one of the most important mycotoxins due to its toxic properties and worldwide distribution which is produced by several Aspergillus and Penicillium species. The knowledge of OTA biosynthetic genes and understanding of the mechanisms involved in their regulation are essential. In this work, we obtained a clear picture of biosynthetic genes organization in the main OTA-producing Aspergillus and Penicillium species (A. steynii, A. westerdijkiae, A. niger, A. carbonarius and P. nordicum) using complete genome sequences obtained in this work or previously available on databases. The results revealed a region containing five ORFs which predicted five proteins: halogenase, bZIP transcription factor, cytochrome P450 monooxygenase, non-ribosomal peptide synthetase and polyketide synthase in all the five species. Genetic synteny was conserved in both Penicillium and Aspergillus species although genomic location seemed to be different since the clusters presented different flanking regions (except for A. steynii and A. westerdijkiae); these observations support the hypothesis of the orthology of this genomic region and that it might have been acquired by horizontal transfer. New real-time RT-PCR assays for quantification of the expression of these OTA biosynthetic genes were developed. In all species, the five genes were consistently expressed in OTA-producing strains in permissive conditions. These protocols might favour futures studies on the regulation of biosynthetic genes in order to develop new efficient control methods to avoid OTA entering the food chain.

UPLC-MS-IT-TOF Identification of Circumdatins Produced by Aspergillus ochraceus

A method based on the combined use of ultraperformance liquid chromatography coupled to mass spectrometry-ion trap-time-of-flight (UPLC-MS-IT-TOF) detection was employed to identify the metabolite production of Aspergillus ochraceus, which is the major cause of food and feed contamination due to ochratoxin A. Under the proposed chromatographic conditions, seven metabolites belonging to the family of circumdatins were separated and identified. Their initial identification was performed through the exact molecular formula, as a function of their accurate mass. Collision-induced dissociation was applied to predict precursor and product ions, and the elemental composition of each compound was obtained. The elimination of nitrogenous groups followed by successive losses of carbonyl groups is the common fragmentation pathway of circumdatins. With the fragmentation data obtained, an UPLC-MS/MS method was created and optimized to detect circumdatins in corn samples.

A Polyketide Synthase Encoded by the Gene An15g07920 Is Involved in the Biosynthesis of Ochratoxin A in Aspergillus niger

The polyketide synthase gene An15g07920 was known in Aspergillus niger CBS 513.88 as putatively involved in the production of ochratoxin A (OTA). Genome resequencing analysis revealed that the gene An15g07920 is also present in the ochratoxin-producing A. niger strain 1062. Disruption of An15g07920 in A. niger 1062 removed its capacity to biosynthesize ochratoxin β (OTβ), ochratoxin α (OTα), and OTA. These results indicate that the polyketide synthase encoded by An15g07920 is a crucial player in the biosynthesis of OTA, in the pathway prior to the phenylalanine ligation step. The gene An15g07920 reached its maximum transcription level before OTA accumulation reached its highest level, confirming that gene transcription precedes OTA production. These findings will not only help explain the mechanism of OTA production in A. niger but also provide necessary information for the development of effective diagnostic, preventive, and control strategies to reduce the risk of OTA contamination in foods.

Ochratoxin A Producing Fungi, Biosynthetic Pathway and Regulatory Mechanisms

Ochratoxin A (OTA), mainly produced by Aspergillus and Penicillum species, is one of the most important mycotoxin contaminants in agricultural products. It is detrimental to human health because of its nephrotoxicity, hepatotoxicity, carcinogenicity, teratogenicity, and immunosuppression. OTA structurally consists of adihydrocoumarin moiety linked with l-phenylalanine via an amide bond. OTA biosynthesis has been putatively hypothesized, although several contradictions exist on some processes of the biosynthetic pathway. We discuss recent information on molecular studies of OTA biosynthesis despite insufficient genetic background in detail. Accordingly, genetic regulation has also been explored with regard to the interaction between the regulators and the environmental factors. In this review, we focus on three aspects of OTA: OTA-producing strains, OTA biosynthetic pathway and the regulation mechanisms of OTA production. This can pave the way to assist in protecting food and feed from OTA contamination by understanding OTA biosynthetic pathway and regulatory mechanisms.

Comparative Ochratoxin Toxicity: A Review of the Available Data

Ochratoxins are a group of mycotoxins produced by a variety of moulds. Ochratoxin A (OTA), the most prominent member of this toxin family, was first described by van der Merwe et al. in Nature in 1965. Dietary exposure to OTA represents a serious health issue and has been associated with several human and animal diseases including poultry ochratoxicosis, porcine nephropathy, human endemic nephropathies and urinary tract tumours in humans. More than 30 years ago, OTA was shown to be carcinogenic in rodents and since then extensive research has been performed in order to investigate its mode of action, however, this is still under debate. OTA is regarded as the most toxic family member, however, other ochratoxins or their metabolites and, in particular, ochratoxin mixtures or combinations with other mycotoxins may represent serious threats to human and animal health. This review summarises and evaluates current knowledge about the differential and comparative toxicity of the ochratoxin group.