Introduction
Fungi of the genus Aspergillus are ubiquitous microorganisms found in air, soil and decaying organic matter. More than 250 species have been classified. While some of them have industrial applications – such as in the production of fermented foods and enzymes – others pose a significant threat to human, animal and plant health due to their ability to produce mycotoxis (Klich et al., 2007).
They are among the primary producers of aflatoxins, which are highly toxic compounds generated under specific environmental conditions. These toxins are associated with numerous issues in the global food chain. As climate change intensifies the environmental conditions favorable for Aspergillus proliferation, the regulation and control of these toxins becomes increasingly critical.
Morphological characteristics
Aspergillus fungi are filamentous organisms characterized by their ability to reproduce asexually through the formation of conidia. Their morphological structure is distinguished by a specialized architecture that facilitates the efficient dispersal of spores. The different structural components are detailed in Image 1.
Image 1. a) Aspergillus morphology b) Aspergillus microscope image
1. Conidiophores:
Conidiophores are elongated, erect structures that emerge from foot cells (basal cells). These structures act as support for the other parts of the conidial apparatus and have a rigid and smooth or rough stipe, depending on the species. Their length and thickness can vary between species, but they always end in a characteristic vesicle.
2. Vesicles:
The vesicle is a globose or clavate structure located at the apex of the conidiophore. Phylalides are borne on its surface, and in some species, metulae are also present. The size and shape of the vesicle are important taxonomic characteristics used in the identification of the fungus.
3. Metulae and Phialides:
- Metulae: They are elongated and conical cells that are arranged radially around the vesicle in species with a biseriate organization. These structures serve as the basis for the phylalids.
- Phylalides: These are specialized structures that originate directly from the vesicles (in uniseriate species) or on the metulae (in biseriate species). Phylalides are responsible producing conidia through a repetitive budding mechanism.
4. Conidia:
Conidia are spherical or subspherical asexual structures arranged in basipetal chains, with the youngest spores form at the base of the chain. These are smooth or rough in texture, depending on the species, and contain melanin, which provides resistance to adverse environmental conditions. They are the dispersal units of the fungus.
5. Basal or foot cells:
The basal or foot cells are the base of the conidiophore and anchor the structure in the substrate. Its shape and size contribute to the support of the conidial apparatus (Klich et al., 2007).
Mycotoxins produced by Aspergillus
Within the genus of Aspergillus there are species which are highly mycotoxin producers, particularly Aspergillus flavus and Aspergillus parasiticus. These toxins do not play a clear role in fungal growth or development and are therefore classified as secondary metabolites. It is believed that their synthesis occurs under specific conditions as a defense or protective response to environmental stress (Carvajal et al., 2013). Additionally, it has also been suggested that these toxins help fungi colonize weakened or damaged plant tissues, thereby enhancing their survival (Varga et al., 2003). Among the most dangerous mycotoxins by the Aspergillus genus are aflatoxins, which are considered potent carcinogens that can contaminate crops such as corn, nuts, wheat, and rice.
These mycotoxins are the most regulated as large-scale contaminations have historically led to significant fatalities. One of the first documented cases occurred in 1961 on a poultry farm in London, where a total of 100,000 turkeys died from the so-called turkey « X » disease after consuming Brazilian peanut meal contaminated with aflatoxins (Blount et al., 1961).
Legislation
Given its toxic potential, the legal limits of AFB1 in raw materials and feed are currently regulated by Commission Regulation (EU) No. 574/2011, which amends Annex I of Directive 2002/32/EC.
| Raw material/feed | Legal limit (ppb) refers to foods with a moisture content of 12% |
|---|---|
| All raw materials for animal nutrition | 20 |
| Compound feedingstuffs for bovine animals, sheep and goats (excluding dairy animals and calves and lambs) | 20 |
| Complete feed for dairy cattle | 5 |
| Complete feed for calves and lambs | 10 |
| Compound feed for pigs, poultry (except young animals) | 20 |
| Other complete feedingstuffs | 10 |
| Other complementary feedingstuffs | 5 |
Table 1. Table of legal limits established in AFB1
Effects of mycotoxins on crops and health
From an agricultural perspective, Aspergillus infections can result in substantial economic losses. For example, Aspergillus flavus infections can reduce total crop yields by 10 to 30% (Ramírez-Camejo et al., 2012). Crop contamination occurs mainly under hot and humid conditions, which compromises both the quality and quantity of agricultural production. Moreover, feed contaminated with mycotoxins often needs to be destroyed, further increasing costs for producers.
In addition, Aspergillus mycotoxins not only compromise food quality but also pose a health risks to both animals and humans. In livestock, consumption of mycotoxin-contaminated feed can lead to liver disease, immunosuppression and reproductive problems, ultimately impacting overall animal productivity. In humans, exposure to aflatoxins has been associated with severe illnesses such as liver cancer, and their presence in food can cause acute food poisoning.
The impact of climate change on mycotoxin production
Climate change is altering temperature and precipitation patterns worldwide, creating more favorable conditions for the proliferation of mycotoxin-producing fungi, such as species form genus Aspergillus. Droughts, heatwaves and changes in growing seasons directly increase crop vulnerability to stress, which can favor fungal infection and, consequently, enhance mycotoxin production.
Higher temperatures and increased humidity create optimal conditions for Aspergillus growth and mycotoxin synthesis. Recent studies have shown an increase in aflatoxin levels in regions where they were not previously a concern, likely due to the expansion of climate zones favorable to the proliferation of these fungi. For example, some European countries, traditionally free of aflatoxins, are beginning to face problems with these toxins as a result of increasing temperatures, figure 2 (Battilani P et al., 2016).
Figure 2. Risk maps of aflatoxin contamination in maize during harvest under 3 different climate scenarios: current, +2 °C, +5 °C.
Mycotoxins produced by fungi of the genus Aspergillus
It has been shown that the Aspergillus genus is capable of generating a wide variety of mycotoxins, (Table 2). Among those included in the following list, aflatoxin stands out due to its toxicity and carcinogen effects (Ráduly et al., 2020).
| Aspergillus species | Aflatoxins | Ochratoxin | Citrinin | Patulin | Cyclopiazonic Acid | Aflatrem | Terrein |
|---|---|---|---|---|---|---|---|
| A. alliaceus | X | ||||||
| A. arachidicola | X | ||||||
| A. arachidicola sp. nov | X | ||||||
| A. bombycis | X | ||||||
| A. carbonarius | X | ||||||
| A. flavus | X | X | X | ||||
| A. korhogoensis | X | ||||||
| A. minisclerotigenes sp. nov. | X | X | |||||
| A. niger | X | ||||||
| A. nomius | X | ||||||
| A. novoparasiticus | X | ||||||
| A. ochraceus | X | ||||||
| A. parasiticus | X | ||||||
| A. pseudotamarii | X | ||||||
| A. rambellii | X | ||||||
| A. terreus | X | X | X | X | |||
| A. toxicarus | X |
Table 2. Species and mycotoxins produced
Among the aflatoxins, aflatoxins B1, B2, G1, and G2 in cultures and M1 in milk stand out. The nomenclatures of B or G correspond to the fluorescence that comes from the molecule B= Blue G= green (Image 3). In the case of aflatoxin M1, it is a metabolite formed through the biotransformation of aflatoxin B1, and is characterized by its accumulation in milk.
Image 3. Fluorescence emitted by aflatoxins present in maize contaminated with Aspergillus
Metabolic pathways and synthesis Aflatoxins
Aflatoxins (AFs) are bisfuranocoumarin compounds produced by more than 16 species of Aspergillus. Among the more than 19 identified analogues, the most important are the aflatoxins of the B series (AFB1 and AFB2) and the G series (AFG1 and AFG2), due to their toxicity and prevalence. The main producers are A. flavus and A. parasiticus. Their biosynthesis is a highly regulated process involving three groups of genes, categorized according to their stage in the metabolic pathway:
- Early pathway genes: aflA, aflB, aflC, hypC, and aflD, which initiate the conversion of hexanoate to norsolorinic acid.
- Intermediate pathway genes: aflG, aflH, aflK, aflV, and aflW, which are responsible for converting intermediates such as averuphin into precursors such as versiconal hemiacetal acetate (HAV).
- Late-pathway genes: aflP, aflQ, hypB, and others, which catalyze final transformations to AFB1, AFB2, AFG1, and AFG2. The regulatory gene aflR controls the expression of many genes in the pathway, while aflS acts as a coactivator in the early stages of biosynthesis.
Regarding specific differences and environmental factors:
- A. flavus mainly produces type B aflatoxins (AFB1 and AFB2), while A. parasiticus produces both type B and type G aflatoxins.
- The inability of A. flavus to synthesize aflatoxin G is due to a deletion in the aflF and aflU genes. However, new strains of A. flavus have been identified that are capable of producing all four aflatoxins.
- Aflatoxin production is influenced by environmental factors and can vary depending on the species and environmental conditions. This knowledge about biosynthetic pathways and genetic regulation allows the development of strategies to mitigate the production of aflatoxins, as well as to better understand metabolic diversity in mycotoxigenic species.
This abstract covers the key aspects of the biosynthesis and regulation of aflatoxins in species of the genus Aspergillus. Figure 3 (Kolawole et al., 2021).
Figure 3. (A) Clustering of biosynthetic aflatoxin genes in Aspergillus species. The arrows indicate the direction of genetic transcription, and the genes that regulate the clustering are marked with a golden arrow. (B) Proposed aflatoxin biosynthetic pathway (Ehrlich et al., 2004; Skory et al., 1992; Yu et al., 2004).
Regulation and strategies to mitigate risk
The regulation of mycotoxins is essential to ensure food safety and protect public health. Many countries have established strict limits on the allowable concentrations of mycotoxins in food and feed. For example, the European Union has set maximum permissible levels of aflatoxins in agricultural products intended for both human consumption and animal feed
However, as climate change increases the prevalence of mycotoxins, the current regulation may become obsolete. A global approach is required that includes active monitoring of environmental conditions and the presence of mycotoxins in crops.
Conclusion
Fungi of the genus Aspergillus and their ability to produce mycotoxins represent an emerging threat in the context of climate change. Global warming is creating more favorable conditions for the proliferation of these fungi, thereby posing significant risks to food security as well as human and animal health.
To mitigate this risk, it is essential to strengthen regulation, promote research and adopt adaptation strategies to protect crops from Aspergillus infections and limit exposure to mycotoxins. Preventive measures include good agricultural practices, such as proper storage management, grain ventilation and pest control, which can significantly reduce the incidence of mycotoxins in animal feed.
Likewise, the use of specific feed additives with anti-mycotoxin properties in feed has proven to be an effective tool. These compounds work by binding mycotoxins in the digestive tract, reducing their absorption and preventing their toxic effects. Such measures not only protect animal health but are also essential for ensuring food safety and sustainability in animal production.
