Introduction
Fungi of the genus Aspergillus are ubiquitous microorganisms, present in the air, soil, and decomposing organic matter, comprising more than 250 species. Although some of them have industrial applications, such as the production of fermented foods or enzymes, others pose a significant threat to human, animal, and plant health due to their production of mycotoxins (Klich, 2007).
These fungi are the main producers of aflatoxins, highly toxic mycotoxins that develop under certain environmental conditions and are linked to multiple problems in the global food chain. With climate change intensifying favorable conditions for the proliferation of fungi of the genus Aspergillus, the regulation and control of these toxins is becoming increasingly crucial.
Morphological characteristics
Aspergillus fungi are filamentous fungi 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 their spores. The different parts are detailed in Image 1.
Image 1. A) Microscope image of Aspergillus (Pickova et al., 2021).
B) Aspergillus morphology (Klich, 2009).
Conidiophores:
Conidiophores are elongated, erect structures that emerge from foot cells (basal cells). These structures act as support for the rest of the conidial apparatus and present a rigid stipe, smooth or rough depending on the species. Their length and thickness may vary among species, but they always end in a characteristic vesicle.
Vesicles:
The vesicle is a globose or clavate structure located at the apex of the conidiophore. Phialides, and in some species, metulae, are inserted in it. Its size and shape are important taxonomic characteristics for fungal identification.
Metulae and Phialides:
- Metulae: Elongated and conical cells arranged radially around the vesicle in species with biseriate organization. These structures serve as a base for the phialides.
- Phialides: Specialized structures that originate directly from the vesicle (in uniseriate species) or from the metulae (in biseriate species). Phialides are responsible for producing conidia through a repeated budding mechanism.
Conidia:
Conidia are spherical or subspherical asexual structures arranged in basipetal chains (the youngest spores form at the base of the chain). They may be smooth or rough, depending on the species, and contain melanin, which provides resistance against adverse environmental conditions. They are the dispersal units of the fungus.
Basal or foot cells:
Basal or foot cells are the base of the conidiophore and anchor the structure to the substrate. Their shape and size contribute to supporting the conidial apparatus (Klich, 2007).
Mycotoxins produced by Aspergillus
Within the Aspergillus genus, there are highly mycotoxin-producing species, among which Aspergillus flavus and Aspergillus parasiticus stand out. These toxins have no clear role in the fungus’s growth or development, classifying them as secondary metabolites. It is believed their synthesis occurs under certain conditions for defense or protection, as a response to environmental stress (Suárez et al., 2013). It has also been suggested that these toxins help fungi colonize weakened or damaged plant tissues, enhancing survival (Varga et al., 2003).
Among the most dangerous mycotoxins produced by the genus Aspergillus are aflatoxins, which are considered potent carcinogens that can contaminate crops such as maize, wheat, and rice.
These mycotoxins are the most heavily regulated, historically associated with large contaminations that have led to numerous deaths. One of the first cases occurred in 1961 on a poultry farm in London, where a total of 100,000 turkeys died from the so-called “X” disease in turkeys after being fed Brazilian peanut meal contaminated with aflatoxins (Blount et al., 1961).
In addition to aflatoxins, the genus Aspergillus has been shown to generate other mycotoxins, as presented in the following table (Ráduly et al., 2020).
| Aspergillus species | Aflatoxins | Ochratoxin | Citrinin | Patulin | Cyclopiazonic Acid | Aflatrem | Terrain |
|---|---|---|---|---|---|---|---|
| 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 1. Aspergillus species and mycotoxins produced.
It should be noted that among aflatoxins, the most important are B1, B2, G1, and G2 in crops, and M1 in milk. The B or G nomenclature corresponds to the fluorescence emitted by the molecule (B = Blue, G = Green) (Image 2). In the case of aflatoxin M1, it is a metabolite obtained after the biotransformation of aflatoxin B1, characterized by its accumulation in milk.
Image 2. Fluorescence emitted by aflatoxins present in maize contaminated with Aspergillus.
Effects of mycotoxins on crops and public health
From an agricultural perspective, Aspergillus infections can cause substantial economic losses. For example, infections caused by Aspergillus flavus can reduce total crop yield by 10 to 30% (Ramírez-Camejo et al., 2012). Crop contamination occurs mainly under warm and humid conditions, degrading both the quality and quantity of production. Moreover, contaminated food often must be destroyed, further increasing costs for producers.
Aspergillus mycotoxins not only affect food quality but also pose risks to animal and human health. In livestock, consumption of contaminated feed can cause liver disease, immunosuppression, and reproductive problems, which in turn affect productivity. In humans, aflatoxin exposure has been linked to serious diseases such as liver cancer, and its presence in food can cause acute food poisoning.
The impact of climate change on mycotoxin production
Climate change is altering temperature and rainfall patterns worldwide, creating more favorable conditions for the proliferation of mycotoxin-producing fungi such as Aspergillus species. Droughts, heatwaves, and shifts in growing seasons directly affect crop vulnerability, which can promote fungal infection and, consequently, mycotoxin production.
Higher temperatures and increased humidity provide an optimal environment for the growth of Aspergillus and the synthesis of mycotoxins. Recent studies have shown a rise in aflatoxin levels in regions where they were previously not a concern, possibly due to the expansion of climatic zones suitable for fungal growth. For instance, some European countries, traditionally free of aflatoxins, are now beginning to face problems with these toxins due to rising temperatures (Battilani et al., 2016).
Image 3. Risk maps of aflatoxin contamination in maize at harvest under three different climate scenarios: current, +2 °C, +5 °C (Battilani et al., 2016).
Metabolic pathways and aflatoxin synthesis
Aflatoxins are bisfuranocoumarin compounds produced by more than 16 Aspergillus species. Aflatoxins of the B series (AFB1 and AFB2) and G series (AFG1 and AFG2) are the most recognized due to their toxicity and prevalence. Their biosynthesis is a highly regulated process involving three groups of genes depending on the stage in the metabolic pathway:
- Early pathway genes: aflA, aflB, aflC, hypC, and aflD, initiating the conversion of hexanoate to norsolorinic acid.
- Intermediate pathway genes: aflG, aflH, aflK, aflV, and aflW, responsible for converting intermediates, such as averufin, into precursors like versiconal hemiacetal acetate (VHA).
- Late pathway genes: aflP, aflQ, hypB and others, catalyzing the final transformations into AFB1, AFB2, AFG1, and AFG2. The regulatory gene aflR controls the expression of many pathway genes, while aflS acts as a coactivator in the early biosynthesis stages.
Regarding specific differences and environmental factors:
- Aspergillus flavus mainly produces B-type aflatoxins (AFB1 and AFB2), while Aspergillus parasiticus produces both B-type and G-type aflatoxins.
- The inability of Aspergillus flavus to synthesize G-type aflatoxins is due to a deletion in the aflF and aflU genes. However, new strains of Aspergillus flavus capable of producing all four aflatoxins have been identified.
- Aflatoxin production is influenced by environmental factors and may vary depending on species and conditions. This knowledge of biosynthetic pathways and genetic regulation allows strategies to mitigate aflatoxin production and better understand metabolic diversity in mycotoxigenic species.
This summary covers the key aspects of the biosynthesis and regulation of aflatoxins in species of the genus Aspergillus (Kolawole et al., 2021):
Image 4. A) Aflatoxin biosynthetic gene cluster in Aspergillus species. Arrows indicate genetic transcription direction; regulatory genes are marked with a golden arrow.
B) Proposed biosynthetic pathway of aflatoxins.
(Ehrlich et al., 2004; Skory et al., 1992; Yu et al., 2004).
Regulation and strategies to mitigate risk
Mycotoxin regulation is essential to ensure food safety and protect public health. In many countries, strict limits exist on the permitted concentration of mycotoxins in food and feed. For example, the European Union has set maximum levels of aflatoxins allowed in agricultural products, both for human consumption and animal feed.
Given its toxic potential, the legal limits established for AFB1, one of the most recognized aflatoxins, are regulated by Commission Regulation (EU) No. 574/2011, which amends Annex I of Directive 2002/32/EC.
| Raw materials/feed | Legal limit (ppb) referring to foods with a moisture content of 12% |
|---|---|
| All raw materials for animal nutrition | 20 |
| Compound feed for cattle, sheep, and goats (except dairy animals, calves, and lambs) | 20 |
| Complete feed for dairy cattle | 5 |
| Complete feed for calves and lambs | 10 |
| Compound feed for pigs and poultry (except young animals) | 20 |
| Other complete feed | 10 |
| Other complementary feed | 5 |
Table 2. Established legal limits for AFB1.
However, as climate change increases the prevalence of mycotoxins, current regulations may become obsolete. A global approach is needed, including active monitoring of environmental conditions and mycotoxin presence in crops.
Conclusion
Fungi of the genus Aspergillus and their ability to produce mycotoxins represent a growing threat in the context of climate change. Global warming is creating more favorable conditions for the proliferation of these fungi, endangering food security and 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, proper storage management, grain ventilation, and pest control. These actions can significantly reduce the incidence of mycotoxins in animal feed.
