Dr. Rui Alexandre Gonçalves
Aquaculture and Mycotoxin Expert
Aquaculture Business Developer – Lucta S.A.
Innovation Division – Feed Additives
UAB Research Park, Bellaterra, Barcelona, Spain
Importance of fungi for the ecosystem is immeasurable!
Fungi are present in all terrestrial habitats, from Antarctica to the hot deserts of Namibia, and in most aquatic environments.
Fungi are opportunistic heterotrophs, they can penetrate solid substrates (e.g., rock, bark, dead branches, bare soil, or grains), and nutritionally exploit almost any food substrate in the world.
Not all fungi are bad!
Several species of fungi, namely from genus Penicillium, play an important role in various industries.
For example, penicillin, produced by P. chrysogenum (formerly P. notatum), discovered by Alexander Fleming in 1929, was probably the most important discovery in the last century as it changed the course of medicine.
Fungi also play a central role in the food industry (cheese and various meat products) and are becoming increasingly important in the biotechnology field, especially on the production of enzymes (e.g. gluconic, citric, and tartaric acids, several pectinases, lipase, amylases, cellulases, and proteases).
Some of the fungi can have a parasitic behaviour and in certain cases can be pathogenic as their ability to penetrate almost any surface can be used to invade host organisms.
Fungi attack almost all known taxa of plants and animals, including shrimp (e.g. Fusarium spp. in penaeids) and fish (e.g. Saprolegniasis).
Focusing on fungi as plant pathogens, they attack all parts and all stages of crop plant development (from root hair to apical buds, grains or fruits).
The fungal infections may be restricted to small leaf spots, or they may be systemic, killing their host very quickly or remaining invisible until it is time to benefit from crucial energy resources, such as those concentrated by the host in anthers, bulbs or seeds.
Not all secondary metabolites are mycotoxins!
Fungal metabolism produces an apparently endless diversity of organic compounds, which are not obviously required for normal growth and metabolism. These are called secondary metabolites.
Not all secondary metabolites are mycotoxins. Simplistically, we could split them into three broad groups:
- ⇰ Toxic to bacteria: antibiotics
- ⇰ Toxic to plants: phytotoxins
- ⇰ Toxic to animals: mycotoxins
Understanding fungal growth and mycotoxin production!
To develop effective strategies to control the impact of fungi and/or mycotoxins in animal production it is essential to understand fungal growth and mycotoxin production behaviour.
Fungal growth and, in consequence, the occurrence of individual mycotoxins is a result of complex interaction of several factors, being environmental conditions -geography and climatic factors- the most relevant (Kuiper-Goodman, 2004; Miraglia et al., 2009; Paterson and Lima, 2010; Paterson and Lima, 2011; Ramirez et al., 2006).
Behind these two main factors (geography and climatic factors) biological requirements such as temperature and water activity (aw) play a critical role in fungal growth and consequent mycotoxin production (CAST, 2003; FAO, 2004; Marth, 1992; Ramirez et al., 2006; Sweeney and Dobson, 1998).
However, fungal growth and mycotoxin production are not simple combinations of optimal temperature and water activity.
FACTORS INFLUENCING FUNGAL GROWTH & MYCOTOXIN PRODUCTION
Environmental conditions (geographic and climatic factors): Temperature & water activity
Ecological conditions: insect attacks, physical plant damage, and stress
Besides the previously mentioned factors contributing to the presence or production of mycotoxins, i.e., environmental (temperature, humidity), ecological conditions (e.g., insect attacks, physical plant damage and general stress) are also key and often beyond human control.
- ⇰ For example, Ramírez et al. (2004) observed that the type of water stress, whether it is caused by osmotic or matric forces, can impact the activity and colonisation of cereal-based substrates by strains of F. graminearum (the main producer of deoxynivalenol).
- ⇰ Several studies on F. graminearum from root and stalk rot of cereals have shown optimum water content and temperature for growth were 0.99–0.98 aw at 20–30°C, changing to 0.95–0.96 aw at 35°C.
However, conditions for optimum fungal growth and maximum toxin production are not the same (Ramírez et al., 2006).
For example, Table 1 shows:
- ⇰ Two species of storage fungi: Aspergillus flavus and Penicillium verrucosum
- ⇰ Two representatives of field fungi: Fusarium verticillioides and F. graminearum
From the table is possible to clearly understand why Aspergillus spp. and Penicillium spp. are in competitive advantage to grow in storage conditions, as they can grow at extreme temperatures and with a lower water activity than Fusarium spp.
In the case of Aspergillus flavus, they occur in warmer climates, whereas
Penicillium verrucosum is adapted to growing in temperate regions, being able to grow at near 0°C.
Table 1. Preferred temperatures and water
activity values for fungal growth.
Mycotoxin producing fungi
Aflatoxins (AF), ochratoxin A (OTA), deoxynivalenol (DON), zearalenone (ZEN), fumonisins (FUM) and ergot alkaloids are the most common mycotoxins found in agriculture commodities and these are responsible for millions of dollars in annual losses worldwide.
These toxins are produced by just a few species from the common genera Aspergillus, Penicillium, Fusarium, and Claviceps.
ASPERGILLUS & PENICILLIUM
All Aspergillus and Penicillium species are either commensals, growing in crops without obvious signs of pathogenicity, or they invade crops after harvest, producing
toxins during drying and storage.
⇰ The most important Aspergillus species, occurring in warmer climates, are A. flavus and A. parasiticus, which produce aflatoxins in maize, groundnuts, tree nuts, etc.
⇰ Penicillium verrucosum also produces ochratoxin A but occurs only in cool temperate climates, where it infects small grains.
FUSARIUM & CLAVICEPS
In contrast, the important Fusarium and Claviceps species infect crops before harvest.
⇰ F. verticillioides is ubiquitous in maize, with an endophytic nature, producing fumonisins, which are generally more prevalent when crops are under drought stress or suffer excessive insect damage.
It has recently been shown that Aspergillus niger also produces fumonisins, and several commodities may be affected.
⇰ F. graminearum, the major producer of deoxynivalenol and zearalenone, is pathogenic for maize, wheat, and barley, and produces these toxins whenever it infects these grains before harvest.
Table 2. Preferred temperatures and water activity values for mycotoxin production.
Feed preservation and mycotoxin management: Complementary but not the same!
Presence of moulds in crops, in raw materials or finished feeds may be the first indication that something is wrong with its hygiene/conservation.
However, it will be hard to correlate the fungal presence with the potential presence of the mycotoxin and vice-versa (Alinezhad et al., 2011; Greco et al., 2015).
Extending the knowledge of fungal growth in crops
There are several reasons why raw material or finished feeds may get mouldy, from improper storage conditions (high humidity, high variations in temperatures leading to condensation, etc.) to a poor manufacturing process (e.g., insufficient drying time, lack of preservatives/anti-moulds, etc.).
In finished feeds, fungal contamination can also originate from an inappropriate selection of ingredients, which can carry fungal spores that are resistant to extrusion/pelleting, having the capacity to germinate afterwards (due to improper storage or poor manufacturing processes).
However, it is important to highlight that the presence of fungi might not represent a mycotoxin threat, but it can be a direct risk for the host.
⇰ For example, Fusarium oxysporum and Fusarium solani, are known as opportunistic pathogens for fish and shrimp (Hatai et al., 1986; Lightner, 1996; Ostland V.E. et al., 1987; Souheil et al., 1999).
Moreover, fungal presence in feed may also reduce its palatability and therefore intake of the feed, leading to economic losses, possible health consequences and unavoidable environmental impact.
Mycotoxins produced on crops, i.e., in the field (which is the case for example of DON and FUM) will remain in raw materials, even after processing, due to their heat stability (Pitt, 2014).
⇰ For example, Fusarium spp. are field fungi usually lacking the ability to grow on dry feed. However, the toxins produced by these fungi species (e.g., DON) will remain stable on the plant raw materials used to manufacture aquafeeds, and in some cases, even be redistributed and concentrated in certain milling fractions (Cheli et al., 2013) e.g., corn vs corn gluten meal (Gonçalves et al., 2018b).
Mycotoxin redistribution and transfer from crops to aquafeeds is well identified and has been observed and reported (Cheli et al., 2013; Gonçalves et al., 2018b).
The fate of mycotoxins during cereal processing, such as sorting, cleaning, milling and thermal processes has been studied by several authors. However, their level in feedstuffs is variable and affected by several factors:
- ⇰ Type of mycotoxins (polar or polar)
- ⇰ Level and extent of fungal contamination
- ⇰ Complexity of the cereal processing technology
Therefore, the use of feed preservatives may reduce the production of mycotoxins from the moment they are added on, however, they will not have any impact on mycotoxins already present in ingredients or feeds produced prior to its addition.
Mycotoxin occurrence is sometimes described to be limited to certain regions, depending on the type of mycotoxin and highly associated, as mentioned before, to the conditions for fungal growth and mycotoxin production.
Is true that environmental conditions will geographically restrict mycotoxin production, however, the globalized feed grain trade may distribute mycotoxins outside of their natural occurrence geographical areas, complicating the estimation of mycotoxin contamination in compound feed.
Some take-home messages
Not all molds produce mycotoxins and even the ones that have that capacity, may be present without producing any toxin. Thus, the confirmation of mold contamination does not mean the presence of mycotoxins.
⇰ As a result, the use of mold inhibitors does not guarantee that feed is free of mycotoxins, as they are also produced in crops and not destroyed during processing.
It is recommended that aquafeed and aquaculture producers regularly monitor raw materials and finished feeds for mycotoxin contamination, either through on-site rapid testing or through an external laboratory that may be equipped with more reliable detection system.