Importance of the factor light
Mycotoxins are toxic metabolites of filamentous fungi and pose a crucial problem for the safety of many food and feed products. According to current estimates, well over 25 percent of the annual world harvest is contaminated with mycotoxins and should be discarded. There are still no comprehensive concepts to effectively and sustainably prevent mycotoxin contamination in food. Only by knowing the optimal growth conditions for fungi and the genetic control of mycotoxin formation, effective prevention strategies can be developed.
Fungal species (or their toxins) frequently found in food and feed are Fusarium (fumonisin, trichothecenes, zearalenone), Aspergillus (ochratoxin, aflatoxins, sterigmatocystin), Alternaria (alternariol, alternariol monomethyl ether/-ethylether, altertoxin, tenuazonic acid) and Penicillium (ochratoxin, citrinin, patulin). Due to the excellent adaptability of fungi to different environments, mycotoxins are particularly clustered in the following products: Cereal products, coffee, cocoa (ochratoxin, trichothecenes); corn (fumonisins); grapes, wine (ochratoxin); spices and nuts (aflatoxin, ochratoxin); cereal products, apples, carrots, grapes, tomatoes (alternariatoxins) products from apples (patulin, citrinin).
Due to their harmful effects on health, EU-wide limits have been set for important mycotoxins. Compliance with these limits is regularly tested by random sampling and is therefore very important for the marketing of certain raw plant products or foodstuffs. However, mycotoxin formation depends on external conditions, such as storage temperature, light, humidity and pH of a food product, or internal factors, such as the growth phase and nutrient status of the fungal cell. Therefore, sometimes mycotoxin contamination cannot be detected even in visibly infested foods. Conversely, however, a food may be contaminated with mycotoxins even though it has hardly any visible fungal infestation. While chemical-analytical methods for the detection of mycotoxins represent an endpoint control, molecular biological methods allow an early control of mycotoxin formation on the genetic level.
Scientists at the Department of Safety and Quality of Fruit and Vegetables of the Max Rubner-Institut in Karlsruhe, are investigating this genetic regulation of mycotoxin formation of different fungal species on relevant foods. Various influencing factors are included in this process. In the course of ongoing research, the complete genomes of important food-relevant mold species were sequenced and comparatively analyzed. Important mycotoxin gene clusters in the fungi could be identified and investigated, and genes and signaling pathways involved in active mycotoxin formation could be characterized. For example, it was found that light of a certain wavelength can have a strong influence on growth and mycotoxin formation in filamentous fungi. Fungi can perceive light of different wavelengths and intensities via so-called light receptors, and downstream signaling cascades are influenced in their activity. These receptors were detected in Fusarium, Aspergillus, Alternaria and Penicillium species and their function was successfully verified by targeted gene silencing.
Light in the blue, white and red wavelength ranges inhibits the growth and mycotoxin formation of, for example, Penicillia and aflatoxin-forming Aspergilli, but yellow light and green light tend to promote them. Fusarium- and ochratoxin-forming aspergilli, in turn, are inhibited in other wavelength ranges because they are able to form light-protective pigments such as carotenoids and melanins. The advantage of this approach is that light in the visible wavelength range hardly causes photooxidative changes in the food, as is the case with UV light, for example, by destroying vitamins and proteins. Other physical or chemical methods of mold prevention include curing and salting, the addition of preservatives and other fungistatic substances, or acidification with ascorbic or citric acid, for example. A simple change in temperature (refrigerator) or humidity can also inhibit the germination of fungal spores to a greater or lesser extent.
It is interesting to note that fungi are not only problematic because of their partial ability to form toxic metabolites such as mycotoxins, and can thus trigger mycotoxicoses in humans and animals. Some of the mycotoxin producers are also plant or human pathogens. Inhaled spores or fungal material can cause clinical pictures such as mycogenic allergies, or can cause invasive mycoses in predisposed individuals. Fungal material and spores from moldy food disposed of in compost can infect crops and ornamentals in the garden and cause plant diseases. In addition, since some fungi are also capable of producing antibiotics, fungi can contribute to the development of antibiotic resistance.