Mycotoxin formation / fungal growth

Fungal colonization and growth and/or mycotoxin production are generally influenced by a variety of factors.9, 10 Therefore, it is not possible to describe a single set of conditions that are favorable for fungal growth and mycotoxin production. The most important conditions are temperature and water activity (available water content to a mold in a substrate). Generally, the optimal temperature for mycotoxin production by many molds range between 20 to 30°C.

The most important factors can be categorized in three groups:

  • plant and environmental factors, including substrate characteristics (e.g. composition, pH, water activity, oxygen content)
  • possible competitive actions (e.g. associated growth of other fungi or microbes)
  • climatic conditions (e.g. temperature, atmospheric humidity)

Generally, in warm (tropical and subtropical) regions aflatoxins are of major concern, while fusariotoxins, such as zearalenone or trichothecenes mainly occur in more moderate climatic regions.2, 5

Furthermore, stress factors such as drought, poor fertilization, high crop densities, weed competition, insect or mechanical damage can weaken the plant’s natural defenses and promote colonization by mycotoxin-producing fungi as well as toxin-formation.

Aspergillus flavus
Aspergillus flavus

The optimum production conditions vary according to substrate, mycotoxin type, temperature and humidity. The Figures 1 to 7 outline the available data about temperature for fungal growth and mycotoxins formation as well as water activity (aw) for fungal growth and mycotoxins formation of the main fungi, which produce mycotoxins, as these parameters mainly influence mycotoxin production.1, 4, 6, 7, 8, 9, 10, 11 But even small changes in aw-values or temperatures can already lead to large changes in the optimum growth rate of the fungi and mycotoxins, that is why the values in the Figures 1, 2, 4 and 5 should be seen as indicator and not as the only, single optimal domain.

Interaction between water activity (aw) and temperature on fungal growth and mycotoxin formation

Interactions between certain aw-values and certain temperatures on the growth of F. verticillioides strains and production of mycotoxins were outlined in the study of Medina et al. (2013).4 The study showed that with an aw of 0.995 the optimum growth rate of F. verticillioides was between 20 and 25 °C, but when the aw-value changed to 0.98 the optimum growing temperature shifted between 30 and 35 °C (Figure 3).4 Opposite conclusions could be outlined for the mycotoxin production. In fact, the optimal temperature and aw for FB1 production were 20°C and an aw of 0.98-0.995. From this it can be concluded that the optimal conditions for production of certain mycotoxins are not the same as for their growth.

Figure 1: Minimum, optimum and maximum temperature range in °C  for fungal growth.
Figure 2: 
Minimum, optimum and maximum water activity (aw) for fungal growth.
Figure 3: Effect of temperature and water activity (aw) on the growth rate of a strain of F. verticillioides. Means of five replicates per treatment (Medina et al., 2013).

Figure 4: Minimum, optimum and maximum temperature range in °C  for mycotoxins formation.
Figure 5:
Minimum, optimum and maximum water activity (aw) for mycotoxins formation.
Figure 6:
The effect of (a) water activity (aw) and (b) temperature on the fumonisin B1 production by a strain of F. verticillioides. Bars indicate standard error of the means (Medina et al., 2013).

Figure 7a:
The effect of water activity (aw) on the fumonisin B2 production by a strain of F. verticillioides. Bars indicate standard error of the means (Medina et al., 2013).
Figure 7b: The effect of temperature on the fumonisin B2 production by a strain of F. verticillioides. Bars indicate standard error of the means (Medina et al., 2013).

References
  1. HUSSEIN, H. S. & BRASEL, J. M. 2001. Toxicity, metabolism, and impact of mycotoxins on humans and animals. Toxicology, 167, 101-134.
  2. MAGAN, N., MEDINA, A. & ALDRED, D. 2011. Possible climate-change effects on mycotoxin contamination of food crops pre- and postharvest. Plant Pathology, 60, 150-163.
  3. MARTH, E. H. 1992. Mycotoxin: Production and control. Food Laboratory News, 35-51.
  4. MEDINA, A., SCHMIDT-HEYDT, M., CÁRDENAS-CHÁVEZ, D. L., PARRA, R., GEISEN, R. & MAGAN, N. 2013. Integrating toxin gene expression, growth and fumonisin B1 and B2 production by a strain of Fusarium verticillioides under different environmental factors. Journal of the Royal Society Interface, 10.
  5. PATERSON, R. R. M. & LIMA, N. 2011. Further mycotoxin effects from climate change. Food Research International, 44, 2555-2566.
  6. POPOVSKI, S. & CELAR, F. A. 2013. The impact of environmental factors on the infection of cereals with Fusarium species and mycotoxin production - A review. Acta Agriculturae Slovenica, 101, 105-116.
  7. RAMOS, A. J., LABERNIA, N., MARÍN, S., SANCHIS, V. & MAGAN, N. 1998. Effect of water activity and temperature on growth and ochratoxin production by three strains of Aspergillus ochraceus on a barley extract medium and on barley grains. International Journal of Food Microbiology, 44, 133-140.
  8. RIBEIRO, J. M., CAVAGLIERI, L. R., FRAGA, M. E., DIREITO, G. M., DALCERO, A. M. & ROSA, C. A. 2006. Influence of water activity, temperature and time on mycotoxins production on barley rootlets. Lett Appl Microbiol, 42, 179-84.
  9. RICHARD, J. L. & PAYNE, G. A. 2003. Mycotoxins in plant, animal, and human systems. Council for Agricultural Science and Technology, Task Force Report No. 139.
  10. SANCHIS, V. 2004. Environmental conditions affecting mycotoxins. In: Magan N. and Olsen M. (Eds.), Mycotoxins in food. CRC Press, Boca Raton Bosten New York, Washington, DC, 174-189.
  11. SWEENEY, M. J. & DOBSON, A. D. W. 1998. Mycotoxin production by Aspergillus, Fusarium and Penicillium species. International Journal of Food Microbiology, 43, 141-158.

Common Mycotoxins