AN OVERVIEW OF MYCOTOXIN DIVERSITY, TOXICITY, AND

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AN OVERVIEW OF MYCOTOXIN DIVERSITY, TOXICITY, AND

REGULATION

http://www.organic-center.org/reportfiles/Mycotoxin_SSR.pdf

Fungi are ubiquitous in the environment and primarily utilize

nonliving organic matter as sources

of energy for growth and reproduction. Fungal growth is typically

triggered by wet conditions and

is accelerated by heat, moisture stress, and humidity. Some fungi

require continued wet conditions to

thrive, others keep growing after moisture levels drop.

There are many different species of fungi. Each has evolved to

exploit a given combination

of environmental conditions. For this reason, wherever there is

heat, some moisture, and a source of

nonliving organic matter (i.e. decomposing crop residues), there

will be fungi.

Why Do Fungi Produce Mycotoxins?

When a fungal spore comes into contact with organic material, it

sends out filament-like

structures called hyphae, which help attach the fungus to its new

home. When the fungus senses

conditions are right (a trigger often linked to moisture levels), it

initiates the decomposition process by

secreting enzymes into its new food source. These enzymes break down

complex organic molecules

in the host tissues into simpler molecules that are more readily

available to the fungi, as well as to other

microorganisms.

As fungal growth breaks down host material, the digested nutrients

are classified into two

categories: primary and secondary metabolites. Primary metabolites

encompass cellulose and other carbon-based compounds

that are used by the fungi and other microorganisms for growth and

reproduction.

The secondary metabolites produced by fungi during the course of

digestion are called

mycotoxins. Fungi produce these biochemicals for a wide array of

reasons, many of which remain

unknown. Mycotoxin production tends to increase when fungal growth

rates slow down and as

fungi move toward dormancy. In such instances, mycotoxin production

appears to be a defensive

reaction. The purpose of the mycotoxins might be to combat the

factors reducing the growth rate of

fungi. Alternatively, fungi may produce mycotoxins to protect

dormant molds and fungal spores from

other, surviving fungal species and bacteria. Perhaps mycotoxins

help protect molds from adverse

environmental conditions (too cold or dry), or from the lack of some

essential nutrient in the substrate

on which the mold is growing.

The strong bitter taste of moldy plant matter and tendency to induce

nausea on species higher

up the food chain may protect the infected material from being

consumed by animals, including humans.

It is obviously an advantage to fungi that humans tend to discard

moldy food and feed, and that animals

learn to avoid spoiled food. All these explanations of why fungi

produce mycotoxins share a common

denominator – attempts by fungi to survive and thrive by gaining or

retaining a competitive edge within

environments crowded by a host of organisms trying to thrive off

available moisture, warmth, and

nutrients.

Overview of Mycotoxins and the Fungi Producing Them

Fungi vary greatly in terms of what organisms and tissues they

attack and how. Mycotoxins are

a byproduct of fungal growth. Mycotoxin potency and production

varies widely across classes of fungi.

Higher organisms, including humans, are not specific targets of

molds or mycotoxins, although one

potential role of mycotoxins may be to deter animals from eating

infected food. Still, both farm animals

and people periodically fall victim to the biochemical warfare waged

between species of fungi and

bacteria competing to exploit the nutrients bound up in dead and

decaying organic matter.

Fungal hyphae on a leaf. Photo coutesy of MacKenzie,

University of Aberdeen ©

" Mycotoxins produced by fungi play a major role in the biochemical

warfare

that unfolds among competing species in virtually every environment

on Earth "

Different fungi produce different types and levels of mycotoxins

depending upon the substrate

the mold is growing on. Molds are nothing more than growing masses

of fungi. In most cases,

mycotoxins are produced and transmitted within the spores created by

molds. The viability of the next

generation of fungi is entirely dependent on the competitiveness and

environmental fate of spores.

Therefore, it makes sense that fungi would provide their spores with

as many biochemical tools as

possible, some of which are classified as mycotoxins, and that

mycotoxin production and levels would

somehow be linked to factors placing fungi under stress.

The fungal species of Aspergillus, Penicillium, Fusarium,

Alternaria, and Claviceps are the main

producers of mycotoxins that periodically pose food safety risks

(Steyn, 1995), as shown in Table 1.

The fungal species within each genera of fungi can produce multiple

mycotoxins, and each mycotoxin

is designed to play a unique role, or roles, in response to specific

combinations of circumstances. This

report focuses on the mycotoxins that appear most frequently in

food: aflatoxins, ochratoxin, fumonisins,

deoxynivalenol, patulin, and the ergot alkaloids.

Mycotoxins are classified in a number of different ways ( et

al., 2003). Physicians tend to

classify mycotoxins in relation to the illnesses they cause, while

medical researchers focus on the organs

they affect in humans – neurotoxins, immunotoxins, hepatotoxins,

nephrotoxins, etc. Organic chemists

favor categories based on chemical structures, while biochemists

prefer to focus on the biosynthetic

pathways that produce the mycotoxins. Mycologists prefer to classify

mycotoxins based on the fungi that

produce them.

Fungi produce compounds other than mycotoxins that are toxic to

other organisms. Some

secondary metabolites of fungi are antibiotics, while others play a

role in triggering plant diseases and are

called phytotoxins. Fungi also produce ethanol, which is not

regarded as toxic at the levels produced in

the natural environment.

Major Mycotoxins Posing Food Safety Risks