Post on 05-Jul-2015
MYCOTOXINS
Introduction
Fungal diseases are common place in plants and animals. In such diseases, the
fungi are actively growing on and invading the body of their hosts. There is another
means by which fungi can cause harm without invading our bodies. When fungi grow on
a living organism or on stored food material that we consume, they may produce harmful
metabolites that diffuse into their food. It is believed that fungi evolved these metabolites
as a means of protecting their food supply by preventing other organisms from eating it.
These metabolites are referred to as mycotoxins, which literally mean "fungus poisons".
Fungi that produce mycotoxins do not have to be present to do harm. If a fungus is
growing for example in a grain storage silo, the environment may have become
unsuitable for the fungus and it dies. Even though the fungus is no longer alive, while it
was growing, if it produced a mycotoxin, it will have poisoned the grains.
A mycotoxin is derived from Greek words ‘mykes’, ‘mukos’ means "fungus" and
from Latin word ‘toxicum’ means "poison". It is a toxic secondary metabolite produced
by an organism of the fungus kingdom, including mushrooms, molds, and yeasts. The
term 'mycotoxin' is usually reserved for the relatively small (MW ~700), toxic chemical
products formed as secondary metabolites by fungi that readily colonize crops in the
field or after harvest.
Most fungi are aerobic (use oxygen) and are found almost everywhere in
extremely small quantities due to the minute size of their spores. They consume organic
matter wherever humidity and temperature are sufficient. One mold species may produce
many different mycotoxins and/or the same mycotoxin as another species.
Growth of fungi on animal hosts produces the diseases collectively called
mycoses, while dietary, respiratory, dermal, and other exposures to toxic fungal
metabolites produce the diseases collectively called mycotoxicoses.
The production of toxins is dependant on the surrounding intrinsic and extrinsic
environments and the toxins vary greatly in their severity, depending on the organism
infected and its susceptibility, metabolism, and defense mechanisms.
Mycotoxins can appear in the food chain as a result of fungal infection of crops,
either by being eaten directly by humans, or by being used as livestock feed. Mycotoxins
greatly resist decomposition or being broken down in digestion, so they remain in the
food chain in meat and dairy products. Even temperature treatments, such as cooking and
freezing, do not destroy mycotoxins.
Mycotoxins and Other Fungal Metabolites
While all mycotoxins are of fungal origin, not all toxic compounds produced by
fungi are called mycotoxins. The target and the concentration of the metabolite are both
important. Fungal products that are mainly toxic to bacteria (such as penicillin) are
usually called antibiotics. Fungal products that are toxic to plants are called phytotoxins
by plant pathologists.
Mycotoxins are made by fungi and are toxic to vertebrates and other animal
groups in low concentrations. Other low molecular weight fungal metabolites such as
ethanol that are toxic only in high concentrations are not considered mycotoxins. Finally,
although mushroom poisons are definitely fungal metabolites that can cause disease and
death in humans and other animals, they are rather arbitrarily excluded from discussions
of mycotoxicology. Molds (i.e., micro-fungi) make mycotoxins; mushrooms and other
macroscopic fungi make mushroom poisons.
The distinction between a mycotoxin and a mushroom poison is based not only
on the size of the producing fungus, but also on human intention. Mycotoxin exposure is
almost always accidental. In contrast, with the exception of the victims of a few
mycological accomplished murderers, mushroom poisons are usually ingested by
amateur mushroom hunters who have collected, cooked, and eaten what is misidentified
as a delectable species.
Mycotoxicoses
Mycotoxicoses is the term used for poisoning associated with exposures to mycotoxins.
The symptoms of a mycotoxicoses depend on: -
The type of mycotoxin
The concentration and length of exposure
Age, health, and sex of the exposed individual
The synergistic effects associated with several other factors such as genetics, diet,
and interactions with other toxics also have been studied. Therefore it is possible that
vitamin deficiency, caloric deprivation, alcohol abuse, and infectious disease status can
all have compounded effects with mycotoxins. In turn, mycotoxins have the potential for
both acute and chronic health effects via ingestion, skin contact, and inhalation. These
toxins can enter the blood stream and lymphatic system; they inhibit protein synthesis,
damage macrophage systems, inhibit particle clearance of the lung, and increase
sensitivity to bacterial endotoxin.
History of Mycotoxins
The existence of mycotoxins was not documented until 1960. However, the
concept that moldy food could lead to illness in people or domestic animals was long
suspected before their existence was demonstrated by science. Long ago, before there
was adequate means of long term storage for perishable goods, food was normally
consumed a short time after it was acquired, but as the world has become more
industrialized and technological advanced, storage of food has become more of an issue.
Food is now commonly stored for long periods of time, giving fungi a greater
opportunity to contaminate our food.
Before 1900, in Italy, researchers there believed consumption of moldy corn by
children led to the development of illness. Some experiments, done at that time, included
the isolation, and growth of the suspected fungus in pure culture, and isolation of toxic
compounds from the fungus that the researchers believed to be the cause of the illness.
However, since the compound was not identified and was not actually isolated from the
moldy corn, it could not be concluded that this compound was the cause of the illness or
that the compound in question was even present on the moldy corn.
There was an extensive outbreak of moldy corn disease in the southeastern
United States in the early 1950's where hundreds of wild pigs foraging in cultivated corn
fields became ill and many died.
It would not be until 1960, when approximately 100,000 turkeys and a lesser
number of other domestic birds died in England, causing losses of approximately several
hundred thousand dollars, before the first mycotoxin was isolated and identified.
Initially, the disease was thought to be caused by a virus and the syndrome was named
"turkey-X disease". The "X" here indicated that the cause of the disease was unknown.
After a great deal of research work, from their isolations, the scientists identified
Aspergillus flavus.
Chemists were also employed in this investigation, and they were able to isolate
and identify the toxin from the oil cake feed. The mycotoxin isolated was named
aflatoxin, the "a" from Aspergillus and "fla" from flavus. Feeding test of food containing
aflatoxin, with various laboratory animals, demonstrated that to varying degrees, all
animals tested were sensitive to aflatoxin. Even consumption of extremely small
amounts of aflatoxin damaged various internal organs and could induce development of
cancer to the liver.
The period between 1960 and 1975 has been termed the mycotoxin gold rush
because so many scientists joined the well-funded search for these toxigenic agents.
Some 300 to 400 compounds are now recognized as mycotoxins, of which approximately
a dozen groups regularly receive attention as threats to human and animal health.
Bioterrorism
Mycotoxins can be used as chemical warfare agents. There is considerable
evidence that Iraqi scientists developed aflatoxins as part of their bio weapons program
during the 1980s. Toxigenic strains of Aspergillus flavus and Aspergillus parasiticus
were cultured, and aflatoxins were extracted to produce over 2,300 liters of concentrated
toxin. The majority of this aflatoxin was used to fill warheads; the remainder was
stockpiled. Aflatoxins seem a curious choice for chemical warfare because the induction
of liver cancer is “hardly a knockout punches on the battlefield”. Even so, the
repugnance caused by the use of chemical and biological weapons is the kind of
emotional response that terrorists seek to elicit. Furthermore, if used against enemies, the
long-term physical and psychological results could be devastating.
Unlike the aflatoxins, trichothecenes can act immediately upon contact, and
exposure to a few milligrams of T-2 is potentially lethal. In 1981, then Secretary of State
Alexander Haig of the United States accused the Soviet Union of attacking Hmong
tribesman in Laos and Kampuchea with a mysterious new chemical warfare agent,
thereby violating the 1972 Biological Weapons Convention. The symptoms exhibited by
purported victims included internal hemorrhaging, blistering of the skin, and other
clinical responses that are caused by exposure to trichothecenes. The purported chemical
warfare agent came to be known as yellow rain.
Mycotoxins and Human Health
Toxicologists tend to concentrate their efforts on hazardous chemicals such as
polyaromatic hydrocarbons, heavy metals, and organic pesticides and they have devoted
less effort to natural products. On the other hand agriculturalists, chemists,
microbiologists, and veterinarians, who are often unfamiliar with the basic principles of
toxicology, have conducted most of the mycotoxin research.
Human exposure to mycotoxins is further determined by environmental or
biological monitoring. In environmental monitoring, mycotoxins are measured in food,
air, or other samples and in biological monitoring, the presence of residues adducts, and
metabolites are assayed directly in tissues, fluids, and excreta.
In general, mycotoxin exposure is more likely to occur in parts of the world
where poor methods of food handling and storage are common, where malnutrition is a
problem, and where few regulations exist to protect exposed populations.
Food Safety and Regulations
Mycotoxin-producing mold species are extremely common, and they can grow on
a wide range of substrates under a wide range of environmental conditions. For
agricultural commodities, the severity of crop contamination tends to vary from year to
year based on weather and other environmental factors. Aflatoxin, for example, is usually
worst during drought years; the plants are weakened and become more susceptible to
insect damage. Mycotoxins occur, with varying severity, in agricultural products all
around the world. The estimate usually given is that one quarter of the world’s crops are
contaminated to some extent with mycotoxins.
Mycotoxins can enter the food chain in the field, during storage, or at later
points. Mycotoxin problems are exacerbated whenever shipping, handling, and storage
practices are conducive to mold growth. The end result is that mycotoxins are commonly
found in foods. Scientists rank mycotoxins as the most important chronic dietary risk
factor, higher than synthetic contaminants, plant toxins, food additives, or pesticide
residues.
The economic consequences of mycotoxin contamination are profound. Crops
with large amounts of mycotoxins often have to be destroyed. Alternatively,
contaminated crops are sometimes diverted into animal feed. Giving contaminated feeds
to susceptible animals can lead to reduced growth rates, illness, and death. Moreover,
animals consuming mycotoxin contaminated feeds can produce meat and milk that
contain toxic residues and biotransformation products. Thus, aflatoxins in cattle feed can
be metabolized by cows into aflatoxin M1, which is then secreted in milk. The ability to
diagnose and verify mycotoxicoses is an important forensic aspect of the mycotoxin
problem.
Many mycotoxins survive processing into flours and meals. When mold-damaged
materials are processed into foods and feeds, they may not be detectable with out special
assay equipment. It is important to have policies in place that ensure that such “hidden”
mycotoxins do not pose a significant hazard to human health.
Since it is normally impracticable to prevent the formation of mycotoxins, the
food industry has established internal monitoring methods. Similarly, government
regulatory agencies survey for the occurrence of mycotoxins in foods and feeds and
establish regulatory limits.
Considerable research has been devoted to developing analytical methods for
identifying and quantifying mycotoxins in food and feeds. The chemical diversity of
mycotoxins and the equally diverse substrates in which they occur pose challenges for
analytical chemistry. Each group of compounds and each substrate have different
chemical and physical properties, so the methods for the separation of toxins from
substrates must be developed on a case-by-case basis. It is, for example, quite a different
matter to assay aflatoxin from peanut butter than it is to identify T-2 toxin from corn.
Mycotoxins are often produced in trace concentrations, so the sensitivity of the detection
systems is also essential.
Complete elimination of any natural toxicant from foods is an unattainable
objective. Therefore, naturally occurring toxins such as mycotoxins are regulated quite
differently from food additives. The Codex Alimentarious Commission, U.S. Food and
Drug Administration, the European Union, the Institute of Public Health in Japan, and
many other governmental agencies around the world test products for aflatoxins and
other mycotoxins and have established guidelines for safe doses.
CLASSIFICATION OF MYCOTOXINS
Mycotoxins are not only hard to define, they are also challenging to classify. Due
to their diverse chemical structures and biosynthetic origins, their myriad biological
effects, and their production by a wide number of different fungal species, classification
schemes tend to reflect the training of the person doing the categorizing.
Clinicians often arrange them by the organ they affect. Thus, mycotoxins can be
classified as hepatotoxins, nephrotoxins, neurotoxins, immunotoxins, and so forth. Cell
biologists put them into generic groups such as teratogens, mutagens, carcinogens, and
allergens. Organic chemists have attempted to classify them by their chemical structures
(e.g., lactones, coumarins); biochemists according to their biosynthetic origins
(polyketides, amino acid-derived, etc.); physicians by the illnesses they cause (e.g., St.
Anthony’s fire, stachybotryotoxicosis), and mycologists by the fungi that produce them
(e.g., Aspergillus toxins, Penicillium toxins).
Some 300 to 400 mycotoxins are now recognized, of which the following groups
regularly receive attention as threats to human and animal health.
1. Aflatoxin
2. Citrinin
3. Ochratoxin
4. Ergot Alkaloids
5. Patulin
6. Fumonisins
7. Trichothecenes
8. Zearalenone
1) AFLATOXIN
Aflatoxins are naturally occurring mycotoxins that are produced by many species
of Aspergillus, such as Aspergillus flavus and A. parasiticus. A. bombycis, A.
ochraceoroseus, A. nomius, and A. pseudotamari are also aflatoxin-producing species,
but they are encountered less frequently. But only about half of A. flavus strains produce
aflatoxins.
Aflatoxins are potent toxic, carcinogenic, mutagenic, immunosuppressive agents,
produced as secondary metabolites by the fungus. After entering the body, aflatoxins are
metabolized by the liver to a reactive intermediate, aflatoxin M1, an epoxide. Aflatoxins
are soluble in methanol, chloroform, actone, acetonitrile.
There are 18 different types of aflatoxins that are identified. Aflatoxin B1 is
considered the most toxic a potent carcinogen. It has been directly correlated to adverse
health effects, such as liver cancer. The presence of Aspergillus in food products does
not always indicate that harmful levels of aflatoxin are also present; it does imply a
significant risk in consumption.
Some important types of aflatoxins are: -
Aflatoxin B1 & B2 are produced by Aspergillus flavus and A. parasiticus
Aflatoxin G1 & G2 are produced by Aspergillus parasiticus
Aflatoxin M1 is metabolite of aflatoxin B1 in humans and animals
Aflatoxin M2 is metabolite of aflatoxin B2 in milk of cattle fed on contaminated
foods
Aflatoxicol is produced by A. flavus
Aflatoxin GM1 is also produced by A. flavus
Parasiticol is also produced by A. flavus
Substrates
Aflatoxin producing members of Aspergillus are common and widespread in
nature. They can colonize and contaminate grains before harvest or during storage. Host
crops are particularly susceptible to infection by Aspergillus following prolonged
exposure to a high humidity environment or damage from stressful conditions such as
drought, a condition which lowers the barrier to entry.
The native habitat of Aspergillus is in soil, decaying vegetation, hay, and grains
undergoing microbiological deterioration and it invade all types of organic substrates
whenever conditions are favorable for its growth. Favorable conditions include 7%
moisture content and high temperature.
Aflatoxins are largely associated with commodities produced in the tropics and
subtropics include cereals (maize, sorghum, pearl millet, rice, wheat), oilseeds (peanut,
soybean, sunflower, cotton), spices (peppers, black pepper, coriander, turmeric, ginger),
and tree nuts (almond, pistachio, walnut, coconut).
Aflatoxins M1, M2 are originally discovered in the milk of animals which are fed
contaminated feed. They metabolically bio-transform aflatoxin B1 into a hydroxylated
form called aflatoxin M1. All sources of commercial peanut butter also contain minute
quantities of aflatoxin.
Mode of Action
Biological activity of aflatoxins in liver cell is represented as: -
1. Interaction with DNA and inhibition of the polymerase responsible
for DNA and RNA synthesis
2. Suppression of DNA synthesis: The inhibition of nucleic acid
biosynthesis may be by inactivation of the synthetic enzyme systems
or because the DNA molecules present in the injured cell no longer
act as suitable models for their duplication.
3. Reduction of RNA synthesis and inhibition of messenger RNA: By
interacting in the first place with DNA, aflatoxins may also affect
the biosynthesis of RNA by preventing the transcription of DNA by
RNA polymerase.
4. Reduction of protein biosynthesis
Due to the biological activity of aflatoxins in liver cell, results can be:
Formation of the so-called fatty liver, connected with the loss of
ability to remove fats from the liver
Coagulopathy caused by inhibition of prothrombin synthesis
Reduced immuno function
Toxicity
The diseases caused by aflatoxin consumption are loosely called aflatoxicoses.
High level aflatoxin exposure produces an acute hepatic necrosis, resulting later in
cirrhosis or carcinoma of the liver. Acute hepatic failure is manifested by hemorrhage,
edema, mental changes and coma. It also results in alteration in digestion, absorption and
metabolism of nutrients. It may also result in death of the infected individual.
In 2004 in Kenya 125 people died and nearly 200 others were treated after eating
aflatoxin contaminated maize. The deaths were mainly associated with homegrown
maize that had not been treated with fungicides or properly dried before storage.
It is observed in male rats that LD50 of aflatoxin B1 is 5.55 mg/kg. It is also noted
that female rat is less sensitive than male rat. LD50 of aflatoxin B1 for female rat is 7.44
mg/kg.
Acceptable Level in Food
The Codex Alimentarious Commission regulates the aflatoxin concentration in
food and feed because of their toxic effects. They proposed that in peanuts maximum
limit of aflatoxin should be 15 µg / kg. The level of aflatoxin should not exceed from 20
ppb (ug/kg) in food commodities. The maximum limit of aflatoxin M1 in milk should be
0.5 µg / kg.
2) CITRININ
Citrinin is a mycotoxin that was first isolated from Penicillium citrinum prior to
World War II, but has also been identified in over a dozen species of Penicillium and
several species of Aspergillus (Aspergillus terreus and Aspergillus niveus). Some of
these species are used to produce human foodstuffs such as cheese (Penicillium
camemberti), sake, miso, and soy sauce (Aspergillus oryzae).
Substrates
It is associated with many other foods such as wheat, rice, corn, barley, oats, rye,
etc. More recently, citrinin has also been isolated from Monascus ruber and Monascus
purpureus, industrial species used to produce red pigments.
Toxicity
Citrinin has been implicated as a contributor to porcine nephropathy. Citrinin acts
as a nephrotoxin in all animal species tested, but its acute toxicity varies in different
species. The LD50 for ducks is 57 mg/kg; for chickens it is 95 mg/kg; and for rabbits it is
134 mg/kg. Rats are killed by parental administration of 35-58 mg/kg of citrinin. Oral
LD50 in rats is 50 mg/kg. Citrinin can also act synergistically with ochratoxin A to
depress RNA synthesis in marine kidneys.
Citrinin is also associated with yellow rice disease in Japan. Although citrinin is
regularly associated with human foods, its full significance for human health is
unknown.
3) OCHRATOXIN
Ochratoxin was discovered as a metabolite of Aspergillus ochraceus in 1965
during a large screen of fungal metabolites that was designed specifically to identify new
mycotoxins. Shortly thereafter, it was isolated from a commercial corn sample in the
United States and recognized as a potent nephrotoxin. Ochratoxin is a mycotoxin that
comes in three secondary metabolite forms, A, B, and C.
Members of the ochratoxin family have been found as metabolites of many
different species of Aspergillus, including Aspergillus alliaceus, A. auricomus, A.
carbonarius, A. glaucus, A. melleus, and A. niger. It is also observed that Penicillium
verrucosum, a common contaminant of barley, is the only confirmed ochratoxin producer
in genus Penicillium.
The three forms differ in that Ochratoxin B (OTB) is a non-chlorinated form of
Ochratoxin A (OTA) and that Ochratoxin C (OTC) is an ethyl ester form Ochatoxin A.
Substrates
Aspergillus ochraceus is found as a contaminant of a wide range of commodities
including beverages such as beer and wine. A. carbonarius is the main species found on
vine fruit, which releases its toxin during the juice making process. Ochratoxin A has
been found in barley, oats, rye, wheat, coffee beans, and other plant products, with barley
having a particularly high likelihood of contamination. It can also be accumulated in the
meat of animals. Thus meat and meat products can be contaminated with this toxin.
The level of ochratoxin produced is influenced by the substrate on which the
molds grow as well as the moisture level, temperature, and presence of competitive
microflora.
Mode of Action
Toxic effects of ochratoxin on the cellular level are connected with the enzymes
of glucose metabolism and of anion transport, leading to intercellular alkalinization.
Ochratoxin interferes with the enzyme phosphorylase causing an increase of glycogen in
the liver. This inhibitory effect is due to competition of the toxin with 3’, 5’ cyclic AMP.
Toxicity
Of the Aspergillus toxins, only ochratoxin is potentially as important as the
aflatoxins. The kidney is the primary target organ. Ochratoxin A is a nephrotoxin to all
animal species studied to date. In addition to being a nephrotoxin, animal studies indicate
that ochratoxin A is a liver toxin, an immune suppressant, a potent teratogen, and a
carcinogen. It causes tumors in the human urinary tract, although research in humans is
limited by confounding factors.
Ochratoxin has been detected in blood and other animal tissues and in milk,
including human milk. It is frequently found in pork intended for human consumption.
Ochratoxin is believed to be responsible for a porcine nephropathy that has been studied
intensively in the Scandinavian countries. In addition, ochratoxin is associated with
disease and death in poultry. Oral LD50 for rats is 59 mg/kg and for dogs it is 0.2 mg/kg.
Acceptable Level in Food
The Codex Alimentarious Commission proposes the maximum level ochratoxin
in wheat, barley, rye and derived products i-e 5 µg / kg. EU suggests a limit of 5 µg / kg
in raw cereals and a limit of 3 µg / kg for processed cereals and 10 µg / kg in dried vine
fruits.
4) ERGOT ALKALOIDS
The ergot alkaloids are among the most fascinating of fungal metabolites. These
compounds are produced as a toxic cocktail of alkaloids in the sclerotia (formation of
compact mass) of species of Claviceps, which are common pathogens of various grass
species. Ergotamine is the principal alkaloid produced by the ergot fungus, Claviceps
purpurea.
Pharmaceutical Use
It is used medicinally for treatment of acute migraine attacks (sometimes in
combination with caffeine), and to induce childbirth and prevent post-partum
haemorrhage.
Mode of Action
Ergot alkaloids are known for their negative effects neuroreceptors. The primary
effect of ergot alkaloids is stimulation of smooth muscle. Ergot alkaloids bind alpha-
adrenoreceptors and further inhibit beta-adrenoreceptors which results in
vasoconstriction. Ergot alkaloids also have been shown to inhibit prolactin secretion in
humans and animals. This effect is attributed to stimulation of the dopamine receptors,
which regulate prolactin.
Toxicity
Human ergotism was common in Europe in the middle Ages in which slow
nervous fever usually occurred in the summer and fall after a severe winter. The
ingestion of ergot sclerotia from infected cereals, commonly in the form of bread
produced from contaminated flour, cause ergotism the human disease historically known
as St. Anthony’s fire.
Two forms of ergotism are usually recognized, gangrenous and convulsive. The
gangrenous form affects the blood supply to the extremities, while convulsive ergotism
affects the central nervous system. Other symptoms that are observed include vomiting,
diarrhea, abdominal distress, headache and general feeling of illness. Modern methods of
grain cleaning have significantly reduced ergotism as a human disease.
5) PATULIN
Patulin is produced by many different molds but was first isolated as an
antimicrobial active principle during the 1940s from Penicillium patulum (later called
Penicillium urticae, now Penicillium griseofulvum). The same metabolite was also
isolated from other species and given the names clavacin, claviformin, expansin, mycoin
c, and penicidin. Patulin is a toxin produced by the P. expansum, Aspergillus,
Penicillium, and Paecilomyces fungal species.
Substarte
Penicillium expansum, the blue mold that causes soft rot of apples, pears,
cherries, and other fruits, is recognized as one of the most common offenders in patulin
contamination. Patulin is also regularly found in unfermented apple juice. It is destroyed
by the fermentation process and so is not found in apple beverages, such as cider.
Mode of Action
In patulin the mechanism of adverse effect is due to the covalent binding of
patulin to cellular nucleophiles, particularly proteins and SH-groups of glutathione. As a
result, covalently cross-linked, over thiol and aminogroups, essentially denatured
proteins such as inhibited protein tyrosin phosphatase are formed.
Pharmaceutical Use
It is tested as both a nose and throat spray for treating the common cold and as an
ointment for treating fungal skin infections. It became apparent that, it shows
antibacterial and antiviral properties.
Toxicity
Patulin is also toxic to both plants and animals, precluding its clinical use as an
antibiotic. Although patulin has not been shown to be carcinogenic, it has been reported
to damage the immune system in animals.
Acceptable Level in Food
In 2004, the EU set limits to the concentrations of patulin in food products. They
currently stand at 50 μg / kg in all fruit juice concentrations, at 25 μg / kg in solid apple
products used for direct consumption and at 10 μg / kg for children's apple products,
including apple juice. Codex Alimentarious Commission suggests 50 μg / liter maximum
limit of patulin in apple juice and apple juice ingredients in ready made soft drinks.
6) FUMONISINS
Fumonisins were first described and characterized in 1988. The most abundantly
produced member of the family is fumonisin B1. They are thought to be synthesized by
condensation of the amino acid alanine into an acetate-derived precursor. Fumonisins are
produced by over 50 species of Fusarium, notably Fusarium verticillioides, Fusarium
proliferatum, and Fusarium nygamai, as well as Alternaria alternata.
Some of the other major types of Fusarium toxins include: -
Beauvercin
Enniatins
Butenolide
Equisetin
Fusarins
Substrate
It infects the grain of developing cereals such as wheat and maize. It is present in
high levels in corn meal and corn grits.
Mode of Action
Fumonisins are both cytotoxic and carcinogenic to animals. The modes of such
actions, however, are not completely understood. However, it is demonstrated that
fumonisin B1 disrupts sphingolipid metabolism by inhibiting sphingosine N-
acyltransferase (ceramide synthase) in rat liver microsomes. It also has been shown that
fumonisin B1 inhibits other intracellular enzymes including protein phosphatases and
argino-succinate synthetase (Jenkins et al., 2000). Therefore, FB1 exerts its cytotoxicity
by inhibiting sphingolipid metabolism, protein metabolism, and the urea cycle. The
carcinogenic role of fumonisin B1 has been linked to the accumulation of sphingoid
bases that cause unscheduled DNA synthesis.
Toxicity
Fumonisins affect animals in different ways by interfering with sphingolipid
metabolism. They cause leuko-encephalomalacia (hole in the head syndrome) in equines
and rabbit, pulmonary edema and hydro-thorax in swine, and hepatotoxic and
carcinogenic effects and apoptosis in the liver of rats. In humans, there is a probable link
with esophageal cancer. The occurrence of fumonisin B1 is correlated with the
occurrence of a higher incidence of esophageal cancer in regions of Transkei (South
Africa), China, and northeast Italy.
The dose of fumonisins in the range of 5-15 mg / kg cause severe inflammation
of the intestinal mucosa and fatty degeneration of the liver in albino rats.
Acceptable Level in Food
The maximum level fumonisins in cereals should be 5 µg / kg as proposed by
Codex Alimentarious Commission.
7) TRICHOTHECENES
The trichothecenes constitute a family of more than sixty metabolites produced
by a number of fungal genera, including Fusarium, Myrothecium, Phomopsis,
Stachybotrys, Trichoderma, Trichothecium, and others. The term trichothecene is
derived from trichothecin, which was the one of the first members of the family
identified. They are commonly found as food and feed contaminants. The trichothecenes
are extremely potent inhibitors of eukaryotic protein synthesis; different trichothecenes
interfere with initiation, elongation, and termination stages.
Diacetoxyscirpenol, deoxynivalenol, and T-2 are the best studied of the
trichothecenes produced by Fusarium species. The macrocyclic trichothecenes are
produced largely by Myrothecium, Stachybotrys, and Trichothecium species.
Substrate
Deoxynivalenol is one of the most common mycotoxins found in grains. It is the
most prevalent and is commonly found in barley, corn, rye, safflower seeds, wheat, and
mixed feeds.
Stachybotrys grows well on all sorts of wet building materials with high cellulose
content, for example, water-damaged gypsum board, ceiling tiles, wood fiber boards, and
even dust-lined air conditioning ducts.
Mode of Action
Cytotoxicity of trichothecenes has been attributed to their potent inhibition of
protein, RNA, and DNA synthesis. When trichothecene binds to active polysomes and
ribosomes, the peptide linkages are interrupted, the initiation and termination sequences
are diminished, and the ribosomal cycle is disrupted.
Toxicity
It has been hypothesized that T-2 and diacetoxyscirpenol are associated with a
human disease called alimentary toxic aleukia. The symptoms of the disease include
inflammation of the skin, vomiting, and damage to hematopoietic tissues. The acute
phase is accompanied by necrosis in the oral cavity, bleeding from the nose, mouth, and
vagina, and central nervous system disorders.
When deoxynivalenol is ingested in high doses by agricultural animals, it causes
nausea, vomiting, and diarrhea. For this reason, deoxynivalenol is sometimes called
vomitoxin or food refusal factor. It is less toxic than many other major trichothecenes.
Stachybotryotoxicosis was first described as an equine disease of high mortality
associated with moldy straw and hay. Until recently, human stachybotryotoxicosis is
considered a rare occupational disease limited largely to farm workers who handle moldy
hay. The presence of Stachybotrys has been associated with pulmonary bleeding in
infants.
Rats can tolerate the intravenous injection of 5 mg trichothecene per kg but are
affected by a dose of 12.5 mg / kg and killed by 500 mg / kg.
8) ZEARALENONE
Zearalenone is a secondary metabolite from Fusarium graminearum. The
reduced form of zearalenone, zearalenol, has increased estrogenic activity.
Substrate
This toxin is found almost entirely in grains and in highly variable amounts
ranging from a few nanograms per gram to thousands of nanograms per gram. The
appearance of mold on grain plants cannot be relied upon to warn of toxin production
because Fusarium-infected grain does not necessarily appear visibly moldy in the
presence of high concentrations of mycotoxins
Pharmaceutical Use
Zearalenone has also been used to treat postmenopausal symptoms in women and
both zearelanol and zearalenone have been patented as oral contraceptives.
Mode of Action
Interaction of zearalenone with estrogen receptors is followed by transportation
of the estrogen-receptor complex into the cellular nucleus, conjugation with chromatin
receptors and a selective transcription of RNA. It results in a number of biochemical
effects like an increase of the muscular content of water and decrease of the lipid
content; enhancing of uterus permeability in relation to glucose, RNA and pre-proteins.
Toxicity
Genotoxicity is a reported concern with respect to zearalenone. Recently,
endocrine (hormone) disrupters have received a lot of public attention. Zearalenone, has
attracted recent attention due to concerns that environmental estrogens have the potential
to disrupt sex steroid hormone functions. Occasional outbreaks of zearalenone
mycotoxicosis in livestock are known to cause infertility.
Acceptable Level
The recommended safe human intake of zearalenone is estimated to be 0.05
µg/kg of body weight per day. Zearalenone levels in foodstuffs are not yet regulated
anywhere.
Other Mycotoxins
Penicillium roqueforti and Penicillium camemberti, species used to manufacture
mold-ripened cheeses, produce a number of toxic metabolites, including penicillin acid,
roquefortine, isoflumigaclavines A and B, PR toxin, and cyclopiazonic acid.
The yellow rice toxins (citrinin, citreoviridin, luteoskyrin, rugulosin, rubroskyrin,
and related compounds) are believed to have exacerbated “Shoshin-kakke”, a particularly
malignant form of beriberi seen in Japan in the early 20th century.
A number of rare and obscure diseases have been hypothesized to be possible
mycotoxicoses, often on extremely meager evidence. These include Kashin-Beck disease
in Russia, mselini joint disease and onylalai in Africa, endemic familial arthritis of
Malnad in India, frontoethmoidal encephalomenin-gocele in Myanmar, sago hemolysis
in Papua New Guinea, and deteriorated sugar cane poisoning in China.
MYCOTOXIN CONTROL STRATEGIES
Methods for controlling mycotoxins are largely preventive. They include good
agricultural practice and sufficient drying of crops after harvest. There is considerable
on-going research on methods to prevent pre-harvest contamination of crops. These
approaches include developing host resistance through plant breeding and through
enhancement of antifungal genes by genetic engineering, use of bio control agents, and
targeting regulatory genes in mycotoxin development. As of now, none of these methods
has solved the problem. Because mycotoxins are “natural” contaminants of foods, their
formation is often unavoidable. Many efforts to address the mycotoxin problem simply
involve the diversion of mycotoxin contaminated commodities from the food supply
through government screening and regulation programs.
In order to control mycotoxin contamination of foods and food products the
following chemical and physical (processing) measures should be adopted.
1. GOOD AGRICULTURAL PRACTICES (GAPS) /
GOOD MANUFACTURING PRACTICES (GMPS)
The first line of defense against the introduction of mycotoxins is at the farm
level and starts with implementation of good agricultural practices to prevent infection.
Preventive strategies should be implemented from pre- through post harvest.
Pre-harvest strategies include maintenance of proper planting/growing conditions
(for example, soil testing, field conditioning, crop rotation, irrigation), antifungal
chemical treatments (for example, propionic and acetic acids), and adequate insect and
weed prevention. Harvesting strategies include use of functional harvesting equipment,
clean and dry collection/transportation equipment, and appropriate harvesting conditions
(low moisture and full maturity).
Post harvest measures include use of drying as dictated by moisture content of the
harvested grain, appropriate storage conditions, and use of transport vehicles that are dry
and free of visible fungal growth. While implementation of these precautions goes a long
way toward reducing mycotoxin contamination of foods, they alone do not solve the
problem and should be an integral part of an integrated HACCP-based management
system.
2. HACCP
Inclusion of mycotoxin control in HACCP plans, an important aspect of an
overall management approach, should include strategies for prevention, control, and
quality from farm-to-fork. In the food industry, post harvest control of mycotoxins has
been addressed via HACCP plans, which include use of approved supplier schemes.
Implementation at pre-harvest stages of the food system needs more attention. Such
action provides a critical front-line defense to prevent introduction of contaminants into
the food and feed supplies. Pre-harvest HACCP programs have been documented for
controlling aflatoxin in corn and coconuts in Southeast Asia, peanuts and peanut
products in Africa, nuts in West Africa, and patulin in apple juice and pistachio nuts in
South America.
3. BIOLOGICAL CONTROL MEASURES
Levels of mycotoxins have reduced in the field and in storage without
intervention. It includes: -
Degradation mechanisms resulting in reduced mycotoxin levels in the field.
Limited research on trichothecenes has suggested the possibility that the
mycotoxin may be metabolized by corn enzymes; and
Decline in trichothecenes levels in grains stored at −18°C to 4°C and
trichothecenes at temperatures greater than 0°C.
The potential for using microorganisms to detoxify mycotoxins has shown
promise. Exposure of trichothecenes to microbes contained in the contents of the large
intestines of chickens completely transformed it in vitro to de-epoxy-DON
(deoxynivalenol, type of trichothecenes), which is 24 times less toxic than DON itself.
4. TRANSGENIC APPROACHES
Current research efforts are focusing on methods to prevent infection at the pre-
harvest stage with emphasis on mechanisms by which the affected plants may inhibit
growth of molds or destroy mycotoxins that they produce. Traditional grain-breeding
strategies to select for preferred genetic traits have been conducted for many years. There
has been limited success with this approach to Fusarium graminearum and Aspergillus
flavus. There are hybrids currently in use that limit mycotoxin production; however, the
potential to reach unacceptable levels remains. Fumonisin production has received less
attention from researchers.
Traditional methods are plagued by many hurdles, however, including
inconsistent, labor-intensive inoculation techniques, lack of single genes and resistant
control genotypes, and the financial implications of evaluating results.
Genetic modification of mold-susceptible plants holds great promise for
controlling this food safety issue. One approach involves increasing production of
compounds (for example, anti-fungal proteinsor secondary metabolites, such as
hydroxamic acids, phenolics, stilbenes) that reduce infection by the microorganism. This
may be accomplished by introducing a novel gene to express the target compound.
Another option is to enhance expression of such a compound by the existing
gene, there by capitalizing on the plant’s own defense mechanisms. For example,
enzymes that catalyze production of anti-fungal could be targeted for expression.
Alternatively, genetic engineering methods to increase production of enzymes that
degrade mycotoxins are also being pursued. Efforts are also under way to engineer plants
to produce compounds that disrupt mycotoxin synthesis. For example, enhanced
expression of α-amylase inhibitor in Aspergillus spp. could result in significantly reduced
aflatoxin levels.
Another avenue for reducing mycotoxin levels would be to reduce insect injury to
plant kernels. Insects play an important role in the proliferation of mold growth in the
field and in storage. Resistance developed through the use of several Bt (Bacillus
thermophilus) genes in corn, wheat, and other cereal grains to minimize insect damage
has led to effective reduction in Fusarium ear rot (F. verticillioides and F. proliferatum)
mycotoxin levels in grain.
CONCLUSION
The variability in mycotoxin contamination and the potential for novel
mycotoxicoses to emerge make the prospects for ongoing significant human
mycotoxicoses likely, especially in low-income countries in which surveillance is less
available because of economical and technological constraints. The human health
consequences of acute aflatoxicosis alone range from death to exacerbated malnutrition,
devastating to the affected populations. Very little is known about the effects of long-
term low-level exposure, especially with regard to co-contamination with multiple
mycotoxins.
Thus, development of low-tech, inexpensive methods for mycotoxin surveillance
is a world health imperative. With several novel approaches being developed, such as
molecular imprint polymers and immuno-assays and bio-assays, adoption of such
methods is within reach. The prevention of mycotoxin contamination of human foods
could have a significant effect on public health in low-income countries, and deserves
significant attention. The food industry should take the lead in these efforts, because it
will lead to improved economic sustainability of the industry, enhanced food safety
efforts, enhanced international trade efforts, and improved public health.
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