Iniyalakshimi.B.R., M.Sc (Agri)
A toxin (Greek toxikon)
poisonous substance produced by living cells or organisms
small molecules, peptides, or proteins that capable of
causing disease on contact with or absorption by body tissues
interacting with biological macromolecules (enzymes or
severity: minor and acute to almost immediately deadly (as
in botulinum toxin).
Microbial toxins are toxins produced by microorganisms,
including bacteria, viruses and fungi.
Different Types of Microbial Toxins
Exotoxins : generated by the bacteria and actively
Endotoxins :are part of the bacteria itself( bacterial
outer membrane) and it is not released until the bacteria
is killed by the immune system.
Clostridium tetani :
rod-shaped, anaerobic , Gram-positive
found as spores in soil or in the gastrointestinal tract of
biological toxin, tetanospasmin
the causative agent of tetanus, a disease characterized
by painful muscular spasms that can lead to respiratory
produces neurotoxins, known as botulinum neurotoxins types
that cause the flaccid muscular paralysis seen in botulism.
Obligate anaerobic spore-former, endospores
commonly found in soil.
Gram-negative, rod-shaped ,an obligate aerobe
has multiple flagella
found in the soil and in water.
strains (CHA0 or Pf-5, ) present biocontrol properties, protecting the roots of
some plant species against parasitic fungi such as Fusarium or Pythium, as
well as some phytophagous nematodes.
produce the secondary metabolite 2,4-diacetylphloroglucinol
Fungi produces two categories of toxins.
One of those produced by some mushrooms (such as Amanita
phalloids, Cortinarius orellanus, Coprinus atrementarius, Inocybe
sp., etc) which are consumed directly as food.
The poisoning caused by mushrooms is called “mycetismus”.
Second type of toxins is those produced by certain molds, which
grow on other food products.
These are reffered as “mycotoxins”. Mycotoxins may be mutagenic
or carcinogenic and may display specific organ toxicity.
Mycotoxins are produced by fungi as secondary metabolites at the
end of the exponential growth phase.
Mycotoxins are generally synthesized via polypeptide route.
Induction of mycotoxicoses i.e. disease of animals and humans caused
by the consumption of feed and food invaded by toxin producing fungi,
thus pose serious threat to human and animal health.
Most mycotoxicoses are caused by common and wide spread fungi as
Aspergillus, Fusarium, Penicillum and Stachybotrys. Aspergillus and
Penicillum produces toxins mostly in stored seeds, hay or commercially
processed food and feeds.
Fusarium produces toxins on corn and other stored grains and
Stachbotrys on straw, hay and other cellulose products used used as
fodder or bedding for animals.
Aflatoxins are naturally occurring mycotoxins. Aflotoxins may
be produced in infected cereal grains, most legumes, groundnut
and other seeds.
The moisture content in the seed or grains is the most
important factor determining whether the aflotoxins producing
fungus will grow or not.
Moisture content >14% favours fungal growth. Aflotoxin , a
polyketide toxin, is a metabolic product of Aspergillus flavus ,
Aspergillus parasiticus and other Aspergillus species.
The polyketides are derived from acetyl CoA and melonyl
CoA. Aflotoxins consists of several chromatographically
distinguishable components such as aflotoxin B1 ,B2, G1,G2,etc.
The main target organ in mammals is the liver so aflatoxicosis
is primarily a hepatic disease.
Several species of Fusarium produce zearalenone and trichothecene
toxins in molded corn. zearalenone (F-2 mycotoxin) is produced by
F.roseum, F.graminearum, F.oxisporum, F.moniliforme and F.tricinctum.
It causes ‘estrogenic syndrome’ in swine.
These are produced by Claviceps purpurea and other Claviceps
species which grow in the heads of cereal grains, produce sclerotinia
and replace grains with the bodies containing ergot alkaloids.
These fungi produce an extensive series of alkaloids , based on the
aminoacid trptophan, dimethyl-allyl pyrophosphate and methyl
methionine and form ergoline ring structure.
The alkaloids are structurally related to lysergic acid diethylamide
(LSD),a well known psycho-active drug. Ergots cause abortion and
smooth muscle contraction which sometimes may lead to the loss of
It is polyketide carcinogenic toxin produced by Penicillium
urticae, P.patulum, P.griseofulvum, P.claviforme, etc,.
It is biosynthesized through acetate – malonate pathway. It is
toxic to bacteria and some fungi, higher plants and animals.
These toxins are produced thye species of stachybotrys on
straw, hay, other fodders and animal feeds, etc. stachybotrys
toxins cause diseases like haemorrhage, necrosis and general
cell damage in domesticated animals.
Several species of Alternaria produce toxins that have
been found in apple, tomato , blue berry, etc. the toxins
produced include alternariol, monomethl ether, alternune,
tenuazoic acid and altertoxin.
Aspergillus and Penicillium produce many other kinds of
toxins such as yellow rice toxins and tremorgenic toxins.
Ochratoxins produced by A.ochraceus causes fatty liver
Effects of Mycotoxins(aflatoxin B1 )
Mycotoxins (100 or 200 μg kg−1 soil) significantly decreased nodule
number, nodule fresh weight and total nitrogenase activity, leading to
reductions in dry matter accumulation and nitrogen yield of the bean
suppressed specific nitrogenase activity.
NADH-dependent glutamate dehydrogenase (NADH-GDH) as well
as glutamate synthase (NADH-GOGAT) activities.
inhibited synthesis of leghaemoglobin, carbohydrate and protein in
the nodule cytosol.
interference with normal nodule physiology and function.
Bacillus thuringiensis (Bt)
Gram-positive, soil-dwelling bacterium
naturally in the gut of caterpillars ( moths, butterflies, )
aerobes capable of producing endospores.
During sporulation produce crystal proteins (proteinaceous
inclusions), δ-endotoxins ( crystal proteins or Cry proteins),
encoded by cry genes(located on the plasmid)
specific activities against insect species of orders
Lepidoptera (moths and butterflies),
Diptera (flies and mosquitoes),
hymenoptera(wasps, bees, ants and sawflies)
use as insecticides, genetically modified crops using Bt
biological alternative to a pesticides.
many crystal-producing Bt strains that do not have
used as specific insecticides under trade names
such as Dipel and Thuricide
Mechanism of Toxicity
1.Ingestion: ICP (Insecticidal crystalline proteins) spore
complexes of Bt by susceptible insect larvae
2. Activation: alkaline pH, in midgut ICP is dissociated to
protoxin is then activated to holotoxin by gut
3.Paralysis: gut becomes paralysed
larva ceases to feed
4.Binding:Binding of the ICP to receptors
major determinant of ICP specificity
5.Pore formation: midgut epithelial cells damaged,
the haemolymph and gut contents can mix.
results in favourable conditions for the Bt spores to
vegetative cells of Bt and the pre-existing microorganisms
in the gut proliferate in the haemocoel causing septicaemia,
6.Mortality of larvae
Effects of Bt Toxins on Soil Ecosystem
Microbial communities- part of complex food webs together
with soil dwelling invertebrates( earthworms , collembolans
,mites ,woodlice ,nematodes )
Carry out processes in soil ecosystem(Nutrient cycling,
decomposition of organic matter, decomposition of agricultural
chemicals , improvement of soil structure).
Mediators of stability of food webs.
Generally, few or no toxic effects of cry protein on invertebrates,
microorganisms and activity of various enzymes in soil.