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 cellular receptors) Severity: minor and acute to almost immediately deadly (as in botulinum toxin). Microbial toxins are toxins produced by microorganisms, including bacteria, viruses and fungi. Abundant dispersal.

1. Microbial Toxins In
The Soil
Amjad Khan Afridi

2. Microbial Toxins
 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 cellular receptors)
 Severity: minor and acute to almost immediately deadly
(as in botulinum toxin).
 Microbial toxins are toxins produced by microorganisms,
 including bacteria, viruses and fungi.
 Abundant dispersal.

3. Different Types of Microbial Toxins
 Bacterial Toxins:
Exotoxins : generated by the bacteria and actively secreted.
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 ofanimals.
 Biological toxin, tetanospasmin.
 The causative agent of tetanus (lockjaw), a disease characterized by
painful muscular spasms that can lead to respiratory failure .

4.  Clostridium botulinum
 Gram-positive, rod-shaped
 Produces neurotoxins, known as botulinum neurotoxins
types A-G,
 That cause the seen in botulism.
 Obligate anaerobic spore-former, endospores
 Commonly found in soil.
Pseudomonas fluorescens
 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 (Insects feeding on
plants.
 Produce the secondary metabolite 2,4-diacetylphloroglucinol (2,4-
DAPG ). natural phenol

5.  Mycotoxin:
 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.

6.  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 as
fodder or bedding for animals.

7. 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.
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.

8. Fusarium toxins:
Several species of Fusarium produce zearalenone (ZEA) and
trichothecene (TC) 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.
(Caused hypoestrogenism: Estrogen deficiency )

9. Ergot alkaloids (Ergotoxin):
 These are produced by Claviceps purpurea and
other Claviceps species which grow in the
heads of, produce sclerotinia and replace
grains with the bodies containing ergot
alkaloids.
 These fungi produce an extensive series of
alkaloids .
 The alkaloids are structurally related to lysergic
acid diethylamide (LSD), a well known
psycho-active drug / hallucinogenic drugs.
 Ergots cause abortion and smooth muscle
contraction which sometimes may lead to the
loss of limbs.

10. Patulin:
 It is polyketide carcinogenic
toxin produced by
Penicillium urticae, P. patulum, P.
griseofulvum, P.claviforme, etc,.
 It is toxic to bacteria and some fungi,
higher plants and animals.
Stachybotrys toxins:
 These toxins are produced thye species of
stachybotrys (black mold) on straw, hay,
other fodders and animal feeds, etc.
 Stachybotrys toxins cause diseases like
haemorrhage, necrosis and general cell
damage in domesticated animals.

11. Alternaria toxins:
 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 disease.

12. 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
/legoglobin (oxygen carrying) , carbohydrate
and protein in the nodule cytosol (cytoplasmic
matrix).
 Interference with normal nodule physiology
and function.

13. 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)

14.  Specific activities against insect species
of orders Lepidoptera (moths and
butterflies), Diptera (flies and
mosquitoes), Coleoptera (beetles),
hymenoptera (wasps, bees, ants and
nematodes, sawflies).
 Use as insecticides, genetically
modified crops using Bt genes.
 Biological alternative to a pesticides.
 Many crystal-producing Bt strains that
do not have insecticidal properties.
 Used as specific insecticides under trade
names such as Dipel and Thuricide

15. Bt crops
Bt
crops
Cry
Protein
Target pests
Maize Cry1Ab European corn borer (Ostrinia nubilalis),Corn stalk
borer ( Sesamia nonagrioides),corn earworm
(Helicoverpa zea Boddie)
Cotton Cry1Ac Bollworm (Earias spp ,Helicoverpa spp, Heliothis
spp, Pectinophora spp)

16. 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 form
protoxin is then activated to holotoxin by gut
proteases.
3. Paralysis: gut becomes paralysed
larva ceases to feed

17. 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
germinate.
vegetative cells of Bt and the pre-existing microorganisms
in the gut proliferate in the haemocoel causing septicaemia,
6.Mortality of larvae

19. Mechanism of Toxicity

20. Thank You
7th April, 2021, Wednesday