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Microbial Toxins in soil

Microbial Toxins in Soil - effects and causal organisms

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Microbial Toxins in soil

  1. 1. Presented by Iniyalakshimi.B.R., M.Sc (Agri) TNAU, Coimbatore.
  2. 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.  Ubiquitous distribution.
  3. 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 of animals.  biological toxin, tetanospasmin  the causative agent of tetanus, a disease characterized by painful muscular spasms that can lead to respiratory failure .
  4. 4.  Clostridium botulinum  gram-positive, rod-shaped  produces neurotoxins, known as botulinum neurotoxins types A-G,  that cause the flaccid muscular paralysis 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.  produce the secondary metabolite 2,4-diacetylphloroglucinol (2,4-DAPG )
  5. 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. 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 used as fodder or bedding for animals.
  7. 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. 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.
  8. 8. Fusarium toxins: 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.
  9. 9. Ergot alkaloids: 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 limbs.
  10. 10. Patulin: 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. Stachybotrys toxins: 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.
  11. 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. 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, carbohydrate and protein in the nodule cytosol.  interference with normal nodule physiology and function.
  13. 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. 14.  specific activities against insect species of orders Lepidoptera (moths and butterflies), Diptera (flies and mosquitoes), Coleoptera (beetles), hymenoptera(wasps, bees, ants and sawflies) nematodes  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. 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. 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. 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
  18. 18. 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.