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Clostridia

Clostridia are a group of about 80 species of strictly anaerobic, Gram-positive, spore-forming bacteria that grow and persist in a wide variety of environments including soil, sediments, decaying plants and compost. They inhabit the gastrointestinal tract of herbivores and wood-eating animals. Because of their ability to produce spores, clostridia can survive in the environment for very long periods in a dormant state. Some species are motile. The group has a bad reputation because of Clostridium tetani and Clostridium botulinum, the causative agent of tetanus and botulism, respectively.7,8 Nevertheless, the vast majority of species in this group are benign and because of their interesting fermentation properties, some species, including solvent-producing C. acetobutylicum, C. beijerinckii, and C. saccharobutylicum, are of interest in industrial biofuel production.

Metabolically, these bacteria are quite versatile and degrade an extremely wide range of organic materials that include carbohydrates, organic acids, alcohols, aromatic compounds, peptides, amino acids, amines, purines, and pyrimidines. Thereby, they effectevely participate in an important part of ecology involving biomass renewal.7

Biomass Renewal Machinery

The bacteria produce complex enzymatic machinery necessary to decompose extremely sophisticated networks of polymers called plant polysaccharides. Numerous Clostridia species have evolved effective strategies for decomposing these molecules. Breakdown of the polymers that compose biomass is generally a very important microbial activity, one that is essential for the continuation of life on this planet. The dark side of this activity is rot in plants. A human equivalent of plant rot is gas gangrene, a serious disease caused by Clostridium perfringens.7

Polysaccharide-fermenting clostridia, including those that can digest cellulose, produce ethanol, formate, acetate, lactate, and succinate. A most remarkable clostridial fermentation is that performed by C. kluyveri. Some Clostridia species are able to fix nitrogen.4

Clostridia produce the highest number of potent toxins responsible for severe diseases in humans and animals. Most of these toxins are pore-forming toxins.

Animal Pathogen

Clostridia species infect many animals. C. colinum occurs consistently in poultry ulcerative diarrheal disease. C. difficile is an important pathogen of horses, pigs and other domestic animals including dogs and cats. Infection with C. difficile in horses leads to fatal necrotizing enterocolitis, with profuse watery diarrhea and dehydration in neonatal foals. It also affects young calves.6 In lambs under 3 weeks of age, C. perfringens causes dysentery. Lambs will cease sucking and start to bleat continuously with or without nervous system signs. Losses can be severe. In very young lambs, death usually occurs 2-12 hours after the onset of the symptoms.8

Human Pathogen

Like other members of Clostridia, the pathogenic clostridia are normal soil inhabitants, with little or no invasive power. The disease they produce results from the production of a variety of highly toxic proteins (exotoxins). Exotoxins are produced by C. botulinum and C. tetani. The immunity to these toxins is achieved by immunization with inactivated toxins (toxoids). A second group of clostridia includes C. perfringens, C. histolyticum, C. novyi, C. septicum, C. sordelii, and C. chauvocii which produce numerous toxins, including histotoxins. Human vaccines against these toxins are not available, although veterinary vaccines have been used to prevent tissue infections.2

Clostridium difficile infection comprises more than 25% of all cases associated with antibiotic-associated diarrhea. After the colonic microbes have been killed by antimicrobials, clostridia spores germinate and release toxins which cause diarrhea and severe inflammation of the colon.6

Recently Clostridium difficile has been isolated from uncooked and ready-to-eat meats.

Clostridium perfringens

Clostridium perfringens is a species of Clostridia most often associated with the feces of warm-blooded animals, but is only present in 13-55% of human feces. Clostridium perfringens are much less prevalent in human feces than bifidobacteria (the third most common bacteria), but their spore-forming ability allows them to survive for significant time periods ouside the enteric environment in water. Clostridium perfringens is used as a bacteria indicator of fecal contamination of water and, sometimes, as an indicator of the protozoan enteric pathogen Giardia, the causative agent of giardiasis.

Clostridium perfringens can survive in deep wounds where it produces enzymes that digest tissue and release large amounts of CO2, a sign of gas gangrene. As a result of large destruction of tissue and blood vessels, human body immune system and antibiotics cannot penetrate. The dead tissue must be removed entirely.3

Clostridium botulinum

Botulism caused by this bacterium is a pure intoxication, resulting from the ingestion of foods in which the organism has previously developed exotoxins. Most reported instances of botulism are associated with contaminated foods canned at home, but the disease has also resulted from failures in commercial canning. Spores of C. botulinum can withstand 5 hours of boiling.5

Clostridium tetani

Clostridium tetani produces a paralytic toxin that causes tetanus. This bacterium is commonly found in soil and in the colon of many mammals including humans. Spores of this pathogen sometimes enter a wound when an object such as splinter contaminated by soil punctures the skin and carries spores into damaged tissue where the spores germinate and release a toxin. This toxin is transported to nerve cells. Symptoms of tetanus develop withing 3 days to several weeks after the infection. In severe cases, respiratory muscle spasms can cause death.5 Spores of Clostridium tetani can remain viable for at least 50 to 70 years.

References
  1. Changing land use patterns in the coastal zone: managing environmental quality in rapidly developing regions. Gary S. Kleppel
  2. Clostridia: molecular biology in the post-genomic era. Gerhard Gottschalk
  3. Microbiology: Diversity, Disease, and the Environment. Abigail A. Salyers, Dixie D. Whitt
  4. The Microbial World. Roger Y. Stanier, John L. Ingraham, Mark L. wheelis, Page R. Painter
  5. The World of Microbes. Howard Gest
  6. Pathogenesis of Bacterial Infections in Animals. Carlton L. Gyles, John F. Prescott, Glenn Songer
  7. Handbook on clostridia. Peter Dürre
  8. Diseases of sheep. Ian D. Aitken

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