Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster (κλωστήρ) or spindle. These characteristics traditionally defined the genus, however many species originally classified as Clostridium have been reclassified in other genera.
- C. difficile, which can overgrow other bacteria in the gut during antibiotic therapy, can cause pseudomembranous colitis.
- C. perfringens, formerly called C. welchii, causes a wide range of symptoms, from food poisoning to gas gangrene. Also responsible for enterotoxemia (also known as "overeating disease" or "pulpy kidney disease") in sheep and goats. C. perfringens also takes the place of yeast in the making of salt rising bread.
Honey sometimes contains spores of Clostridium botulinum, which may cause infant botulism in humans one year old and younger. The bacteria produce botulinum toxin, which eventually paralyzes the infant's breathing muscles. Adults and older children can eat honey safely, because the clostridia do not compete well with the other rapidly growing bacteria present in the GI tract.
Clostridium is sometimes found in raw swiftlet birds' nests, a Chinese delicacy. Nests are washed in a sulfite solution to kill the bacteria before being imported to the U.S. 
Neurotoxin types Neurotoxin production is the unifying feature of the species C. botulinum. Seven types of toxins have been identified and allocated a letter (A-G). Most strains produce one type of neurotoxin but strains producing multiple toxins have been described. C. botulinum producing B and F toxin types have been isolated from human botulism cases in New Mexico and California. The toxin type has been designated Bf as the type B toxin was found in excess to the type F. Similarly, strains producing Ab and Af toxins have been reported. Organisms genetically identified as other Clostridium species have caused human botulism; Clostridium butyricum producing type E toxin and Clostridium baratii producing type F toxin. The ability of C. botulinum to naturally transfer neurotoxin genes to other clostridia is concerning, especially in the food industry where preservation systems are designed to destroy or inhibit only C. botulinum but not other Clostridium species.
C. thermocellum can utilize lignocellulosic waste and generate ethanol, thus making it a possible candidate for use in ethanol production. It also has no oxygen requirement and is thermophilic, reducing cooling cost.
The anaerobic bacterium C. ljungdahlii, recently discovered in commercial chicken wastes, can produce ethanol from single-carbon sources including synthesis gas, a mixture of carbon monoxide and hydrogen that can be generated from the partial combustion of either fossil fuels or biomass. Use of these bacteria to produce ethanol from synthesis gas has progressed to the pilot plant stage at the BRI Energy facility in Fayetteville, Arkansas.
Genes from C. thermocellum have been inserted into transgenic mice to allow the production of endoglucanase. The experiment was intended to learn more about how the digestive capacity of monogastric animals could be improved. Hall et al. published their findings in 1993.
Non-pathogenic strains of clostridia may help in the treatment of diseases such as cancer. Research shows that clostridia can selectively target cancer cells. Some strains can enter and replicate within solid tumours. Clostridia could, therefore, be used to deliver therapeutic proteins to tumours. This use of Clostridia has been demonstrated in a variety of preclinical models.
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