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The toxins function by damaging the intestinal mucosa and cause the symptoms of C. difficile infection, including pseudomembranous colitis. TcdA is one of the largest bacterial toxins known. With a molecular mass of 308 kDa, it is usually described as a potent enterotoxin , [ 3 ] but it also has some activity as a cytotoxin . [ 4 ]
Without either toxin A or toxin B, C. difficile may colonize the gut, but is unlikely to cause pseudomembranous colitis. [45] The colitis associated with severe infection is part of an inflammatory reaction, with the "pseudomembrane" formed by a viscous collection of inflammatory cells, fibrin , and necrotic cells.
Clostridioides difficile (syn. Clostridium difficile) is a bacterium known for causing serious diarrheal infections, and may also cause colon cancer. [4] [5] It is known also as C. difficile, or C. diff (/ s iː d ɪ f /), and is a Gram-positive species of spore-forming bacteria. [6]
There are different plasmid sizes of C. difficile. The detected molecular weights range from 2.7x10 6 to 100x10 6, but plasmid sizes show no correlation with toxicity. In order to detect the toxin B level in C. difficile, clinicians extensively use cell culture assays derived from stool specimens from patients with PMC.
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations.
Natural selection is inherently involved in the process of speciation, whereby, "under ecological speciation, populations in different environments, or populations exploiting different resources, experience contrasting natural selection pressures on the traits that directly or indirectly bring about the evolution of reproductive isolation". [54]
The variation affects the speed and rate of evolutionary change through the selection and modulation of phenotypic variations. [28] Ultimately, this results in a lower amount of diversity observed throughout evolution, as the majority of phenotypes do not persist beyond a few generations due to their inferior morphology, biochemical makeup, or ...
Niches of similar species are segregated (as the result of natural selection) in order to prevent interspecific hybridisation, because hybrids are less fit. (Many cases of niche segregation explained by interspecific competition are better explained by this mechanism, i.e., " reinforcement of reproductive barriers ") (e.g., Rohde 2005b).