As noted already the etiologic agent of Asiatic or epidemic cholera is a comma-shaped bacterium, V. cholerae. It is gram-negative, made motile by means of a single terminal or polar flagellum - or motility organ. This bacterium was seen in the excreta and intestinal contents of cholera victims by Filippo Pacini and described by him so accurately in a report he published in Florence in 1854 that it still carries the name he gave it, although the finding made little impact at the time.

The bacterium can be grown in the laboratory in simple media and in an alkalinity greater than that tolerated by most other bacteria. This latter characteristic is of significance for its growth in the human small intestine, as will be duly noted. On the other hand, it is more sensitive to acidity. This bacterium survives and multiplies in the environment outside the human body, notably in any relatively uncon-taminated alkaline environment. It does not regularly infect animals; its host range is limited to humans. And it can be carried and spread by humans, especially convalescents, in the absence of overt disease. The bacteria produce and secrete a toxin, which is the actual cause of the symptoms that constitute the disease.

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