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Vibrio
ORGANISM:
- Genus: Vibrio
- Species: cholerae
GENERAL CONCEPTS:
- The name Vibrio derives from the Latin because these curved rods possess a single polar flagellum and appear "to vibrate".
- V. cholerae was first isolated in pure culture in 1883 by Robert Koch.
- V. cholerae produces the disease cholera, defined as "a metabolic disturbance of the epithelial cells of the small bowel". Cholera is caused, in part, by a potent enterotoxin (choleragen) and is usually a disease of poor sanitation.
- Humans are the only natural host for this organism and there have been 6 great pandemics of cholera.
- Two biotypes of V. cholerae are described: Classic and El Tor.
- Three serotypes (Ogawa, Inaba and Hikojima) are also recognized.
DISTINCTIVE PROPERTIES:
- The genus Vibrio is composed of Gram negative, curved rods that are motile by means of a single polar flagellum.
- These organisms are sensitive to acid pH but tolerate alkaline pH (9.0-9.6) very well.
- The El Tor biotype produces less toxin but colonizes better and is more resistant to environmental factors.
PATHOGENESIS:
- The acid sensitivity of V. cholerae means that a large dose is required to produced disease. Indeed, 1011 vibrios given orally fail to produce illness but if bicarbonate (e.g. Alka-Selzer�) precedes the inoculation, then only 104 are required.
- Cholera is a disease of the small intestine, unlike most other enteric illnesses. The bacteria penetrate the mucus layer and adhere to the mucosal cells where they subsequently produce toxin.
- The potent enterotoxin choleragen is well defined. This 84 kD protein enterotoxin is composed of 2 major domains; the A domain controls its biologic activity while the B domain binds the toxin to cellular receptors (GM1 receptor). Binding via B leads to penetration of the A1 peptide, which enzymatically transfers ADP-ribose from NAD to a GTP regulatory protein. This leads to activation of adenylate cyclase and the overproduction of cyclicAMP, which causes hypersecretion of chlorides and water (click the image to animate). A cholera patient may secrete 20 liters of fluid per day with 108 vibrios per ml!
HOST DEFENSES:
- Gastric acidity plays an important role in preventing cholera infection. Also, genetic factors may be important since the toxin must bind to specific cellular receptors in order to function. Secretory antitoxin IgA may also play a role.
EPIDEMIOLOGY:
- Three patterns of cholera have been observed: heavily endemic, neoepidemic and limited outbreaks.
- Transmission of disease is primarily via water that has been contaminated by human feces. Carriage of vibrios is rare.
DIAGNOSIS:
- Clinical: Gastrointestinal symptoms and the presence of a rice water stool are presumptive for cholera. Direct examination of the feces may indicate vibrios.
- Laboratory: Isolation of V. cholera employs thiosulfate-citrate-bile salts-sucrose (TCBS) agar, which is selective for the organism.
CONTROL:
- Sanitary: Sanitary measures are most important in controlling cholera outbreaks. The shear volume of liquid and concentration of viable organisms precludes measures for decontamination.
- Immunological: An experimental vaccine using a modified strain has been attempted. This strain, called "Texas Star", produces a toxin that has the B domain but not the A (biologically active) domain. Ingestion of this organism should allow production of sIgA against the cell-binding moiety (thereby preventing binding) without producing overt disease. In addition, researchers have recently suggested that a cholera vaccine might be incorporated into french fries!
- Chemotherapeutic: Most important to the patient is the replacement of lost fluid and salts. Death from cholera results as a consequence of extreme and rapid dehydration. Moderate or broad spectrum antibiotics may be used to reduce the output of viable organisms.
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