Chapter 16 Vibrios, campylobacters and Wolinella
Bacteria belonging to these three genera (together with others such as the genus Helicobacter) are morphologically similar, being Gram-negative curved bacilli. They are enteric pathogens of humans or part of the normal flora. Because of their unusual growth requirements (formate and fumarate needed), they have to be cultured in special media.
The genus Vibrio includes two important human pathogens, but their natural habitat is water. Vibrio cholerae causes cholera, while Vibrio parahaemolyticus causes a less severe diarrhoea. The main symptom of cholera is watery diarrhoea that can be fatal as a result of severe dehydration, water and electrolyte loss.
Grows in alkaline conditions (pH 8.5–9.2 approximately): selective media for culture such as thiosulphate-citrate-bilesucrose (TCBS) medium are based on this property. This, together with biochemical tests and serology, helps identification. Serotyping is based on the somatic O antigens. All diarrhoea-producing strains of V. cholerae are designated as O1 and are subdivided into three major serotypes – the Ogawa, Inaba and El Tor strains.
V. cholerae has the ability to colonize the intestinal tract in very high numbers and about 108 cells per millilitre are seen in patients’ faeces. The cells attach to but do not invade the intestinal mucosa. Pathogenicity is due to secretion of an enterotoxin, which binds to ganglioside receptors on mucosal cells. After a lag period of 15–45 min, adenylate cyclase is activated and the cyclic adenosine monophosphate concentration inside the intestinal cells increases. This in turn leads to excretion of electrolytes and water and subsequent diarrhoea, leading to severe dehydration.
Intravenous administration of fluids and electrolytes is essential for recovery. Oral administration of a solution containing glucose and electrolytes (oral rehydration therapy) is successful, but the patient must be capable of consuming the liquid by mouth. Severely ill patients are generally too weak to ingest fluids. Antibiotics (usually tetracycline) do not affect the disease outcome once the enterotoxin attaches to the intestinal cells, but they prevent subsequent attacks by reducing the number of toxin-producing V. cholerae cells in the intestine.