Members of the genus Vibrio are all Gram-negative straight or curved rods, which do not form spores and are motile, usually by a single polar flagellum. Most are oxidase and catalase positive and ferment glucose without gas production. They occur naturally in fresh water and marine environments and some may be pathogenic to humans. Many of the pathogenic species, with the notable exception of Vibrio cholerae, are adapted to salt or brackish water habitats and are halophilic to some degree, being unable to grow in the absence of sodium chloride.

Three species are considered to be important human pathogens - V. cholerae, V. parahaemolyticus and V. vulnificus. All three have the potential to be foodborne, and are most often associated with the consumption of raw, or undercooked, shellfish. A number of other species have infrequently been isolated from the stools of people suffering from gastroenteritis and are considered to be occasional human pathogens. These include V. alginolyticus, V. fluvialis, V. furnissii, V. hollisae, V. metschnikovii and V. mimicus. These species are not generally regarded as significant foodborne pathogens.

Vibrio cholerae
V. cholerae is the cause of outbreaks and epidemics of cholera, a serious and potentially fatal gastrointestinal infection, which is still a major health problem in parts of the developing world. Humans are the principal reservoir for V. cholerae and cholera is usually associated with poor hygiene and polluted water supplies, but it may also be foodborne.

Contamination of fruits, vegetables and other foods usually occurs via an infected food handler, or by the use of polluted water in food preparation. Not all strains of V. cholerae cause cholera and the main virulence factor involved in the disease is the cholera enterotoxin (CT). Strains causing classic epidemic cholera generally belong to one of two serogroups, O1 or O139, although other strains have been reported to cause occasional isolated cases of cholera-like disease.

Non-O1/O139 strains can cause a less severe form of diarrhoeal disease and are more common clinical isolates in developed countries. These isolates are typically related to consumption of contaminated shellfish, especially raw oysters. Non-O1/O139 V. cholerae strains are not uncommon in certain estuarine waters and may contaminate shellfish harvested from polluted sites.

V. cholerae will grow rapidly in temperature-abused processed foods and will also survive for extended periods in chilled and even frozen foods, but it does not survive desiccation for more than 48 hours. The cells are not heat-resistant and are readily destroyed by cooking and pasteurisation.

Vibrio parahaemolyticus
V. parahaemolyticus is the species most likely to be associated with foodborne disease in humans. It can cause mild to moderate gastrointestinal infections, which are usually self limiting and rarely fatal. It is not normally involved in epidemics or large outbreaks, but is an important source of foodborne disease, especially in Japan and other Asian countries.

Infection is almost always associated with consumption of seafood and V. parahaemolyticus is very common in marine coastal waters in tropical regions and in temperate regions during the warmer months of the year. There is also evidence that it may be becoming more common in colder waters during the summer, possibly as a consequence of rising sea temperatures. As with V. cholerae, not all strains of V. paraheamolyticus are capable of causing human disease. Pathogenicity is associated with a thermostable haemolysin, called the Kanagawa phenomenon. Almost all isolates from cases of food poisoning are Kanagawa positive strains.

V. paraheamolyticus is part of the normal microflora of coastal and estuarine waters in almost all temperate regions and may be present in comparatively high numbers when the water temperature is at its highest during the summer. At these times, shellfish may easily become contaminated, but fortunately most environmental isolates are Kanagawa negative and do not cause illness. The possibility of infection means that shellfish from high-risk waters should not be eaten raw.

Most cases of V. paraheamolyticus infection in Europe can be traced to imported seafood, but there is evidence to suggest that it is becoming more common in domestically harvested shellfish. V. paraheamolyticus differs from V.cholerae in that it is an obligate halophile and will not grow unless a salt concentration of at least 0.5% is present. In fact it will grow at salt levels as high as 10%. Like V. cholerae it will grow rapidly in temperature abused foods and survives chilled and frozen storage, but not drying or mild heat processes.

Vibrio vulnificus
V. vulnificus is an occasional cause of serious infections, which may sometimes be foodborne. However, wound infection following contact with the marine environment is more likely. Foodborne infections can take the form of gastroenteritis in healthy adults, but in vulnerable individuals the pathogen can cause primary septicaemia, which is very serious and has a mortality rate of more than 50%. V. vulnificus is now the leading cause of death in the USA, related to consumption of seafood and is almost always associated with raw oysters from the Gulf of Mexico, which are thought to have a very high contamination rate . Only about 90 cases of V. vulnificus infection are reported each year in the USA and major outbreaks have not been recorded, but the high death rate means that it is regarded as a serious pathogen.

Elsewhere, sporadic cases have been identified in Europe, Korea and Taiwan. Like V. parahaemolyticus, V. vulnificus is a common part of the natural microflora of coastal waters in tropical and temperate regions worldwide, especially during the summer months. This being so, given the low number of cases of human infection, it is likely that only certain strains are pathogenic to man. V. vulnificus is a halophile requiring at least 0.5% salt to grow and will tolerate levels of up to 5%. It will multiply in live oysters, but not at temperatures of less than 13^oC. Like other species it resists low temperatures for some time, but is destroyed by cooking and is not resistant to desiccation.