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Shigella spp.

S. sonnei, S. flexneri, S. boydii, S. dysenteriae

General characteristics

Shigella are Gram-negative, nonmotile, nonsporeforming rod-shaped bacteria. The illness caused by Shigella (shigellosis) accounts for less than 10% of the reported outbreaks of food borne illness. Shigella rarely occurs in animals; principally a disease of humans except other primates such as monkeys and chimpanzees. The organism is frequently found in water polluted with human faeces.

Disease symptoms

Shigellosis (bacillary dysentery).

Symptoms -- Abdominal pain; cramps; diarrhoea; fever; vomiting; blood, pus, or mucus in stools; tenesmus.

Onset time -- 12 to 50 hours.

Infective dose -- As few as 10 cells depending on age and condition of host. The Shigella spp. are highly infectious agents that are transmitted by the faecal-oral route.

The disease is caused when virulent Shigella organisms attach to, and penetrate, epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly, and spread to contiguous epithelial cells resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin (very much like the verotoxin of E. coli O157:H7).


Serological identification of culture isolated from stool.

Associated foods

Salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk and dairy products, and poultry. Contamination of these foods is usually through the oral-oral route. Faecally contaminated water and unsanitary handling by food handlers are the most common causes of contamination.


Shigella is heat-sensitive and will be killed by thorough heating (over 70 °C). Raw or undercooked foods and cross-contamination, when cooked material comes into contact with raw produce or contaminated materials (cutting boards), are the main causes of infection. Proper cooking and hygienic food handling thus can prevent Shigella infections to a large extend.

Risk populations

Infants, the elderly, and the infirm are susceptible to the severest symptoms of disease, but all humans are susceptible to some degree.


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