Shigella

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Shigella are Gram-negative, non-motile, non-spore forming rod-shaped bacteria closely related to Escherichia coli. The causative agent of human shigellosis, Shigella also cause disease in other primates, but not in other mammals.

Classification

Shigella species are classified by four serogroups:

Serogroup A: S. dysenteriae (12 serotypes)
Serogroup B: S. flexneri (6 serotypes)
Serogroup C: S. boydii (18 serotypes)
Serogroup D: S. sonnei (1 serotype)

Group A–C are physiologically similar; S. sonnei (group D) can be differentiated on the basis of biochemical metabolism assays.

Pathogenesis

Shigella infection is typically via ingestion (fecal–oral contamination); depending on age and condition of the host, as few as 10 bacterial cells can be enough to cause an infection. Shigella cause dysentery that results in the destruction of the epithelial cells of the intestinal mucosa in the cecum and rectum. Some strains produce enterotoxin and Shiga toxin, similar to the verotoxin of E. coli O157:H7.

The most common symptoms are diarrhea, fever, nausea, vomiting, stomach cramps, and straining to have a bowel movement. The stool may contain blood, mucus, or pus (eg dysentery). In rare cases, young children may have seizures. Symptoms can take as long as a week to show up, but most often begin two to four days after ingestion. Symptoms usually last for several days, but can last for weeks.

Severe dysentery can be treated with ampicillin, TMP-SMX, or fluoroquinolones such as ciprofloxacin.

References

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