Shigella are a familiy of enteric pathogens consisting of four different species that are a common cause of diarrheal disease. Shigella are usually transmitted person to person but may also be transmitted via food, often from an infected food worker. The majority of illness from Shigella is short lived and does not require specific antibiotic therapy; however, some forms can be life threatening.

MICROBIOLOGY

Shigella belongs to the family Enterobacteriaceae and closely resembles Escherichia coli at the genetic level. Four species of Shigella—Shigella dysenteriae, Shigella flexneri, Shigella boydii, and Shigella sonnei—are differentiated by group-specific polysaccharide antigens of lipopolysaccharide, designated A, B, C, and D, respectively. Shigella dysenteriae consists of 10 antigenic types, of which type 1 produces a potent cytotoxin known as Shiga toxin. Shigella flexneri is divided into 6 types and 14 subtypes, and S. boydii into 18 serologic types. Although there is only 1 S. sonnei serotype, there are at least 20 colicin types. Shigellae are biochemically very similar, and differentiation among species is based primarily on serologic methods using group- and type-specific antisera.

EPIDEMIOLOGY

Shigellosis occurs throughout the world with varying species distribution. Shigella dysenteriae and S. flexneri are the predominant species in developing countries, whereas S. sonnei is the major isolate in developed countries, accounting for more than three quarters of the isolates in the United States. Shigella flexneri is being seen more frequently in the United States in homosexual men and is thought to be transmitted sexually in this group.