Shigella.

Shigella strains capable of producing illness possess a 140 megadalton plasmid that determines its property of epithelial cell penetration. In the classic presentation, there is a descending intestinal tract infection beginning with the passage of small numbers of large-volume stools and progressing to the passage of many small-volume stools that contain blood and mucus. Shigellae are efficiently transmitted to contacts in person-to-person spread and only 200 viable cells are sufficient to produce illness in susceptible persons. Shigellosis should be suspected in persons with severe diarrhea, in those with fever, when bloody stools are passed, or when stool smears contain many leukocytes. The current treatment of choice is TMP-SMX or, in adults, a quinolone (norfloxacin or ciprofloxacin).