Shigella Shigella is a genus of Gram - negative , non - spore forming rod - shaped bacteria closely related to " Escherichia coli " and " Salmonella " . The causative agent of human shigellosis , " Shigella " causes disease in primates , but not in other mammals . It is only naturally found in humans and apes . During infection , it typically causes dysentery . The genus is named after Kiyoshi Shiga , who first discovered it in 1898 . Classification . " Shigella " species are classified by four serogroups : Group " A " – " C " are physiologically similar ; " S . sonnei " ( group " D " ) can be differentiated on the basis of biochemical metabolism assays . Three " Shigella " groups are the major disease - causing species : " S . flexneri " is the most frequently isolated species worldwide and accounts for 60 % of cases in the developing world ; " S . sonnei " causes 77 % of cases in the developed world , compared to only 15 % of cases in the developing world ; and " S . dysenteriae " is usually the cause of epidemics of dysentery , particularly in confined populations such as refugee camps . Pathogenesis . " Shigella " infection is typically via ingestion ( fecal – oral contamination ) ; depending on age and condition of the host as few as 100 bacterial cells can be enough to cause an infection . " Shigella " causes 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 . Both Shiga toxin and verotoxin are associated with causing hemolytic uremic syndrome . " Shigella " invade the host through epithelial cell s of the large intestine . Using a Type III secretion system acting as a biological syringe , the bacterium injects IpaD protein into cell , triggering bacterial invasion and the subsequent lysis of vacuolar membranes using IpaB and IpaC proteins . It utilizes a mechanism for its motility by which its IcsA protein triggers actin polymerization in the host cell ( via N - WASP recruitment of Arp2 / 3 complexes ) in a " rocket " propulsion fashion for cell - to - cell spread . The most common symptoms are diarrhea , fever , nausea , vomiting , stomach cramps , flatulence , and constipation . The stool may contain blood , mucus , or pus . In rare cases , young children may have seizure s . 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 . Shigella is implicated as one of the pathogenic causes of reactive arthritis worldwide . Severe dysentery can be treated with ampicillin , TMP - SMX , or fluoroquinolone s such as ciprofloxacin and of course rehydration . Each of the " Shigella " genomes includes a virulence plasmid that encodes conserved primary virulence determinants . The " Shigella " chromosome s share most of their genes with that of " E . coli " K12 strain MG1655 Identification . " Shigella " species are negative for motility and are non - lactose fermenters . ( However , " S . sonnei " can ferment lactose ) . They typically do not produce gas from carbohydrates ( with the exception of certain strains of " S . flexneri " ) and tend to be overall biochemically inert . " Shigella " should also be Urea Hydrolysis negative . When inoculated to a Triple Sugar Iron slant they react as follows : K / A , gas - , H2S - . Indole reactions are mixed , positive and negative , with the exception of " S . sonnei " which is always indole negative .