Shigella is a genus of Gram-negative, facultative anaerobic, nonspore-forming, nonmotile, rod-shaped bacteria closely related to Salmonella. The genus is named after Kiyoshi Shiga, who first discovered it in 1897. 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. Shigella is one of the leading bacterial causes of diarrhea worldwide. Insufficient data exist, but conservative estimates suggest Shigella causes about 90 million cases of severe dysentery, with at least 100,000 of these resulting in death each year, mostly among children in the developing world.
The diagnosis of shigellosis is made by isolating the organism from diarrheal fecal sample cultures. Shigella species are negative for motility and are generally not lactose fermenters, but 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 (TSI) slant, they react as follows: K/A, gas -, and H2S -. Indole reactions are mixed, positive and negative, with the exception of S. sonnei, which is always indole negative. Growth on Hektoen enteric agar will produce bluish-green colonies for Shigella and bluish-green colonies with black centers for Salmonella.
Prevention and Treatment
Hand washing before handling food and thoroughly cooking all food before eating decreases the risk of getting shigellosis. Severe dysentery can be treated with ampicillin, TMP-SMX, or fluoroquinolones, such as ciprofloxacin, and of course rehydration. Medical treatment should only be used in severe cases or for certain populations with mild symptoms (elderly, immunocompromised, food service industry workers, child care workers). Antibiotics are usually avoided in mild cases because some Shigella species are resistant to antibiotics, and their use may make the bacteria even more resistant.
Antidiarrheal agents may worsen the sickness, and should be avoided. For Shigella-associated diarrhea, antibiotics shorten the length of infection. Currently, no licenced vaccine targeting Shigella exists. Shigella has been a longstanding World Health Organisation target for vaccine development, and sharp declines in age-specific diarrhoea dysentery attack rates for this pathogen indicate natural immunity does develop following exposure; thus, vaccination to prevent the disease should be feasible. Several vaccine candidates for Shigella are in various stages of development.