What are Shigella?

Natural Habitat and Features

Shigella was named for Kiyoshi Shiga, a Japanese physician and bacteriologist, who first isolated S. dysenteriae in 1896. There are four recognized species. Other Shigella spp. that were named in the first half of the twentieth century have either been moved to other genera (for example, Shigella galinarum is now Salmonella galinarum) or subsumed into one of the four recognized species (for example, S. ambigua is now S. dysenteriae type II). These bacteria are closely related to Escherichia coli
and share most genes with E. coli strain K12.

Shigella spp. infections occur only naturally in primates. Other animals are not normally infected, but infections have been induced in a few animals used as models for the disease and its treatment. Infection is usually fecal to oral and can occur anytime food or water becomes contaminated with infected feces. Infection also can occur during certain sex practices. Good hygiene, including thorough handwashing with soap and water after fecal elimination, is the best way to prevent shigellosis.

The most common site for bacterial growth is in the large intestine. A dose of as few as ten organisms is enough to cause infection in
some persons. Symptoms usually occur twenty-four to forty-eight hours after
ingestion of Shigella-contaminated food or water.
Shigella spp. thrive in the large intestine and, because of
their virulence plasmid, have the ability to invade intestinal
epithelial cells. This invasion leads to the most common symptoms of shigellosis:
diarrhea, abdominal cramps, nausea, and vomiting. In addition, blood, pus, and
mucus are often present in the stool because invasion often leads to ulceration of
the colonic epithelium.

The disease is usually self-limiting and lasts about one week in healthy
adults, by which time the immune system disposes of the invading
bacteria. In infants, small children, and those debilitated from other illnesses,
the disease may last longer; dehydration caused by diarrhea and
vomiting can lead to severe problems. Damaging enterotoxins and Shiga toxins are coded by the virulence plasmid and are produced by most strains.
Shiga toxins can cause hemolytic uremic syndrome, which leads
to anemia and kidney failure. Some non- Shigella spp. also
contain Shiga toxins because plasmid transfer can occur across species lines. A
small percentage of persons with S. flexneri may develop
Reiter’s
syndrome, which causes joint pain, eye irritation, and
painful urination. Reiter’s syndrome can last months or even years and can lead to
difficult-to-treat chronic arthritis.

Persons often remain infective for up to two weeks after the dysentery symptoms
abate because bacteria remain in the intestine for one to two weeks after
recovery. For several years after infection, persons may retain immunity against
the particular Shigella strain that infected them, but they are
susceptible to other strains. Rehydration is the main treatment for shigellosis;
however, antibiotic therapy can be used in severe cases. The antibiotics of choice
are ampicillin, trimethoprim/sulfamethoxazole, nalidixic acid, ciprofloxacin, and
azithromycin. Because resistance plasmids are easily transferred between members
of the Enterobacteriaceae, antibiotic resistant
Shigella strains are becoming more widespread. Various
Shigella vaccines have been investigated and live attenuated
vaccines have been tested; however, no broad-spectrum Shigella
vaccine is available.

S. dysenteriae has twelve serotypes and is the major cause of
epidemic dysentery. This disease usually occurs in less-developed countries with poor
sanitation and is often seen in Africa, Southeast Asia, and the Indian
sub-continent. Fecal to oral transmission usually occurs because of
sewage-contaminated water. S dysenteriae type I causes the most
severe form of shigellosis. It is especially severe in malnourished and otherwise
debilitated persons; life-threatening complications often occur. More than 30
percent of dysentery cases worldwide are caused by S.
dysenteriae.

S. flexneri, with six serotypes, is the most common cause of
endemic dysentery worldwide and accounts for more than 60 percent of shigellosis
in less-developed countries. The main sources are contaminated water and food
caused by poor sanitation and the use of human waste to fertilize crop plants.
Neither S. dysenteriae nor S. flexneri are major
pathogens in areas where good sanitation leads to the
availability of clean water and proper disposal of fecal wastes. In the developed
world, less than 15 percent of shigellosis cases can be traced to S.
flexneri and even less to S. dysenteriae. S.
sonnei has a single serotype and is biochemically different than the
other Shigella spp. The only known reservoir of S.
sonnei is the human intestinal tract, and this bacterium does not
survive for extended periods in other locations. It is most commonly transmitted
by infected food handlers who have poor hygiene and is the most common cause of
endemic shigellosis in developed countries. Approximately 77 percent of
shigellosis in developed countries and 70 percent in the United States is caused
by S sonnei, which is less virulent than other
Shigella spp. and causes a milder form of shigellosis.
Occasional outbreaks have occurred in the United States and have been traced to
S. sonnei-infected food handlers.

S. boydii, with twenty-three known serotypes, is the most genetically diverse of the Shigella spp. As in other Shigella spp., most S. boydii serotypes are similar to E. coli; however, there are some that seem to share genes with Vibrio cholerae. Although S. boydii has worldwide distribution, it is most common on the Indian subcontinent. It affects all primates, including humans, and can survive for extended periods in the soil. It is responsible for only a small percentage of human shigellosis in the rest of the world.

Garrity, George M., ed. The Proteobacteria. Vol. 2 in Bergey’s Manual of Systematic Bacteriology. 2d ed. New York: Springer, 2005. This volume describes the Proteobacteria in detail.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. This text outlines many common bacteria and describes their natural history, pathogenicity, and other characteristics.

Niyogi, S. K. “Shigellosis.” Journal of Microbiology 43 (2005): 133-143. This article reviews shigellosis and its effects on human health.

Romich, Janet A. Understanding Zoonotic Diseases. Clifton Park, N.Y.: Thomson Delmar Learning, 2008. This book has a good section on shigellosis and its causes and treatments.