Shigella is a genus of Gram-negative bacteria that is similar in behavior and habitat to Escherichia coli . The bacterium is named after its discoverer, Japanese scientist Kiyoshi Shiga. The bacteria were discovered over 100 years ago.
Some strains of the bacteria can produce toxins, including the so-called Shiga toxin, which is very similar to the destructive verotoxin of Escherichia coli O157:H7. Indeed, strain O157:H7 is now presumed to have arisen by virtue of a genetic recombination between strains of Shigella and Escherichia coli in the intestinal tract, which resulted in the acquisition of the verotoxin by Escherichia coli.
The similarity between Shigella and Escherichia coli extends to the structure of the bacteria and their utilization of certain compounds as nutrients. The similarity is so pronounced that Shigella has been regarded as a strain of Escherichia coli. However, this is now known not to be the case. Shigella does not produce gas from the utilization of carbohydrates, while Escherichia coli does.
Shigella is one of a group of bacteria, which includes Escherichia coli, that inhabits the intestinal tract of humans and other warm blooded animals. Most strains of the bacterium are innocuous. However, the strains that possess the destructive toxins can do much damage to the intestinal wall and other areas of the body.
There are a number of Shigella species that are noteworthy to humans. Shigella sonnei, which is also known as group D Shigella, is the cause of almost 70 percent of the reported cases of food-borne Shigella illness in the United States each year. Shigella flexneri, which is also called group B Shigella, is responsible for virtually all the remaining cases of food-borne illness. In underdeveloped countries of the world, the bacterium Shigella dysenteriae type 1 is epidemic in its scope.
The illness that is caused by Shigella species is called shigellosis. The illness is classified as a bacillary dysentery . An estimated 300,000 cases of shigellosis occurs in the United States each year. Production of the toxins following the ingested of Shigella -contaminated food produces the illness. The illness is characterized by pain in the abdomen, cramps, diarrhea that can become bloody as intestinal cells are damaged, vomiting, and fever. These symptoms typically begin from 12 hours to three days after consuming food that is contaminated with the microorganism. Contamination usually results from the exposure of the food to feces-contaminated water or from improper hygiene prior to the handling of the food. Both are routes of transfer of fecal material to the food. The amount of fecal material need not be great, as studies have proven that only 10 living Shigella are required to establish an infection in humans.
The infection tends to be fairly short in duration and clears without any therapeutic intervention. In some people, however, the primary infection can be the prelude to very damaging infections of the kidney and the joins. The latter infection, which is caused by Shigella flexneri, is known as Reiter's syndrome. This can persist for years. During this time, infections by other strains of Shigella are possible.
Shigellosis results from the attachment of the bacteria to epithelial cells that line the intestinal tract, and the entry of the bacteria into the cells. Within the host cells, the bacteria divide and can then spread laterally to infect other host cells. The interior location of the bacteria protects them from any host immune response or from antibiotics . Additionally, some strains of Shigella produce the toxins that can damage the epithelial cells.
The establishment of an infection is easier in people whose immune systems are compromised. For example, shigellosis is a significant problem in those afflicted with acquired immunodeficiency syndrome.
Treatment for Shigella infections is not always clinically prudent. Many infections, while very inconvenient and painful, pass relatively quickly. Management of the symptoms, particularly ensuring proper hydration, is preferred in immunocompetent people, as opposed to antibiotic therapy. The reason is that the bacteria can rather readily acquire resistance to antibiotics, which can make eradication of the bacteria even harder. Also, the antibiotic resistant bacteria can be excreted in the feces of the infected individual, and may then spread the resistant strain to other people.
Prevention of the spread of infection involves proper hygiene and thorough cooking of foods.
See also Enterobacteriaceae; Enterotoxin and exotoxin; Food safety