Shigella - AbsoluteAstronomy.com

This article is about the genus. For the disease, see shigellosis
Shigellosis
Shigellosis, also known as bacillary dysentery or Marlow Syndrome, in its most severe manifestation, is a foodborne illness caused by infection by bacteria of the genus Shigella. Shigellosis rarely occurs in animals other than humans and other primates like monkeys and chimpanzees...

Shigella is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

of Gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

, nonspore forming

Endospore

An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...

, non-motile, rod-shaped bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

closely related to Escherichia coli
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

and Salmonella
Salmonella
Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

. The causative agent of human shigellosis

Shigellosis

Shigellosis, also known as bacillary dysentery or Marlow Syndrome, in its most severe manifestation, is a foodborne illness caused by infection by bacteria of the genus Shigella. Shigellosis rarely occurs in animals other than humans and other primates like monkeys and chimpanzees...

, Shigella causes disease in primate

Primate

A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s, but not in other mammals. It is only naturally found in humans and apes. During infection, it typically causes dysentery

Dysentery

Dysentery is an inflammatory disorder of the intestine, especially of the colon, that results in severe diarrhea containing mucus and/or blood in the faeces with fever and abdominal pain. If left untreated, dysentery can be fatal.There are differences between dysentery and normal bloody diarrhoea...

. The genus is named after Kiyoshi Shiga

Kiyoshi Shiga

was a Japanese physician and bacteriologist.-Biography:Shiga was born in Sendai, Miyagi Prefecture, though his original family name was Satō. He graduated from the Medical School of Tokyo Imperial University in 1896 and went to work at the Institute for the Study of Infectious Diseases under Dr....

, who first discovered it in 1898.

Phylogenetic studies indicate that Shigella is more appropriately treated as subgenus

Subgenus

In biology, a subgenus is a taxonomic rank directly below genus.In zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic name and the specific epithet: e.g. the Tiger Cowry of the Indo-Pacific, Cypraea tigris Linnaeus, which...

of Escherichia
Escherichia
Escherichia is a genus of Gram-negative, non-spore forming, facultatively anaerobic, rod-shaped bacteria from the family Enterobacteriaceae. In those species which are inhabitants of the gastrointestinal tracts of warm-blooded animals, Escherichia species provide a portion of the...

, and that certain strains generally considered E. coli – such as EHEC O157:H7

Escherichia coli O157:H7

Escherichia coli O157:H7 is an enterohemorrhagic strain of the bacterium Escherichia coli and a cause of foodborne illness. Infection often leads to hemorrhagic diarrhea, and occasionally to kidney failure, especially in young children and elderly persons...

– are better placed in Shigella (see Escherichia coli#Diversity for details).

Classification

Shigella species are classified by four serogroups

Serotype

Serotype or serovar refers to distinct variations within a subspecies of bacteria or viruses. These microorganisms, viruses, or cells are classified together based on their cell surface antigens...

Serogroup A: S. dysenteriae
Shigella dysenteriae
Shigella dysenteriae is a species of the rod-shaped bacterial genus Shigella. Shigella can cause shigellosis . Shigellae are Gram-negative, non-spore-forming, facultatively anaerobic, non-motile bacteria.S...

(12 serotypes)
Serogroup B: S. flexneri
Shigella flexneri
Shigella flexneri is a species of Gram-negative bacteria in the genus Shigella that can cause diarrhea in humans. There are several different serogroups of Shigella; S. flexneri belongs to group B. S. flexneri infections can usually be treated with antibiotics although some strains have become...

(6 serotypes)
Serogroup C: S. boydii
Shigella boydii
Shigella boydii is a Gram-negative bacteria of the genus Shigella. Like other member of the genus, S. boydii is a non-motile, non-sporeforming, rod-shaped bacteria which can cause dysentery in humans through fecal-oral contamination....

(23 serotypes)
Serogroup D: S. sonnei
Shigella sonnei
Shigella sonnei is a species of Shigella. Together with Shigella flexneri, it is responsible for 90% of shigellosis. Shigella sonnei is named for the Danish bacteriologist Carl Olaf Sonne....

(1 serotype)

Groups 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

Dysentery

that results in the destruction of the epithelial cells of the intestinal mucosa in the cecum

Cecum

The cecum or caecum is a pouch, connecting the ileum with the ascending colon of the large intestine. It is separated from the ileum by the ileocecal valve or Bauhin's valve, and is considered to be the beginning of the large intestine. It is also separated from the colon by the cecocolic...

and rectum

Rectum

The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...

. Some strains produce enterotoxin

Enterotoxin

An enterotoxin is a protein toxin released by a microorganism in the intestine. Enterotoxins are chromosomally encoded exotoxins that are produced and secreted from several bacterial organisms. They are often heat-stable, and are of low molecular weight and water-soluble...

and shiga toxin

Shiga toxin

Shiga toxins are a family of related toxins with two major groups, Stx1 and Stx2, whose genes are considered to be part of the genome of lambdoid prophages. The toxins are named for Kiyoshi Shiga, who first described the bacterial origin of dysentery caused by Shigella dysenteriae. The most common...

, similar to the verotoxin of E. coli O157:H7 and other verotoxin-producing Escherichia coli

Verotoxin-producing Escherichia coli

Verotoxin-producing Escherichia coli comprise strains of the bacterium Escherichia coli that, when infecting humans, have been linked with the severe complication hemolytic-uremic syndrome . They are known by a number of names, including enterohemorrhagic E. coli , Shiga-like toxin-producing E...

. Both shiga toxin and verotoxin are associated with causing hemolytic uremic syndrome. As noted above, these supposed E. coli strains are at least in part actually more closely related to Shigella than to the "typical" E. coli.

Shigella invade the host through the M-cells in the gut epithelia of the large intestine

Large intestine

The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...

, as they cannot enter directly through the epithelial cells. Using a Type III secretion system acting as a biological syringe, the bacterium injects IpaD protein into cells, triggering bacterial invasion and the subsequent lysis

Lysis

Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....

of vacuolar

Vacuole

A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain...

membranes using IpaB and IpaC proteins. It uses a mechanism for its motility by which its IcsA protein triggers actin polymerization in the host cell (via N-WASP

Wiskott-Aldrich syndrome protein

The Wiskott–Aldrich Syndrome Protein is a 502-amino acid protein that is expressed in cells of the hematopoietic system. In the inactive state, WASp exists in an auto-inhibited conformation with sequences near its C-terminus binding to a region near its N-terminus...

recruitment of Arp2/3 complexes) in a "rocket" propulsion fashion for cell-to-cell spread.
The most common symptoms are diarrhea

Diarrhea

Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...

, fever

Fever

Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

, nausea

Nausea

Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...

, vomiting

Vomiting

Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...

, stomach cramps and flatulence

Flatulence

Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply gas...

. The stool may contain blood, mucus, or pus. In rare cases, young children may have seizure

Seizure

An epileptic seizure, occasionally referred to as a fit, is defined as a transient symptom of "abnormal excessive or synchronous neuronal activity in the brain". The outward effect can be as dramatic as a wild thrashing movement or as mild as a brief loss of awareness...

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

Reactive arthritis

Reactive arthritis , is classified as an autoimmune condition that develops in response to an infection in another part of the body. Coming into contact with bacteria and developing an infection can trigger the disease. Reiter's syndrome has symptoms similar to various other conditions collectively...

worldwide.

Each of the Shigella genomes includes a virulence plasmid

Plasmid

In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

that encodes conserved primary virulence determinants. The Shigella chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s share most of their genes with those of E. coli K12 strain MG1655

Diagnosis

Shigella species are negative for motility and are not 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 (TSI) slant

TSI 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. Growth on Hektoen enteric agar
Hektoen enteric agar
Hektoen enteric agar is a selective and differential agar primarily used to recover Salmonella and Shigella from patient specimens. HE contains indicators of lactose fermentation and H2S production; as well as inhibitors to prevent the growth of gram positive bacteria...

will produce bluish-green colonies for Shigella and bluish-green colonies with black centers for Salmonella
Salmonella
Salmonella is a genus of rod-shaped, Gram-negative, non-spore-forming, predominantly motile enterobacteria with diameters around 0.7 to 1.5 µm, lengths from 2 to 5 µm, and flagella which grade in all directions . They are chemoorganotrophs, obtaining their energy from oxidation and reduction...

.

Treatment

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. Antibiotics are usually avoided in mild cases because some Shigella are resistant to antibiotics, and their use may make the germ even more resistant. Antidiarrheal agents may also worsen the sickness.

For Shigella-associated diarrhea, antibiotics shorten the length of infection.

External links

Shigella genomes and related information at PATRIC, a Bioinformatics Resource Center funded by NIAID
World Health Organization. Shigella. Available at: http://www.who.int/topics/shigella/en/.
CDC. Shigellosis: General information. Available at: http://www.cdc.gov/nczved/divisions/dfbmd/diseases/shigellosis/.

Classification

Pathogenesis

Diagnosis

Treatment

External links

See also