Shigella Dysenteriae: Characteristics, Morphology, Culture, Diseases

Shigella dysenteriae is a bacterium of the Enterobacteriaceae family that is characterized in that it has a bacillus shape, is flagellate, is stained pink with Gram stain, does not form spores, nor does it produce gas when metabolizing carbohydrates.

This bacterium belongs to serogroup A of the genus Shigella . This genus is widely distributed worldwide, representing the main cause of bacterial or bacillary dysentery. In addition to serogroup A, there are three other main subgroups in this genus represented by the letters B to D ( S. flexneri, S. boydii, and S. sonnei, respectively).

Bacterial dysentery, also known as shigellosis, is an acute infection of the lining of the intestine that presents as diarrhea accompanied by fever, nausea or vomiting, colic and tenesmus, which mainly affects children under five years of age and the elderly, with high rates of morbidity and mortality worldwide.

Shigella dysenteriae is the most virulent species of the genus and humans are the only reservoir for this and the other Shigella . The source of the infection is infested people and the spread can be directly through the fecal-oral route, or indirectly through contaminated food or water. Treatment is through antibiotics.

characteristics

The body shape of Shigella dysenteriae is that of an elongated rod (bacillus). Its cell wall is simple and unable to retain crystal violet and iodine during the Gram staining technique, which is why it only acquires a pinkish coloration, which defines it as a Gram negative species.

It is a facultative anaerobic species, which means that it can survive both in aerobic conditions and in conditions of lack of oxygen. Furthermore, this species is non-mobile due to the lack of a flagellum, and it never forms spores as a reproductive strategy.

In laboratory tests, it is negative to lactose and lysine, which implies that it does not react with either of these two elements.

Unlike other enteric bacteria, Shigella dysenteriae and other congeneric bacteria do not produce gas when metabolizing carbohydrates.

Bacteria of this species are very resistant to acids, due to this they survive gastric acids very well, and a few organisms that reach the gastrointestinal tract are capable of colonizing it.

Despite not forming spores, this species shows great resistance. For example, it is able to survive for about two months in soil and for more than a week in water.

Taxonomy

Shigella dysenteriae is a bacterium belonging to the phylum Proteobacteria, Gammaproteobacteria class, Enterobacteriales order and the Enterobacteriaceae family. It is included within the genus Shigella , of which it is the type species.

Researchers estimate that the first isolation of a representative of this genus was made by Chantenesse and Widal in 1888, however, the first description of the genus was made by the Japanese researcher Kiyoshi Shiga nine years later, during a dysentery epidemic in Japan. .

Shiga gave a very detailed description of a species that he named ” Bacillus dysenteriae .” The researchers later relocated this species to a new genus, dedicated to Shiga, and today it is known as Shigella dysenteriae type 1.

Shigella dysenterae has 13 different serotypes, of which serotype 1 is the one with the greatest clinical importance. This is characterized by not fermenting mannitol, lacking catalase, and presenting a very active ß-galactosidase enzyme.

Morphology

Shigella dysenterae , like other congeneric bacteria, is a small bacillus, with a size that does not exceed 1.5 μm in length and 0.8 μm in diameter.

Its cell wall is represented by a thin layer of peptidoglycans, without teichoic and lipoteichoic acids, nor does it present a capsule.

The species is non-mobile due to the absence of a flagellum. It also does not present fimbriae or pilis. In solid culture medium it grows only around the inoculation site.

Shigella dysenterae has a plasmid of approximately 220 kb that, according to scientists, is of vital importance in the invasion process

Culture

There are different culture media for the isolation and identification of bacteria of the genus Shigella . To increase the probability of isolating these bacteria, some culture broths can be previously used, such as:

Gram Broth – Negative (GN)

This broth contains sodium deoxycholate and sodium citrate, both of which inhibit the growth of Gram positive bacteria. In addition, it contains mannitol and tryptose as a carbon source, which favor the growth of bacteria that use these sugars, such as Salmonella spp. and Shigella spp., while limiting those that ferment dextrose, such as Proteus .

Substances such as nitrogen, vitamins, minerals and amino acids are also part of this broth. The samples in this broth are cultured for short periods of time (4-6 hours) at a temperature of 35 ° C.

Selenite broth F and tetrathionate broth

These broths inhibit the growth of most Gram negative bacteria, but favor the growth of other bacteria, such as Shigella and Salmonella . The cultures are maintained for slightly longer periods (6-12 hours) and allow increasing the number of organisms of these genera in the case of patients with mild infections.

After increasing the number of organisms in the sample, different media (support, differential, moderately selective or highly selective) can be used. The most commonly used means are the following:

Salmonella-Shigella (SS) agar

It is a moderately selective and differential medium, made with peptones, lactose, Na thiosulfate, Na citrate, NH4 citrate and iron III. Meanwhile, the bright green substrates and ox bile are inhibitors of some other bacterial species.

The samples should be cultured at 35-37 ° C and have an incubation time of 18-24 hours. In these cultures, Shigella species lack the enzymes that allow them to metabolize lactose, for which reason they will grow as transparent, convex colonies, 2 to 4 mm in diameter.

Although this culture method is selective and differential for Salmonella spp. and Shigella spp., can inhibit the growth of some strains of Shigella dysenteriae serotype 1, so it is recommended to use it in conjunction with another medium, such as MacConkey Agar.

MacConkey agar

This culture medium contains bile salts and crystal violet, which are compounds that inhibit the growth of Gram positive bacteria. It also contains peptone and lactose, which allows the separation of Gram negative bacteria into fermenters and non-fermenters.

All Shigella species are unable to ferment lactose because they lack the enzymes galactosidopermease and galactosidase. Because of this, the colonies appear colorless or transparent, convex and small (up to 4 mm in diameter), after one day of incubation.

Lifecycle

The only reservoir for Shigella dysenteriae is humans, although the bacteria can survive for up to 11 days in water and about two months in dirty laundry. When it enters the human digestive system, it can tolerate gastric acids, requiring a very small number of cells to infect a new host.

Once inside the new host, the bacterium attaches to the walls of the small intestine and begins to reproduce asexually by fission. During this period there is no clinical manifestation of the disease. Virulence is determined by the presence of a plasmid essential for colonization of the intestinal epithelium.

The initial entry of the bacteria to the mucosa of the colon is via the M cells in the follicles associated with the epithelium, from which it migrates without causing damage, at that time it is capable of infecting the cells of the intestinal epithelium through the basolateral membranes, never through the ciliated edges.

In the cytoplasm of the epithelial cells, the bacteria multiply and spread to other cells, causing tissue death as well as an inflammatory response. The disease is aggravated by the production of Shiga toxin by S. dysenteriae and is associated with hemolytic uremic syndrome and other serious conditions.

During the time that the bacteria remain in the host, some bacteria will be released with the stools and can colonize a new person to start the cycle again.

Diseases

Shigella dysenteriae is the most virulent of all species in the genus Shigella . All of them can cause shigellosis or bacillary dysentery, but in the case of S. dysenteriae it can present various types of complications, mainly toxic megacolon and uremic-hemolytic syndrome.

Shigellosis

Shigellosis or bacillary dysentery is an acute intestinal infection caused by bacteria of the genus Shigella . The disease is transmitted from a contaminated person to a healthy person in a direct fecal-oral way, or indirectly by ingesting contaminated food or water. It can also be transmitted through sexual contact.

Symptoms of the disease include an abrupt rise in body temperature that can reach 40 ° C, nausea with or without vomiting, generally bloody diarrhea, with mucus and with a sensation of incomplete evacuation, abdominal pain and headache. Dehydration is a frequent complication in these cases.

In some cases, shigellosis can present symptoms that can be mistakenly attributed to meningoencephalitis.

Hemolytic uremic syndrome

Disease produced by toxins released by different bacteria, including Shigella dysenteriae , which affects the blood vessels of the kidney mainly, but can also attack the nervous and gastrointestinal systems.

Symptoms of the syndrome include kidney failure, as well as a decreased number of thrombocytes and hemolytic anemia. Besides Shigella, other bacteria can cause the syndrome, such as Escherichia coli and Salmonella species .

Toxic megacolon

Megacolon is defined as an increase in the normal diameter of the colon by more than six centimeters. When it occurs acutely it is called toxic megacolon. Causes of this disease include ulcerative colitis, Crohn’s disease, and Salmonella, Shigella, Campylobacter, Yersinia, and Trypanosoma infections .

References

  1. Student presentation on Shigella dysenteriae . Recovered from: web.uconn.edu.
  2. Shigella dysenteriae . On Wikipedia. Recovered from: en.wikipedia.org.
  3. S. León-Ramírez (2002). Shigellosis (bacillary dysentery). Health in Tabasco.
  4. LM Bush & MT Pérez. Shigellosis (bacillary dysentery). Recovered from: msdmanuals.com.
  5. AA Nash, RG Dalziel & JR Fitzgerald (2015). Attachment to and entry of microorganisms into the body, in Mims’ pathogenesis of infectious disease. 6 th edition. Elsevier.
  6. M. Gil. Salmonella-Shigella agar : foundation, preparation and uses. Recovered from: lifeder.org.
  7. C. Lyre. Gram negative bacteria: general characteristics, structure, diseases, examples. Recovered from: lifeder.org.

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