Shigella flexneri It is a Gram negative, immobile Enterobacteriaceae bacillus that does not form spores, does not release gas as a result of carbohydrate fermentation and cannot metabolize (neither lactose nor lysine). Other characteristics that it shares with the other species of the genus are the absence of a capsule and that it is facultative anaerobic.
This bacterium belongs to serogroup B of the genus Shigella. It is the species of the genus most commonly isolated from patients with bacterial dysentery in developing countries. Of this species, 8 serotypes have been described, of which 2a, 1b, 3a, 4a and 6 are among the most virulent.
Shigellosis or bacterial dysentery caused by Shigella flexneri is more frequent but less severe than that caused by S. dysenteriae. This disease, which mainly affects the elderly and infants under 5 years of age, is associated with high rates of morbidity and mortality throughout the world..
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- Shigella flexneri It has a bacillus shape, it is Gram negative and facultative anaerobic, it lacks flagella and capsule, in addition, this species does not sporulate.
- The metabolism of Shigella flexneri It is incapable of fermenting lactose, nor does it metabolize lysine, but it does metabolize catalase. Does not generate gas as a product of sugar metabolism.
- It does not ferment adonitol, nor does it form acetylmethyl carbinol. It is acid tolerant, which is why it survives very well the pH of the stomach.
- Contains numerous virulence genes, which are related to a 220 kb virulence plasmid.
- Their DNA is 70 to 75% similar to that of Escherichia coli.
Shigella flexneri is a Proteobacteria included in the class Gammaproteobacteria, order Enterobacteriales, family Enterobacteriaceae, genus Shigella. This species was described by Castellani and Chalmers in 1919.
This species currently has eight identified serotypes of which the most virulent are those identified as 1b, 2a, 3a, 4a, and 6. All of them present in developing countries, except for 2a, which is more frequent in developing countries. industrialized countries.
Shigella flexneri it is a short bacillus, with a length of 0.7 to 1.5 μm in length, as well as a diameter of 0.3 to 0.8 μm.
Its cell wall is represented by a thin layer of peptidoglycans, without teichoic and lipoteichoic acids. It also does not present a capsule.
The species is non-mobile due to the absence of a flagellum, but it does present type 1 fimbriae, a characteristic that differentiates it from other congeneric species. In solid culture medium grows only around the inoculation site.
Shigella flexneri presents a plasmid of approximately 220 kb, which is of vital importance in the invasion process.
Gram negative broth and selenite cystine broth are recommended to inhibit the growth of Gram positive bacteria and promote the growth of Gram Negative bacteria that metabolize mannitol and tryptose, such as Salmonella spp., and Shigella spp. The incubation time in these broths is 16 hours.
The temperature at which the broth must be kept to achieve maximum bacterial growth is 35 ° C, however, to favor the development of Shigella flexneri, some authors suggest that the ideal incubation temperature is 42 ° C.
The Gram negative broth, according to some authors, shows poor results to recover and allow the growth of colonies of Shigella sonnei Y S. flexneri stressed (with acidic and spicy solutions).
As culture media for the different species of Shigella Salmonella-Shigella (SS) Agar, MacConkey Agar are mainly used. The first of these is moderately selective and in it grow colonies of Shigella colorless, convex and no more than 4 mm in diameter.
MacConkey Agar, on the other hand, inhibits the growth of Gram positive bacteria and separates the fermenting Gram negative bacteria from the non-fermenting ones. In this environment, the colonies of Shigella show an appearance similar to that obtained on SS agar.
Other culture media used for the cultivation of S. flexneri include Hektoen Enteric Agar (HEA), XLD Agar (Xylose Lysine Deoxycholate agar), DCA Agar (Deoxycholate Citrate agar) and Tergitol-7 agar.
The bacteria Shigella flexneri It only reproduces inside the human being, although it can survive for some time outside its host. The infection is transmitted directly from an infected organism to a healthy one, or it can occur indirectly through the ingestion of contaminated food or water.
A few specimens of this species are necessary to infect a new host due to the fact that they are tolerant to the acid of gastric juices. The pathogenesis of this species is related to its ability to invade and replicate in the colon epithelium, which it manages to reach through the basolateral membranes..
What Shigella flexneri lacks a flagellum, it causes the actin filaments of the infected cell to polymerize to promote their displacement in the cell's cytosol. As long as the bacterial infection remains, some bacteria are expelled to the outside of the body during bowel movements..
As already noted, these bacteria can survive for some time outside the host. If some of these bacteria contaminate water or food, they will infect a new host and the cycle will repeat..
Bacteria of the genus Shigella they are all responsible for basilar dysentery, also known as shigellosis. This disease is an acute infection of the epithelium and it can cause bloody diarrhea, high fevers, rectal bleeding, as well as nausea, vomiting, headaches and even death.
Shigella flexneri is endemic in most developing countries, and despite being less virulent than S. dysenteriae, it causes greater mortality than this or any of the other species of the genus. Its high incidence in these countries is due to inadequate water treatment, malnutrition and also to the cost of medicines.
Shigellosis can be treated with antibiotics, however some strains of Shigella flexneri have developed resistance to many antibiotics, mainly the cheapest and most commonly used in developing countries.
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