Shigella flexneri is an immobile Gram negative Enterobacteriaceae bacillus that does not form spores, does not release gas from 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.
Shigella flexneri. Taken and edited from: Y tambe.
Shigellosis or bacterial dysentery caused by Shigella flexneri is more common 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.
characteristics
- Shigella flexneri is rod-shaped, Gram negative and facultative anaerobic, lacks flagella and capsule, in addition, this species does not sporulate.
- The metabolism of Shigella flexneri is unable to ferment lactose, neither does it metabolize lysine, but it does metabolize catalase. It does not generate gas as a product of the metabolism of sugars.
- It does not ferment adonitol, nor does it form acetylmethyl carbinol. It is acid tolerant, so it survives the pH of the stomach very well.
- Contains numerous virulence genes, which are related to a 220 kb virulence plasmid.
- Its DNA has a similarity of 70 to 75% with that of Escherichia coli.
Taxonomy
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 the industrialized countries.
Morphology
Shigella flexneri is a short bacillus, 0.7 to 1.5 μm long, as well as 0.3 to 0.8 μm in diameter.
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 flagellum, but it does present type 1 fimbriae, a characteristic that differentiates it from other congeneric species. In solid culture medium it grows only around the inoculation site.
Shigella flexneri has a plasmid of approximately 220 kb, which is of vital importance in the invasion process.
Culture
Gram negative broth and selenite cystine broth are recommended to inhibit the growth of Gram positive bacteria and favor 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 maintained to achieve the maximum growth of the bacteria is 35 ° C, however, to favor the development of Shigella flexneri, some authors suggest that the ideal incubation temperature is 42 ° C.
Gram negative broth, according to some authors, shows poor results to recover and allow the growth of stressed Shigella sonnei and S. flexneri colonies (with acidic and pungent solutions).
Salmonella-Shigella (SS) Agar, MacConkey Agar are mainly used as culture media for the different species of Shigella. The first of these is moderately selective and on it grow colorless, convex colonies of Shigella, no more than 4 mm in diameter.
Culture of bacteria of the Shigella genus. Taken and edited from: Prakash R1, Bharathi Raja S, Devaraj H, Devaraj SN..
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. On this medium, the Shigella colonies show an appearance similar to that obtained on SS agar.
Other culture media used for growing S. flexneri include Hektoen Enteric Agar (HEA), XLD Agar (Xylose Lysine Deoxycholate agar), DCA Agar (Deoxycholate Citrate agar), and Tergitol-7 agar.
Lifecycle
The Shigella flexneri bacteria only reproduce 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 water or food.
A few specimens of this species are necessary to infect a new host due to the fact that they are tolerant of acid from 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.
As Shigella flexneri lacks a flagellum, it causes the actin filaments of the infected cell to polymerize to promote their movement in the cell's cytosol. As long as the bacterial infection remains, some bacteria are flushed out 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 the water or food, they will infect a new host and the cycle will repeat.
TSI (triple sugar iron) agar slant results. From left: 1) uninoculated (as a control); 2) P. aeruginosa: Glc (-), Lac / Suc (-), H 2 S (-), Gas (-); 3) E. coli: Glc (+), Lac / Suc (+), H 2 S (-), Gas (+); 4) Salmonella Typhimurium: Glc (+), Lac / Suc (-), H 2 S (+), Gas (+); 5) Shigella flexneri: Glc (+), Lac / Suc (-), H 2 S (-), Gas (-). Taken and edited from: Y_tambe.
Diseases
Bacteria of the genus Shigella 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 higher 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 the cost of medicines.
Shigellosis can be treated with antibiotics, however, some Shigella flexneri strains have developed resistance to many antibiotics, mainly the cheapest and most commonly used in developing countries.
References
- M. Uyttendaele, CF Bagamboula, E. De Smet, S. Van Wilder & J. Debevere (2001). Evaluation of culture media for enrichment and isolation of Shigella sonnei and S. flexneri. International Journal of Food Microbiology.
- AV Jennison & NK Verma (2003). Shigella flexneri infection: pathogenesis and vaccine development. FEMS Microbiology Reviews.
- Shigella flexneri. On Wikipedia. Recovered from: en.wikipedia.org.
- S. León-Ramírez (2002). Shigellosis (bacillary dysentery). Health in Tabasco.
- LM Bush & MT Pérez. Shigellosis (bacillary dysentery). Recovered from: msdmanuals.com.
- 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.
- M.Gil. Salmonella-Shigella agar: rationale, preparation and uses. Recovered from: lifeder.org.