Shigella sonnei It is a bacterium of the Enterobacteriaceae family that is characterized by having a short bacillus shape, lacking flagellum, fimbriae and capsule. Other characteristics of the species include that it is lactose and lysine negative and catalase positive, in addition, it does not sporulate or release gas when metabolizing carbohydrates.
This bacterium belongs to serogroup D of the genus Shigella and it has only one serotype identified to date. It is the most common species of the genus in developed countries and is increasingly isolated from patients in developing countries. This species, along with S. flexneri, is responsible for 90% of shigellosis cases.
In addition to causing shigellosis, Shigella sonnei can cause bacteremia, urinary tract infection, vulvovaginitis, rectal prolapse, reactive arthritis, and various other complications.
Although direct contamination via fecal buccal and infection by ingestion of contaminated water or food is still the most common form of infection, contagion through sexual contact is becoming more and more frequent..
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Shigella flexneri is short rod-shaped, about twice its length in length.
Its cell wall is made up of peptidoglycans arranged in a simple layer (Gram negative), not surrounded by an extracellular capsule and it is an immobile species because it lacks a flagellum, nor does it have fimbriae. It can grow and carry out its metabolic activities both in the presence and in the absence of oxygen.
It reproduces asexually by fission and does not produce spores. It is lactose and lysine negative, catalase positive, and acid tolerant. Your metabolism does not release gas when fermenting carbohydrates.
Its habitat is the human colon, this being the only reservoir of the species.
Shigella sonnei was first described to science as Bacterium sonnei by Levine in 1920, and relocated to the genre Shigella by Weldin in 1927. This species is taxonomically located in the phylum Proteobacteria, class Gammaproteobacteria, order Enterobacteriales, family Enterobacteriaceae.
This is a clonal species and researchers estimate that it first appeared in Europe around 1500 years BC. C. It is a very conservative species and only one serotype is known..
Shigella sonnei it is a rod-shaped bacterium, with a maximum length of 1.7 μm and a diameter close to half this length. Its cell wall is simple, composed of peptidoglycans, without teichoic and lipoteichoic acids, without extracellular capsule, flagella or fimbriae..
Internally, a double-stranded circular chromosome and a virulence plasmid of approximately 220 Kb are observed..
For the cultivation of Shigella A preculture in Gram negative broth or selenite cystine broth is recommended for a period of 16 hours and at a temperature of 35 ° C to inhibit the growth of Gram positive bacteria and favor that of Gram negative bacteria..
Subsequently, various culture media can be used, which have different selective capacity. According to some researchers, the most commonly used culture media for the isolation of enteropathogenic Enterobacteriaceae, including Shigella, ordered in increasing order according to their selectivity are the following:
It is a selective and differential medium in which methylene blue inhibits the growth of Gram positive bacteria, as well as some Gram negative bacteria. On the other hand, eosin indicates the presence or absence of fermentation by showing changes in coloration with pH.
This culture medium is also selective and differential. Contains bile salts and crystal violet, responsible for inhibiting the growth of Gram positive bacteria. Fermenting and non-fermenting bacteria can be differentiated and detected for their part, using lactose as a substrate and neutral red as a pH indicator..
This is a selective medium that is commonly used to isolate species of Salmonella and of Shigella, from both clinical and food samples. The substrates used are xylose and lysine, while the indicator is phenol red.
This culture medium allows to differentiate colonies of Salmonella of those of Shigella due to the color changes that may occur over time. On the one hand, the colonies of Shigella will always remain red, while those of Salmonella they will first change to yellow and then they will return to red.
This selective culture medium is used mainly to isolate colonies of Salmonella and of Shigella of stool samples. The substrates it uses are different carbohydrates such as lactose, sucrose and salicin and it also contains peptone. This medium allows the growth of other bacterial species, but does not discriminate between them.
This is a moderately selective and differential medium, which can inhibit some strains of Shigella dysenteriae serotype 1, therefore it is advisable to use it simultaneously with another culture medium.
This agar contains bright green and ox bile which inhibit the growth of some species of bacteria..
An important factor to take into account is that when working with media such as MacConkey, Hektoen or SS, which use lactose as a substrate, it is that Shigella sonnei it can spontaneously secrete the virulence plasmid. Due to this, it can give slightly lactose positive results, as well as present two morphological types after 24 to 48 hours.
The only reservoir of Shigella sonnei It is the human being. The life cycle of the bacterium begins when it invades the epithelium of the colon. Once it manages to colonize it, the bacteria begin to replicate and cause the disease known as shigellosis. The bacteria can replicate many times.
Shigellosis is a self-limited disease that lasts for several days. During this period, the bacteria can divide numerous times and can eventually be shed by the host..
Once evacuated, the bacteria can last up to about three months in the environment, and if some of them are accidentally ingested by another host, they will infect it to start the cycle again.
Bacteria of the genus Shigella they are all responsible for an acute infection of the colon epithelium known as shigellosis or bacillary dysentery, whose symptoms include rectal bleeding, diarrhea, fevers, among others. It is associated with high levels of morbidity and mortality.
Shigella sonnei It is the main cause of shigellosis in developed countries, however, in the last two decades it has become more frequent in developing countries. The forms of contagion are oral fecal route, by ingestion of contaminated water or food, as well as by sexual contact between an infected person and a healthy person..
Shigellosis by Shigella sonnei It can present some complications such as rectal prolapse, reactive arthritis, toxic megacolon and hemolytic uremic syndrome. In addition, infections outside the gastrointestinal tract may occur, such as bacteremia, urinary tract infection, and vulvovaginitis..
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