SHIGELLA SONNEI: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE, DISEASES - BIOLOGY - 2024

Shigella sonnei is a bacterium of the Enterobacteriaceae family that is characterized by having the shape of a short bacillus, lacking a 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 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, together with S. flexneri, is responsible for 90% of shigellosis cases.

Shigella sonnei on SIM agar, showing a negative result for H2S production, a negative result for the indole test (after adding the Kovac reagent), and a negative result for motility. Taken and edited from: A doubt.

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 infection and infection by ingesting contaminated water or food is still the most common form of infection, infection through sexual contact is becoming more and more frequent.

characteristics

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.

Morphology

Shigella sonnei 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.

Culture

For the culture 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 development 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 culture media most frequently used for the isolation of enteropathogenic Enterobacteriaceae, including Shigella, ranked in increasing order according to their selectivity are the following:

Eosin Methylene Blue Agar (EMB)

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.

MacConkey agar

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.

Xylose-lysine-deoxycholate (XLD) agar

This is a selective medium that is commonly used to isolate Salmonella and Shigella species from both clinical and food samples. The substrates used are xylose and lysine, while the indicator is phenol red.

This culture medium makes it possible to differentiate Salmonella colonies from Shigella colonies due to the color changes that can occur over time. On the one hand, Shigella colonies will always remain red, while Salmonella colonies will first turn yellow and then return to red.

Hektoen enteric agar

This selective culture medium is mainly used to isolate Salmonella and Shigella colonies from 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.

Shigella sonnei culture after 48 hours on Hektoen enteric agar (HEK). Taken and edited from: Todd Parker, Ph.D., Assoc Director for Laboratory Science, Div of Preparedness and Emerging Infections at CDC.

Salmonella-Shigella Agar (SS)

This is a moderately selective and differential medium, which can inhibit some strains of Shigella dysenteriae serotype 1, for which it is recommended 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, Shigella sonnei 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.

Lifecycle

The only reservoir for Shigella sonnei is humans. The life cycle of the bacteria 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.

Diseases

The bacteria of the Shigella genus 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 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.

Shigella sonnei shigellosis can present some complications such as rectal prolapse, reactive arthritis, toxic megacolon, and hemolytic uremic syndrome. In addition, infections outside the gastrointestinal tract, such as bacteremia, urinary tract infection, and vulvovaginitis, may occur.

References

1. 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.
2. Shigella sonnei. On Wikipedia. Recovered from: en.wikipedia.org.
3. S. León-Ramírez (2002). Shigellosis (bacillary dysentery). Health in Tabasco.
4. M. Gil. Salmonella-Shigella agar: rationale, preparation and uses. Recovered from: lifeder.org.
5. V. Hinic, H. Seth-Smith, M. Stöckle, D. Goldenberger & A. Egli Adrian (2018). First report of sexually transmitted multi-drug resistant Shigella sonnei infections in Switzerland, investigated by whole genome sequencing. Swiss Medical Weekly.
6. JM Guevara, R. Cipriani, D. Giraldo, E. Mezarina, I. Sánchez, Z. Villagómez, A. Antezana, R. Alagón & R. Carranza (2014). Shigella sonnei: Is a change happening in our environment? Annals of the Faculty of Medicine.