Shigella sonnei: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes


Shigella sonnei is a bacterium that is responsible for causing shigellosis, a type of bacterial infection affecting the digestive system. Shigellosis is commonly characterized by symptoms such as diarrhea, abdominal pain, fever, and occasionally, bloody stools. It is a specific serotype of the Shigella genus, which also includes other species such as Shigella flexneri, Shigella boydii, and Shigella dysenteriae.

It is primarily transmitted through the fecal-oral route, meaning that it spreads when people consume food or water contaminated with the bacterium. This can occur due to poor hygiene practices, such as inadequate handwashing after using the toilet or handling contaminated objects. The bacteria can survive in various environments and can be particularly prevalent in areas with inadequate sanitation facilities or crowded conditions.

Upon ingestion, S. sonnei bacteria invade the lining of the intestines, causing inflammation and damage to the cells. This results in the characteristic symptoms of shigellosis, including watery or bloody diarrhea, cramps, and fever. The infection can range from mild to severe, with severe cases potentially leading to complications such as dehydration, rectal prolapse, or even seizures in young children.

Diagnosis of Shigella sonnei infection is typically done through stool sample analysis, where the bacterium can be identified and confirmed through laboratory tests. Antibiotic susceptibility testing is also performed to determine the appropriate antibiotics for treatment, as antibiotic resistance in Shigella species has become a growing concern.

The treatment of Shigella sonnei infections often involves antibiotics to help control the infection and reduce the duration of illness. However, due to increasing antibiotic resistance, healthcare providers must carefully select appropriate antibiotics based on susceptibility testing results. Additionally, supportive care, such as oral rehydration therapy, is crucial in managing dehydration and maintaining fluid and electrolyte balance.

Prevention of Shigella sonnei infection focuses on practicing good hygiene and sanitation measures, including regular handwashing with soap and clean water, especially before handling food or eating. Proper sanitation and safe handling of food and water sources are also essential to prevent contamination. In some cases, vaccination against Shigella is being explored as a potential preventive measure, although there is currently no widely available vaccine specifically targeting S. sonnei.


Shigella sonnei is a gram-negative bacterium that belongs to the family Enterobacteriaceae. Morphologically, it appears as small, non-motile rods (bacilli). Here are some key characteristics of S. sonnei morphology:

Shigella sonnei colony morphology on CLED agar of Urine Culture
Fig. Shigella sonnei colony morphology on CLED agar of Urine Culture
  1. Shape: S. sonnei bacteria are straight rods with a cylindrical shape. They are typically about 0.5 to 0.8 micrometers in diameter and 1.5 to 3 micrometers in length.
  2. Gram Staining: It is gram-negative, which means it does not retain the crystal violet stain during the Gram staining procedure. Instead, it takes up the counterstain safranin, appearing pink or red under a microscope.
  3. Capsule: It does not produce a capsule. A capsule is a protective layer surrounding some bacteria, but Shigella species generally lack this structure.
  4. Flagella: It is non-motile and does not possess flagella, which are whip-like appendages used for bacterial motility.
  5. Spore Formation: S. sonnei does not form spores. Spores are dormant and highly resistant structures formed by some bacteria under unfavorable conditions.
  6. Arrangement: S. sonnei bacteria typically appear as single cells, arranged singly or in short chains. They do not form complex arrangements like clusters or long chains.
Gram Negative Rods (GNRs) of Shigella sonnei
Fig. Gram Negative Rods (GNRs) of Shigella sonnei


Shigella sonnei is a highly pathogenic bacterium and is the most common cause of shigellosis, a gastrointestinal infection characterized by diarrhea and other associated symptoms. Here are some key aspects of the pathogenicity of S. sonnei:

  1. Invasion of Intestinal Epithelium: It has the ability to invade the epithelial cells lining the intestines. It uses various virulence factors to penetrate the intestinal mucosa and gain access to the underlying tissues. Once inside the cells, it multiplies and spreads to adjacent cells, causing tissue damage and inflammation.
  2. Production of Shigatoxin: It can produce Shigatoxin, also known as Shiga toxin. This toxin is responsible for the severe symptoms associated with shigellosis, including bloody diarrhea and kidney damage. Shigatoxin inhibits protein synthesis in host cells, leading to cell death and tissue damage.
  3. Inflammatory Response: The invasion of S. sonnei into the intestinal epithelium triggers a strong inflammatory response. This leads to the release of pro-inflammatory cytokines and chemokines, causing inflammation and tissue destruction in the intestines. The inflammatory response contributes to the symptoms of shigellosis, such as abdominal pain and fever.
  4. Cell-to-Cell Spread: It has the ability to spread from cell to cell without exiting the host cell. This mechanism allows the bacterium to evade the immune system and continue to cause infection within the host.
  5. Ability to Survive in Phagocytes: S. sonnei can survive and replicate within phagocytes, which are immune cells responsible for engulfing and destroying bacteria. This intracellular survival allows the bacterium to evade immune defenses and potentially disseminate to other tissues.
  6. Adherence and Colonization: It produces adhesins that aid in its attachment to the intestinal epithelial cells. This allows the bacterium to colonize the intestines and establish infection.

Lab Diagnosis

The laboratory diagnosis of Shigella sonnei infection typically involves several methods to accurately identify the bacterium. Here are some common laboratory techniques used for the diagnosis of S.sonnei:

Shigella sonnei in wet mount of Tryptone soy broth microscopy
Fig. Shigella sonnei in wet mount of Tryptone soy broth microscopy
  1. Stool Sample Collection: A fresh stool sample is collected from the patient suspected of having a Shigella sonnei infection. It is important to collect the sample as early as possible during the acute phase of the illness.
  2. Microscopy: The stool sample is examined microscopically for the presence of Shigella sonnei. Gram staining may be performed to visualize the gram-negative rods. S. sonnei appears as straight rods with a pink or red color due to its gram-negative nature.
  3. Culture: The stool sample is inoculated onto selective culture media that promote the growth of Shigella species while inhibiting the growth of other bacteria. Commonly used selective media for Shigella sonnei include MacConkey agar and Xylose Lysine Deoxycholate (XLD) agar. The plates are then incubated at an appropriate temperature (usually around 37°C) for 24-48 hours.
  4. Biochemical Tests: After growth on the selective media, biochemical tests are performed to confirm the identification of S. sonnei. These tests may include testing for the fermentation of sugars, production of specific enzymes, and other biochemical characteristics that differentiate S. sonnei from other bacteria.
  5. Serological Testing: Serological tests may be employed to identify specific antigens associated with Shigella sonnei. The most common serological test used for Shigella species is the slide agglutination test, which involves mixing the bacterial isolate with specific antibodies (antisera) against S.sonnei. If agglutination occurs, it indicates the presence of Shigella sonnei antigens.
  6. Molecular Methods: Polymerase Chain Reaction (PCR) techniques may be used for rapid and specific detection of Shigella sonnei DNA in clinical samples. PCR-based assays can target specific genes or regions of the bacterial genome and provide highly accurate and rapid results.


The treatment of Shigella sonnei infection, also known as shigellosis, typically involves a combination of supportive care and antibiotic therapy. Here are the main aspects of treatment for S. sonnei:

  1. Supportive Care: Rehydration is a critical component of treatment for shigellosis as the infection can lead to dehydration due to diarrhea. Oral rehydration therapy (ORT) with a solution containing a proper balance of water, electrolytes, and glucose is often the first line of treatment. In severe cases, intravenous fluids may be necessary to correct dehydration.
  2. Antibiotic Therapy: Antibiotics are commonly prescribed to shorten the duration of illness, reduce symptoms, and limit the spread of S. sonnei. However, it is important to note that antibiotic resistance has become a growing concern, and the choice of antibiotics should be guided by susceptibility testing. The recommended antibiotics for Shigella sonnei infections include fluoroquinolones (e.g., ciprofloxacin) and azithromycin. Trimethoprim-sulfamethoxazole and ampicillin are also used in some cases, but their effectiveness may be limited due to increasing resistance.
  3. Avoidance of Antidiarrheal Medications: Antidiarrheal medications, such as loperamide or bismuth subsalicylate, should generally be avoided in cases of Shigella sonnei infection. These medications can prolong the duration of the infection by preventing the elimination of the bacteria from the body. However, in specific circumstances, healthcare providers may consider using antidiarrheal medications under medical supervision.
  4. Symptom Management: Over-the-counter medications such as acetaminophen or ibuprofen may be used to alleviate fever and abdominal pain. However, it is important to consult a healthcare professional before using these medications, especially in cases involving children or individuals with underlying medical conditions.

It is crucial to complete the full course of prescribed antibiotics, even if symptoms improve before the treatment duration is completed. This helps ensure the eradication of the bacteria and minimizes the risk of antibiotic resistance development.

In some cases, healthcare providers may decide to hospitalize patients with severe shigellosis, particularly if complications such as high fever, dehydration, or the presence of underlying health conditions are present.


Prevention of Shigella sonnei infection, or shigellosis, involves implementing several measures to reduce the risk of transmission. Here are key strategies for preventing S. sonnei:

  1. Good Hygiene Practices: Proper hand hygiene is crucial in preventing the spread of Shigella sonnei. Wash your hands thoroughly with soap and clean water, especially after using the toilet, changing diapers, and before handling or consuming food. Encourage children to practice good handwashing techniques as well.
  2. Safe Food Handling: Practice safe food handling and preparation techniques. Wash fruits and vegetables thoroughly before consumption. Cook food thoroughly, particularly meats, and avoid cross-contamination by using separate cutting boards and utensils for raw and cooked foods. Ensure food is stored at proper temperatures to prevent bacterial growth.
  3. Safe Water Sources: Ensure access to safe and clean drinking water sources. If the quality of water is uncertain, consider using bottled water or purifying water through boiling, filtration, or chlorination methods.
  4. Sanitation: Maintain good sanitation practices, particularly in areas with inadequate sanitation facilities. Promote the use of clean and sanitary toilets or latrines to prevent fecal contamination of the environment.
  5. Personal Protective Measures: Encourage the use of proper hygiene measures, such as wearing gloves when handling feces or contaminated materials, especially in healthcare or childcare settings.
  6. Travel Precautions: If traveling to areas with a higher risk of Shigella sonnei infection, take additional precautions. Drink only bottled or treated water, avoid consuming raw or undercooked foods, and maintain good personal hygiene practices.
  7. Community Awareness and Education: Promote awareness and education about Shigella sonnei infection within communities, schools, and healthcare facilities. Emphasize the importance of proper hand hygiene, safe food handling, and sanitation practices.
  8. Vaccination: While there is no widely available vaccine specifically targeting Shigella sonnei, research and development of shigellosis vaccines are ongoing. Consult with healthcare professionals for updates on vaccine availability and recommendations.


  • Shigella sonnei is a gram-negative bacterium that causes shigellosis, a common gastrointestinal infection.
  • It is a non-motile, non-spore-forming bacterium that appears as straight rods under the microscope.
  • It is primarily transmitted through the fecal-oral route, often due to poor hygiene practices or contaminated food and water.
  • The infection is characterized by symptoms such as diarrhea (often bloody), abdominal cramps, fever, and occasionally vomiting.
  • It can invade the intestinal epithelium, leading to tissue damage and inflammation.
  • It produces Shigatoxin, which contributes to the severity of symptoms and can lead to kidney damage.
  • Diagnosis of S. sonnei involves laboratory methods such as microscopy, culture, biochemical tests, serological testing, and molecular methods.
  • Treatment typically involves supportive care with rehydration therapy and the use of appropriate antibiotics, guided by susceptibility testing.
  • Prevention involves practicing good hygiene, including proper handwashing, safe food handling, ensuring safe water sources, sanitation, and community awareness.
  • Vaccines specifically targeting S. sonnei are currently under development.
  • Prompt diagnosis, treatment, and prevention measures are crucial in reducing the spread of Shigella sonnei and minimizing the impact of shigellosis.

Further Readings

  1. “Shigellosis” – Centers for Disease Control and Prevention (CDC): This webpage by the CDC provides detailed information on Shigella infections, including Shigella sonnei. It covers various aspects such as symptoms, transmission, diagnosis, treatment, and prevention. You can find it at:
  2. “Shigella sonnei: From Food Poisoning to Cellular Microbiology” – Frontiers in Microbiology: This review article delves into the cellular microbiology of Shigella sonnei, focusing on its pathogenesis, virulence factors, and host interactions. It provides insights into the molecular mechanisms of infection and highlights potential targets for therapeutic interventions. The article is available at:
  3. “Epidemiology and Clinical Features of Shigella sonnei Infections among Children in Israel” – Journal of Clinical Microbiology: This study explores the epidemiology and clinical characteristics of Shigella sonnei infections in children in Israel. It provides valuable insights into the prevalence, antimicrobial resistance patterns, and risk factors associated with Shigella sonnei infections. The article can be accessed here:
  4. “Phenotypic and Molecular Characterization of Shigella sonnei Isolates from Outbreaks in Different Regions of Brazil” – PLoS ONE: This research article investigates the phenotypic and molecular characteristics of Shigella sonnei isolates from multiple outbreaks in different regions of Brazil. It sheds light on the genetic diversity, antimicrobial resistance profiles, and virulence factors of Shigella sonnei strains circulating in Brazil. The full article is available at:

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