What is Shigella?

What is Shigella?

Shigella is a genus of Gram-negative bacteria that causes bacillary dysentery (shigellosis), a highly contagious intestinal infection characterized by bloody diarrhea, fever, and abdominal cramps. 1, 2

Bacterial Classification and Species

Shigella belongs to the Enterobacteriaceae family and is now classified within the Escherichia tribe due to genetic and phenotypic similarities 3. There are four distinct species:

• S. sonnei: Predominant in industrialized countries; causes the mildest disease
• S. flexneri: Common in developing countries
• S. dysenteriae: Most dangerous; occurs in outbreaks with high mortality rates, particularly in catastrophic situations
• S. boydii: Less common globally

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Clinical Presentation

Shigellosis typically progresses through two phases:

1. Initial watery diarrhea with abdominal cramps and fever
2. Dysentery phase with bloody diarrhea, characterized by frequent scant bloody stools, fever, abdominal cramps, and tenesmus (painful straining)

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The disease can range from mild to severe, with potential life-threatening complications 3.

Pathogenesis and Virulence

Shigella is an invasive bacterial pathogen with multiple virulence mechanisms 1:

• Invasion of epithelial cells via Type III Secretion System
• Toxin production (particularly Shiga toxin in S. dysenteriae)
• Induction of severe inflammation and inflammatory destruction of the intestinal epithelium
• Subversion of host immune responses to promote infection and limit adaptive immunity
• Remodeling of host cytoskeleton and intracellular motility

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Epidemiology and Transmission

Shigellosis is a significant global public health problem:

• Annual worldwide cases: Estimated at 160 million 3
• Major cause of diarrhea-related morbidity and mortality in developing countries, especially among children
• Extremely low infectious dose: As few as 10-100 organisms can cause disease
• Transmission: Fecal-oral route (hand-to-mouth), highly contagious in poor and crowded communities
• Foodborne transmission: Increasingly recognized; third most reported foodborne bacterial pathogen in the U.S. (2002 data) 5

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Antibiotic Resistance

Multidrug resistance is a serious and widespread problem with Shigella strains 6, 3. Strains can rapidly acquire resistance in endemic and epidemic settings, necessitating periodic antibiotic susceptibility testing 6.

Clinical Significance

Shigella serves as an outstanding model organism for studying bacterial pathogenesis and has contributed significantly to understanding:

• Bacterial invasion mechanisms
• Host-pathogen interactions
• Inflammatory bowel disease pathways
• Virulence gene regulation

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The bacterium's ability to disrupt intestinal homeostasis and cause inflammatory destruction of the gut epithelium provides insights into inflammatory bowel diseases such as Crohn's disease and ulcerative colitis 4.