Shigella
Summary
Shigella is a gram-negative, non-lactose fermenting, facultative anaerobic bacillus that causes bacillary dysentery (bloody diarrhea with mucus). It is highly infectious (low infectious dose), transmitted via fecal-oral route, and is a major cause of invasive diarrheal illness worldwide, especially in children in developing countries.
Detail
Shigella species (S. dysenteriae, S. flexneri, S. sonnei, S. boydii) are gram-negative rods in the Enterobacteriaceae family. Unlike Salmonella, Shigella is non-motile, does not produce H2S, and has a very low infectious dose (as few as 10-100 organisms) because it is acid-resistant, allowing survival through the stomach.
Pathophysiology: Shigella invades the M cells of Peyer's patches and colonic epithelium via the Type III secretion system, causing lysis of the phagosome and intracellular replication. It spreads cell-to-cell using actin-based motility (similar to Listeria), causing mucosal destruction, inflammation, and microabscesses primarily in the colon (unlike Salmonella which favors the ileum). Shiga toxin (produced by S. dysenteriae serotype 1) inhibits the 60S ribosomal subunit, halting protein synthesis and causing cell death; it can also cause hemolytic uremic syndrome (HUS) via damage to renal endothelium (though less commonly than EHEC).
Clinical presentation: Presents with fever, abdominal cramps, and bloody, mucoid diarrhea (dysentery). Fecal-oral transmission is common in daycare centers, contaminated food/water, and among men who have sex with men. Unlike Salmonella, Shigella rarely causes bacteremia because it stays localized to the gut mucosa.
Lab diagnosis: Stool culture shows non-lactose fermenter on MacConkey agar; H2S negative (distinguishes from Salmonella). PCR/multiplex GI panels increasingly used.
Treatment: Supportive care with fluids; antibiotics (azithromycin, ciprofloxacin, or ceftriaxone) shorten duration and reduce transmission, especially in severe cases. Antimotility agents (e.g., loperamide) are contraindicated because they can worsen the disease by prolonging toxin exposure and increasing risk of toxic megacolon.
Complications: Reactive arthritis (part of reactive arthritis triad with conjunctivitis and urethritis, associated with HLA-B27), Shigella-associated HUS, and rarely Reiter syndrome.
High-yield board points: Compare with Salmonella (motile, H2S+, can cause bacteremia, associated with poultry/eggs) and EHEC O157:H7 (also produces Shiga-like toxin, causes HUS via free toxin, not invasion). Shigella invades cells; ETEC/EHEC do not.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- Harrison's Principles of Internal Medicine
- UWorld Step 1 QBank
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.