Shigella is one of those “small bug, big consequences” organisms: a tiny Gram-negative rod that can turn a couple of contaminated hand-to-mouth moments into classic bloody diarrhea, intense cramps, and a Step 1–worthy set of mechanisms (M cells, actin rockets, and Shiga toxin). If you can quickly recognize Shigella’s clinical pattern and why it causes it, you’ll pick up easy points across micro, GI path, pharm, and public health.
Where Shigella Fits (Big-Picture Classification)
Shigella is a Gram-negative, non-motile, non–lactose fermenting rod in the Enterobacteriaceae family (classically oxidase-negative and facultatively anaerobic).
Core ID Features (memorize these)
- Gram stain: Gram-negative rod
- Motility: Non-motile (contrast with Salmonella: motile)
- Lactose fermentation on MacConkey: Non–lactose fermenter (pale colonies)
- Reservoir: Humans only (no animal reservoir—important epidemiology clue)
- Infectious dose: Very low (as few as ~10–100 organisms) → easy person-to-person spread
First Aid cross-reference: Microbiology → Gram-negative bacteria → Enteric bacilli (Shigella, Salmonella, etc.) and GI → Infectious diarrhea syndromes.
Definition in One Line
Shigella causes inflammatory dysentery: fever + abdominal cramps + bloody, mucoid diarrhea due to invasion of colonic mucosa (and sometimes toxin-mediated effects).
Transmission & Epidemiology (Why It Spreads So Easily)
Transmission
- Fecal–oral, often from person-to-person spread
- Contaminated food/water can occur, but the classic exam stem is close-contact transmission
High-yield settings
- Daycare centers
- Institutions (nursing homes, shelters)
- Crowded living conditions
- Travel and poor sanitation
Why low infectious dose matters
- Shigella is relatively acid-resistant, so it can survive gastric acid better than many enteric pathogens.
- Result: a tiny inoculum can cause disease → outbreaks in close quarters.
Pathophysiology: The Step 1 Mechanism Map
Think of Shigella disease in two layers:
- Invasion and inflammation (main driver of dysentery)
- Toxin effects (classically Shiga toxin; especially S. dysenteriae)
1) Invasion of the colon (key mechanism)
Shigella targets the colon and invades via:
- M cells (Peyer patch-associated microfold cells) in the intestinal epithelium
- After uptake, Shigella:
- Escapes the phagosome
- Spreads cell-to-cell using actin polymerization (“actin rockets”)
- Triggers intense neutrophilic inflammation → mucosal ulceration → blood and pus in stool
HY association: “Invades M cells + actin-based intracellular spread” is a shared theme with Listeria (actin rockets), but Shigella is Gram-negative and causes dysentery.
2) Shiga toxin (the “toxin” piece)
Some Shigella (notably S. dysenteriae) produce Shiga toxin, which:
- Inactivates the 60S ribosomal subunit by removing an adenine from 28S rRNA
- Stops protein synthesis → cell death
Clinically high yield consequence: increased risk of HUS (hemolytic uremic syndrome), especially in children.
Step-style link: Shiga toxin mechanism = same ribosomal target as EHEC’s Shiga-like toxin (also hits 60S), but the organisms differ in colonization and stool findings.
First Aid cross-reference: Micro → Bacterial protein synthesis inhibitors/toxins (Shiga toxin: 60S) and Renal → HUS.
Clinical Presentation (How It Shows Up on Exams)
Typical timeline & symptoms
- Incubation: often 1–3 days
- Early: watery diarrhea may occur
- Progresses to dysentery:
- Fever
- Tenesmus (painful urge to defecate)
- Abdominal cramps
- Small-volume bloody, mucoid stools
Classic stool pattern
- Fecal leukocytes: present
- Occult blood: positive
- Suggests inflammatory diarrhea (invasion/destruction of mucosa)
Exam stem giveaways
- Daycare outbreak
- Sibling with similar symptoms
- “Bloody diarrhea + fever + tenesmus”
- “Low infectious dose” or “person-to-person spread”
Complications (High-Yield Associations)
1) Hemolytic uremic syndrome (HUS)
More classically tied to EHEC, but Shigella (esp. S. dysenteriae) can also cause it.
HUS triad:
- Microangiopathic hemolytic anemia
- Thrombocytopenia
- Acute kidney injury
2) Seizures in children
- Shigellosis is a notable infectious cause of febrile seizures in kids.
3) Reactive arthritis
Post-infectious autoimmune phenomenon (think “can’t see, can’t pee, can’t climb a tree”):
- Conjunctivitis/uveitis
- Urethritis/cervicitis
- Arthritis
Often associated with HLA-B27 and classically triggered by GI pathogens like Shigella, Salmonella, Campylobacter, Yersinia.
Shigella vs Similar Bugs (Quick Differentiation Table)
| Organism | Motility | Lactose fermenter? | Key clinical clue | Stool | Notable toxin/complication |
|---|---|---|---|---|---|
| Shigella | No | No | Daycare/institutions, low infectious dose, tenesmus | WBCs + blood | Shiga toxin → 60S, possible HUS |
| Salmonella (non-typhoidal) | Yes | No | Poultry/eggs, reptiles | WBCs ± blood | Can cause osteomyelitis in sickle cell |
| EHEC | Yes | Yes (E. coli) | Undercooked beef, leafy greens | Blood, typically no WBCs, less fever | Shiga-like toxin → HUS |
| Campylobacter jejuni | Yes (darting) | No | Undercooked poultry, unpasteurized milk | WBCs + blood | Guillain-Barré, reactive arthritis |
| Entamoeba histolytica (protozoa) | — | — | Travel, liver abscess | Blood, fewer WBCs | Flask-shaped ulcers |
Step 1 trick: Bloody diarrhea + fever + fecal WBCs pushes you toward invasive/inflammatory bacteria (Shigella, Salmonella, Campylobacter) rather than toxin-only watery diarrhea.
Diagnosis (What You’d Actually Order—and What NBME Loves)
Clinical diagnosis (often enough)
In mild cases, treat supportively based on classic presentation and epidemiology.
Stool studies (when to test)
- Severe disease, immunocompromised, outbreak investigation, persistent symptoms, or dysentery
Options:
- Stool culture (traditional)
- Multiplex PCR stool panels (common in real life; fast)
- Stool exam: fecal leukocytes and occult blood support inflammatory diarrhea
Lab pearls
- Shigella is a non–lactose fermenter on MacConkey agar.
- Oxidase-negative is typical for Enterobacteriaceae (often not directly tested for Shigella specifically, but useful framework).
Treatment (Step 1/2 Practical Approach)
1) Supportive care (always)
- Oral rehydration is the cornerstone
- Correct electrolytes
- IV fluids if severe dehydration
2) Antibiotics (select patients; can shorten course & reduce shedding)
Antibiotics may be used for:
- Severe dysentery
- High-risk patients (young children, elderly, immunocompromised)
- Public health reasons (reducing transmission)
Common options (resistance patterns matter):
- Azithromycin
- Ceftriaxone
- Ciprofloxacin (in adults; be mindful of resistance and age/pregnancy considerations)
Board-style nuance: Choice depends on local resistance and patient factors; what’s “right” on an exam is usually recognizing that antibiotics can be indicated for Shigella (unlike EHEC, where antibiotics can increase HUS risk).
3) Avoid antimotility agents in dysentery
- Loperamide/diphenoxylate can worsen invasive disease by prolonging contact time and increasing complications.
High-Yield Memory Hooks (What to Recall Under Time Pressure)
- Shigella = invasive dysentery → fever + tenesmus + bloody, mucoid diarrhea
- Low infectious dose → daycare/institution outbreaks, person-to-person
- Invades M cells and spreads via actin polymerization
- Shiga toxin (esp. S. dysenteriae) → inhibits 60S by targeting 28S rRNA → can contribute to HUS
- Fecal WBCs present (inflammatory diarrhea)
- Treat with rehydration ± antibiotics (and avoid antimotility agents)
Rapid-Fire USMLE-Style Check Yourself
-
Child with fever, tenesmus, and bloody diarrhea after daycare outbreak → likely organism?
Shigella -
Mechanism of toxin-associated protein synthesis inhibition (Shigella dysenteriae)?
Inactivates 60S ribosomal subunit by damaging 28S rRNA -
Why is Shigella so contagious?
Very low infectious dose + acid resistance → easy fecal–oral spread -
Stool finding that supports invasive/inflammatory diarrhea?
Fecal leukocytes and blood