Bit · Micro
Salmonella vs Shigella
Two gram-negative non-lactose-fermenting rods that cause bloody diarrhoea and look almost identical on initial culture. The pivot is motility, H₂S, animal reservoir, and infectious dose.
Mechanism
Both are gram-negative rods in the Enterobacteriaceae, both are non-lactose-fermenters, both invade the colon, both cause dysentery. Differences are mechanistic:
- Salmonella — motile (peritrichous flagella), produces H₂S on culture media. Many serovars; reservoirs include poultry, eggs, reptiles (pet turtles), unpasteurised milk. Invades through M cells of Peyer's patches and is taken up by macrophages — produces a monocytic inflammatory response. S. typhi causes typhoid (rose spots, relative bradycardia, hepatosplenomegaly). High infectious dose (~10⁵).
- Shigella — non-motile, H₂S negative. No animal reservoir — human-to-human, fecal-oral, often kids in daycare. Very low infectious dose (~10–100 organisms — much more contagious). Invades M cells but produces a neutrophilic response. Shiga toxin can trigger HUS (especially S. dysenteriae type 1).
Differentiator Table
| Salmonella | Shigella | |
| Motility | Motile (flagella) | Non-motile |
| H₂S on TSI | Positive (black) | Negative |
| Reservoir | Animals — poultry, eggs, reptiles, milk | Humans only |
| Infectious dose | High (~10⁵) | Very low (~10–100) |
| Cell invasion | M cells → Peyer's patches → macrophages | M cells of colon → cytoplasm spread |
| Stool inflammatory response | Monocytic | Neutrophilic (PMNs prominent) |
| Bacteremia | Common (esp. S. typhi) | Rare |
| HUS risk | No | Yes (Shiga toxin, esp. S. dysenteriae type 1) |
| Antibiotics | Avoid in uncomplicated non-typhoidal (prolongs carriage); treat typhoid and severe disease | Antibiotics shorten course; treat to limit spread |
The Pivot
Four quick questions:
- Animal exposure (pet turtle, eggs, poultry)? Salmonella.
- Daycare or institutional outbreak with tiny inoculum? Shigella.
- H₂S black on TSI and motile? Salmonella.
- HUS in a child after bloody diarrhoea? Shigella (or EHEC O157:H7 — the other Shiga-toxin producer).
Antibiotic rule reversal: treat Shigella, mostly don't treat non-typhoidal Salmonella (prolongs carriage).
NBME-Style Stem
A 4-year-old boy in daycare develops fever, abdominal cramps, and bloody diarrhoea. Three other children in his classroom developed similar symptoms over the past week. Stool culture grows non-lactose-fermenting, non-motile, H₂S-negative gram-negative rods. Which of the following is the most likely organism?
Concept Anchor
Salmonella and Shigella both invade the colon, but one swims (motile, H₂S+) and comes from animals; the other doesn't move (non-motile, H₂S−), comes only from people, and needs only a handful of organisms to start an outbreak.