Salmonella vs Shigella
How to tell Salmonella and Shigella apart when a board question makes them look alike.
Salmonella is motile and H2S-positive; Shigella is nonmotile, H2S-negative, and low-dose contagious.
Side by side
Motile H2S-positive non-lactose fermenter linked to poultry, eggs, reptiles, and animal reservoirs.
Nonmotile H2S-negative low-inoculum organism causing person-to-person dysentery with cramps and tenesmus.
Salmonella invades intestinal mucosa and can survive within macrophages; systemic spread occurs in high-risk hosts or typhoid disease.
Shigella invades colonic M cells, spreads cell-to-cell, and triggers inflammatory dysentery; Shiga toxin can add severity.
H2S production, motility, animal-food exposure, or reptile contact points Salmonella.
Daycare outbreak, very low infectious dose, nonmotility, and tenesmus point Shigella.
The trap
Both are gram-negative invasive enteric rods causing inflammatory diarrhea with fecal leukocytes, and bloody diarrhea can occur with either. The discriminating clues are exposure history, motility, and antibiotic strategy. Animal-source food exposure (raw poultry, eggs, reptiles), motile organism, H2S-positive on TSI agar, no person-to-person spread in healthy adults equals Salmonella (antibiotics often PROLONG shedding in nontyphoidal disease, so reserve for severe cases, bacteremia, immunocompromised, or sickle cell). Day-care or close-contact spread with very low inoculum (less than 100 organisms), nonmotile, H2S-negative, Shiga toxin causing HUS in children equals Shigella (treat with ciprofloxacin or azithromycin to shorten illness and reduce transmission).
Watch it explained
The repair move
For invasive enteric rods, ask: animal-food reservoir or human-only low-dose spread?
Try a free question
CoreA patient develops fever, abdominal cramps, and inflammatory diarrhea after eating undercooked eggs. Stool culture grows a motile, non-lactose-fermenting gram-negative rod that produces H2S. Which organism is most likely?
Show answer and explanation
Answer: A. Salmonella species. Nontyphoidal Salmonella is motile, H2S-positive, non-lactose-fermenting, and linked to poultry/eggs/reptiles.
Why the other choices are tempting
- B. Shigella is nonmotile, H2S-negative, and has a very low infectious dose with dysentery.
- C. V cholerae causes painless rice-water diarrhea from toxin-mediated secretion.
- D. C jejuni is curved, grows well at 42 C, and is linked to poultry with bloody diarrhea.
- E. ETEC causes traveler watery diarrhea, not H2S-positive invasive Salmonella.
Flashcards
Tap a card to flip it.
What is the fastest separator for Salmonella vs Shigella?
Salmonella is motile and H2S-positive; Shigella is nonmotile, H2S-negative, and low-dose contagious.
What clue should make you choose Salmonella?
Motile H2S-positive non-lactose fermenter linked to poultry, eggs, reptiles, and animal reservoirs.
Quick answers
What is the fastest way to tell Salmonella from Shigella?
Salmonella is motile and H2S-positive; Shigella is nonmotile, H2S-negative, and low-dose contagious.
What is the key clue for Salmonella?
Motile H2S-positive non-lactose fermenter linked to poultry, eggs, reptiles, and animal reservoirs.
What is the key clue for Shigella?
Nonmotile H2S-negative low-inoculum organism causing person-to-person dysentery with cramps and tenesmus.
What is the common trap with Salmonella vs Shigella?
Both are gram-negative invasive enteric rods causing inflammatory diarrhea with fecal leukocytes, and bloody diarrhea can occur with either. The discriminating clues are exposure history, motility, and antibiotic strategy. Animal-source food exposure (raw poultry, eggs, reptiles), motile organism, H2S-positive on TSI agar, no person-to-person spread in healthy adults equals Salmonella (antibiotics often PROLONG shedding in nontyphoidal disease, so reserve for severe cases, bacteremia, immunocompromised, or sickle cell). Day-care or close-contact spread with very low inoculum (less than 100 organisms), nonmotile, H2S-negative, Shiga toxin causing HUS in children equals Shigella (treat with ciprofloxacin or azithromycin to shorten illness and reduce transmission).