Shigella

CASE 1

• A 12 year old boy has mid-abdominal cramps and watery diarrhea followed 12 hours later by lower abdominal pain and stools with both blood and mucus. He has developed a fever to 38.7 degrees with mild chills.

• A Gram stain of the stool is performed and shows numerous Gram-negative bacilli, abundant neutrophils and erythrocytes (below). A culture of the stool grows Shigella flexneri.

• The patient is treated with oral ciprofloxacin and returns to normal health in 2-3 days.

Questions:

1. To what other member of the Enterbacteriaceae is Shigella most closely related (in fact, some have argued the addition of only four genetic elements make them indistiguishable)?
2. What is tenesmus? What causes it?
3. How will Shigella appear on MacConkey media, on an SS agar or Hektoen plate?
4. How does Shigella enter the colonic mucosa?
5. How does Shigella spread across the colonic mucosa?
6. Why is there blood and mucus in the stool? What would you see if you performed a proctoscopic examination?
7. What is the natural history of this infection?
8. Is there any difference between the infections caused by the four species of Shigella?