Shigella

Question 1

What are the key characteristics of Shigella?

Answer 1

1. Gram-negative rod – Most effective enteric pathogen
2. Non-motile – Unlike Salmonella
3. Facultative anaerobe – Can survive with or without oxygen
4. Produces Shiga toxin – Causes intestinal damage
5. Belongs to Enterobacteriaceae family

Question 2

What are the morphological features of Shigella?

Answer 2

1. Shape & Size – Rod-shaped, 0.4–0.6 μm by 1–3 μm
2. Gram stain – Pink (Gram-negative)
3. No capsule or spores – Unlike some enteric bacteria

Question 3

What are the four groups of Shigella?

Answer 3

1. Group A – S. dysenteriae
2. Group B – S. flexneri
3. Group C – S. boydii
4. Group D – S. sonnei

Question 4

How is Shigella distinguished from Salmonella?

Answer 4

1. No gas production – On glucose fermentation
2. No H₂S production – Unlike Salmonella
3. Non-motile – Lacks flagella

Question 5

What diseases does Shigella cause?

Answer 5

1. Shigellosis – Gastroenteritis caused by Shigella
2. Bacillary dysentery – Severe form with blood & mucus in stool
3. Dysentery – Inflammatory diarrhea

Question 6

What are the clinical findings of Shigella infection?

Answer 6

1. Early symptoms – Fever, abdominal cramps, nausea
2. Diarrhea – Watery at first, then bloody with mucus
3. Severe cases – Dehydration, painful bowel movements, tenesmus

Question 7

What is unique about Shigella’s infectious dose (ID50)?

Answer 7

Very low ID50 – As few as 100 bacteria can cause infection

Question 8

How does Shigella cause disease?

Answer 8

1. Invades M cells in intestinal epithelium
2. Enters cytoplasm and spreads to adjacent cells
3. Invades mucosal cells, causing inflammation
4. Leads to cell death and bloody diarrhea
5. Some strains produce Shiga toxin for additional damage

Question 9

What is the mechanism of Shiga toxin?

Answer 9

1. Encoded by bacteriophages – Like E. coli O157:H7
2. Binds to 60S ribosomal subunit – Blocks protein synthesis
3. Kills intestinal cells – Worsens inflammation and diarrhea

Question 10

What are the ‘Four Fs’ of Shigella transmission?

Answer 10

1. Fingers – Contaminated hands
2. Flies – Carry bacteria to food
3. Food – Contaminated by infected handlers
4. Feces – Fecal-oral transmission

Question 11

What is the host range of Shigella?

Answer 11

1. Humans are primary hosts – No animal reservoir
2. Fecal-oral spread – Direct contact or contaminated water/food

Question 12

What samples and microscopy methods are used to diagnose Shigella?

Answer 12

1. Samples – Feces, rectal swab
2. Microscopy – Gram stain (pink), methylene blue stain (shows neutrophils)

Question 13

What are the key culture characteristics of Shigella?

Answer 13

1. Non-lactose fermenting – Colorless on MacConkey’s agar
2. No gas or H₂S production – Alkaline slant, acid butt on TSI
3. Growth on special media – MacConkey’s, EMB, TSI, Hektoen’s agar

Question 14

How is Shigella infection treated?

Answer 14

1. Rehydration therapy – First-line treatment
2. Mild cases – No antibiotics needed
3. Severe cases – Fluoroquinolones (ciprofloxacin), TMP-SMX

Question 15

Why are anti-peristaltic drugs contraindicated in Shigella infections?

Answer 15

They slow bowel movement, increasing toxin absorption and worsening disease

Question 16

What are the key preventive measures against Shigella?

Answer 16

1. Hygiene & sanitation – Proper sewage disposal, handwashing
2. Water safety – Chlorination, avoid swallowing swimming water
3. Food safety – Prevent fecal contamination of food & water