Shigella

Question 1

Disease

Answer 1

Enterocolitis (dysentery)

Question 2

Properties/Characteristics

Answer 2

• Non-lactose-fermenting Facultative gram-negative rods.
• Nonmotile
• don’t produce H2S
• O antigens in the cell wall divide into 4 groups (A-D)

Question 3

Epidemiology

Answer 3

• human colon; no animal carriers
• fecal–oral route (4F’s= fingers, flies, food, feces)
• very low ID50
• Children in mental institutions and daycare centres experience outbreaks of shigellosis.

Question 4

Pathogenesis

Answer 4

• Invades ileum & colon mucosa but doesn’t penetrate farther; sepsis is rare.
• Enterotoxin Shiga Toxin in cell wall encoded by lysogenic bacteriophages.
• similar enterohemorrhagic E. coli O157:H7 strains toxin (enterocolitis & HUS).
• Infectious dose lower because resistant to stomach acid.
• No chronic carrier state.

Question 5

Clinical Findings

Answer 5

• 1-4 day Incubation period-> fever & abdominal cramps-> diarrhea (1st watery later
  blood & mucus)
• Varies from mild to severe depending on 2 major factors: Shigella species & patient age (young & elderly most severely affected)
• Serum agglutinins appear after recovery; not protective-> organism doesn’t enter blood.

Question 6

Laboratory Diagnosis

Answer 6

• Gram-stained smear & culture.
  Non-lactose-fermenting colourless colonies on EMB or MacConkey’s agar.
• TSI agar-> alkaline slant with acid butt, no
  gas or H2S.
• Anti-O antibody in agglutination test with methylene blue stain on fecal sample for neutrophils presence.

Question 7

Treatment

Answer 7

• fluid & electrolyte replacement
• severe->ciprofloxacin
• Resistance-> plasmid-encoded enzymes (β-lactamase-> degrades ampicillin & mutant pteroate synthetase-> reduces sulfonamides sensitivity).

Question 8

Prevention

Answer 8

• Public health measures (sewage disposal, water supply chlorination, stool cultures for food handlers, & handwashing before food handling).
• No Prophylactic drugs or vaccine.