Diphtheria

Question 1

1) Classification

Answer 1

Family: Corynbacteriaceae
Genus: Corynebacterium
Species: diphtheria

Question 2

2) Morphology

Answer 2

Gram positive rods 
Club shaped 
Pleomorphic
Metachromatic granules
Arranged in sharpe angles or parallel (resembles the shape of Chinese letters)

Question 3

3) Growth characteristics

Answer 3

Obligate aerobe
Fastidious
Slow growing even on enriched media
Ferment carbohydrate into lactic acid

Question 4

4) Antigenic structure

Answer 4

Extracellular protein- diphtheria protein

Question 5

5) Virulence factor

Answer 5

• heat-labile diphtheria toxin
• invasion of mucosal membrane
• destruction and superficial inflammation
• necrotised epithelium forms “grey pseudomembrane” over the tonsils, pharynx and larynx

Question 6

6) Disease

Answer 6

Diphtheria of the respiratory tract- fever and pseudomembrane commonly over the tonsils, pharynx and larynx

Wound and skin diphtheria

Question 7

7) Microbiological diagnosis

Answer 7

DME:
Gram-stained smear -> Gram-positive “club-shaped” arranged in “picket fence” or “Chinese letter”

Methylene blue stain: blue rods with darker blue metachromatic granules

Culture:

Italian BLT

• blood agar plate: small, grey, granular colonies
• Loeffler slant medium: luxuriant growth of typical “diphtheria-like” morphology
• Tellurite agar: brown to black colonies

Specific tests: 
- carbohydrate fermentation tests 
- tests for toxigenicity: 
In vivo tests
In vitro rest- gel diffusion precipitation test

Question 8

8) Treatment and prevention

Answer 8

• early administration of diphtheria antitoxin
• penicillin or erythromycin recommended
• children are actively immunised with diphtheria toxoid (given as DPT)