MORPHOLOGY OF CORYNEBACTERIUM DIPHTHERIAE
Shape – Corynebacterium diphtheriae is a Thin, slender rod-shaped (bacillus) bacterium with a tendency to clubbing at one or both the ends due to the presence of metachromatic granules which may present at one or both the ends.
Size – The size of Corynebacterium diphtheriae rods is about 3–6 µm × 0.6–0.8 µm.
Arrangement Of Cells – Corynebacterium diphtheriae is arranged in the angular fashion resembling English letters ‘V’ or ‘L’ which is also called as “Chinese letter pattern” or “Cuneiform arrangement”. This unique arrangement of the C. diphtheriae is due to the incomplete separation of the daughter cells after cell division (by Binary fission).
Motility – C. diphtheriae is a Non-motile bacterium.
Flagella – Corynebacterium diphtheriae is a Non-flagellated bacterium
Spores – C. diphtheriae is a Non-sporing Bacterium.
Capsule – Corynebacterium diphtheriae is a non-capsulated bacterium.
Gram Staining Reaction – C. diphtheriae is a Gram +ve bacterium (but tend to decolorized easily).
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CULTURE REQUIREMENTS OF CORYNEBACTERIUM DIPHTHERIAE
⇒ Special requirements – Corynebacterium diphtheriae requires enriched media for the growth and readily grows in a media containing Blood, Serum or Egg, commonly Loeffler’s serum slope & Tellurite Blood Agar medium is used for the cultivation of Corynebacterium diphtheriae in diagnostic laboratories. Also, on the ordinary medium like Nutrient Agar medium, the growth of C. diphtheria is scanty in amount.
⇒ Optimum temperature – C. diphtheriae grows best at 37°C
⇒ Optimum pH – 7.4-7.6
⇒ Oxygen requirements – Corynebacterium diphtheriae is an Aerobic and Facultative anaerobic bacterium i.e. can tolerate high levels of oxygen as well as readily grows in an environment with the low level of oxygen too.
⇒ There are various culture media used for the cultivation of Corynebacterium diphtheriae in the laboratory and most commonly the Loeffler’s Serum Slope medium is used, the other media employed for the isolation of Corynebacterium diphtheriae are as follows –
- Nutrient Agar medium (Growth is scanty)
- Tellurite Blood Agar medium (selective medium)
- Hiss’s Serum Water
- Tinsdale medium is also a selective culture medium for the isolation of Corynebacterium diphtheriae
- The liquid medium (Nutrient Broth medium, TSB medium, etc. can be used but the growth is scanty in amount)
⇒ The Tellurite Blood Agar medium which is the Selective medium for Corynebacterium diphtheriae can be prepared by incorporating Potassium Tellurite (0.04%) in the Blood Agar medium for the growth of Corynebacterium diphtheriae.
The Tinsdale Medium which is also a Selective medium for C. diphtheriae can be prepared in the laboratory by incorporating Bovine serum, Cysteine and Potassium tellurite in Sheep’s Blood Agar medium for the growth of Corynebacterium diphtheriae in the laboratory.
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CULTURE CHARACTERISTICS OF CORYNEBACTERIUM DIPHTHERIAE
| Cultural Characteristics | Loeffler’s Serum Slope | Tinsdale Agar medium |
|---|---|---|
| Shape | Circular | Circular with a brown halo surrounding the colony |
| Size | 0.5-1 mm | 0.5-1 mm |
| Elevation | Low Convex | Low Convex |
| Surface | Glistening | Glistening |
| Color | White or Creamy | Black or Brown colonies |
| Structure | Opaque | Opaque |
| Hemolysis | ----- | Weak β- Hemolysis or No hemolysis (γ - Hemolysis) |
The growth on tellurite blood agar medium varies considerably as per the biotype of Corynebacterium diphtheriae which is as follows:
| Cultural Characteristics of C. diphtheriae on Tellurite Blood Agar medium | C. d. gravis | C. d. intermedius | C. d. mitis |
|---|---|---|---|
| Shape | Circular & flat | Circular | Circular & flat |
| Size | 1-2 mm | 1 mm | Variable size of colony |
| Elevation | Raised center, crenated edges with radial striations (daisy head colony) | Central elevation | Central elevation |
| Surface | Glistening | Glistening | Glistening |
| Color | Grey | Grey – Black | Shiny Black |
| Structure | Opaque | Opaque | Opaque |
| Hemolysis | Weak β- Hemolysis or No hemolysis (γ - Hemolysis) | No hemolysis (γ - Hemolysis) | Weak β- Hemolysis or No hemolysis (γ - Hemolysis) |
Similarly in liquid culture media like Hiss’s Serum water, and Nutrient broth or Glucose broth, the growth of the bacterium occurs as a turbidity with a surface pellicle formation which may vary as per the biotype of the C. diphtheriae (the C. diphtheriae – gravis, intermedius and mitis described below) which is further analyzed for the morphology (under the microscope), gram reaction, biochemical tests, and Corynebacterium diphtheriae specific tests.
| Cultural Characteristic | C. d. gravis | C. d. intermedius | C. d. mitis |
|---|---|---|---|
| Growth in Hiss’s Serum water or in Broth culture | Surface pellicle | Turbidity (in 24 hr.), Clearing (48 hr.) with fine granular sediment | Diffused turbidity throughout the content |
That’s All about the Morphology & Culture Characteristics of Corynebacterium diphtheriae
Further Reading:
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