Corynebacterium diphtheriae is the causative agent of diphtheria in humans. It appears as small, gram-positive bacilli that form short chains or clumps. It produces a toxin that causes a local pseudomembrane and can lead to systemic effects on the heart, nerves, and other tissues. Diagnosis involves isolating the organism from throat or skin lesions and confirming its identity and toxin production through culture and biochemical tests. Treatment involves antibiotics and diphtheria antitoxin to prevent complications.

1. Corynebacterium diptheriae

2. Introduction
• Causative agent of Diptheria in humans.
• Small, pleomorphic (club-shaped),
gram-positive bacilli that appear in short
chains (“V” or “Y” configurations) or in clumps
resembling “Chinese letters”.
• Cells contain metachromatic granules
(visualize with methylene blue stain)
• Lipid-rich cell wall contains
meso-diaminopimelic acid, arabino-galactan
polymers, and short-chain mycolic acids.

3. • Morphology :
• Thin, slender, gram positive, non sporing, non-
motile bacilli with average size 3-6umx0.6-0.8um.
• club shaped due to presence of metachromatic
granules at one or both ends, also called volutin or
babes-Ernst granules.
• Appear in short chains (“V” or “L” configurations) or
in clumps resembling “Chinese letters” or cuneiform
arrangements.

4. • Special stains like albert [ malachite green &
toludine blue], neisser or polychrome
methylene blue are used for staining.
• The bacilli look green and metachromatic
granules look bluish black by using albert
stain.
• There are 3 biotypes- gravis, intermedius and
mitis.

6. • Cultural characteristics:
• Media enriched with blood serum or egg.
• Hiss serum water:
• Liquid media containing serum- growth seen as
turbidity and pellicle formation.
• Loefflers serum slope:
• Rapid growth 6-8 hours-small circular, white or
creamy and glistening.
• Tellurite blood agar:
• Potassium tellurite 0.04%- act as selective media
• Black colonies due to reduction of tellurite to
tellurium.
• Colonies appears after 48 hrs.

7. Tellurite blood agar

8. • Tinsdale agar
• contains sheep’s blood, bovine serum, cystine
and potassium tellurite is selective medium.
• Brown halo surrounding the colony is
differentiating feature.

9. • Biochemical reactions:
• Ferment glucose and maltose with acid but no gas.
• Don’t ferment lactose, mannitol or sucrose
• Don’t hydrolyse urea or form phosphatase.
• Gelatin is not liquified

10. Toxins:
Exotoxin: diptheria toxin

11. • Pathogenesis:
• Most common in children of 2-10 years.
• Incubation period-3-4 days
• Mode of transmission- droplet spread
• The diphtheria may be following clinical types:
1. Faucial
2. Laryngeal
3. Nasal
4. Conjunctival
5. Otitic
6. Vulvovaginal
7. Cutaneous mainly around mouth and nose.
Faucial diphtheria is the commonest type.
The toxin has both local as well as systemic effects.

12. • Local effects:
• Bacilli remain confined to the site of entry
usually upper respiratory tract
• Multiply and produce toxin.
• Toxin causes local necrotic changes along with
superficial inflammatory reactions
• The necrosed epithelium together with
fibrinous exudates, leucocytes, erythrocytes
and bacteria constitute pseudomemebrane.
• Which makes problem in swallowing.

14. • Systemic effects:
• Toxin diffuses to blood streams and causes toxemia
• The toxin has got affinity for cardiac muscle,
adrenals and nerve endings.
• It acts systematically on the cells of these tissues
• The bacilli themselves do not play any part in the
systemic effects because they neither penetrate in
to the tissue nor pass into blood stream producing
bacteremia.

15. • Complications:
• Local
• The pseudomembrane may extend to the larynx
which may lead to laryngeal obstruction, asphyxia
and death.
• Systemic
• Diphtheritic myocarditis which may terminate in
heart failure and death.
• Polyneuropathy and post diphtheritic paralysis of
palatine and ciliary muscles
• Degenerative changes in adrenal, kidney and liver
may occur.

16. • Laboratory diagnosis:
• Isolation of organism
• Demonstration of its toxicity by virulence tests.

17. • Collection of specimens:
• two Swabs from lesions e.g. throat, nose, larynx,
ear, conjunctiva, or skin.
• Direct microscopy:
• Smears are stained with both gram and albert stain.
• Diphtheria bacilli show beaded slender green rods
in typical chinese letter pattern.

18. • Culture:
• The swabs are inoculated on the following culture
media:
• Loefflers serum slope:
• Growth appears within6-8 hours.
• Subculture is done on tellurite blood agar and plate is
incubated at 37 c for 48 hours.
• Tellurite blood agar:
• These plates have to be incubated at 37c for at least 48
hours before declaring these as negative, as growth
may sometimes be delayed.
• Blood agar:
• Useful for differentiating streptococcal or
staphylococcal pharyngitis which may simulate
diphtheria.

19. • Colony morphology and staining:
• Loefflers serum slope: small circular, white or
creamy and glistening.
• Tellurite blood agar: Black or grey coloured
colonies.
• Smears are stained with both gram and albert stain.
• Diphtheria bacilli show beaded slender green rods
with bluish black metachromatic granules, in typical
chinese letter pattern.
• Gram stain is done to identify vincents spirochaetes
and fusiform bacilli.

20. • Biochemical tests:
• Hiss serum water is used for testing fermentation of
carbohydrates.
• Ferment glucose and maltose with acid but no gas.
• Don’t ferment lactose, mannitol or sucrose
• Don’t hydrolyse urea or form phosphatase.
• Gelatin is not liquified.

21. • Virulence tests:
• These tests demonstrate the production of
exotoxin by bacteria isolated on culture.
• Virulence testing may be done by:
- In vivo: Guinea pigs and rabbits- by subcutaneous
or intracutaneous.
- In vitro: Eleks gel precipitation test and tissue
culture tests

23. • Prophylaxis:
• Active immunisation
- Started at 6 weeks of age by toxoid in combination
with pertusis and tetanus vaccine [DPT, Triple
vaccine].
- 3 doses are given by intramuscular route at an
interval of 4-6 weeks.
- Booster dose at 18 months and 5 years.

24. • Passive immunisation- 500-1000 units of
antitoxin[Anti diphtheric serum,ADS] is
administered subcutaneously.
• Combined immunisation
• Schick test:
• Done to demonstrate circulating diptheria antitoxin.

25. Treatment:
• Erythromycin (orally or by injection) for 14
days (40 mg/kg per day with a maximum of 2
g/d), or
• Procaine penicillin G given intramuscularly for
14 days (300,000 U/d for patients weighing
<10 kg and 600,000 U/d for those weighing
>10 kg).
• Patients with allergies to penicillin G or
erythromycin can use rifampin or clindamycin.