Corynebacterium Diphtheria

  • 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)
  • 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.
  • Special stains like albert [ malachite green & toludine blue], neisser or polychrome methylene blue are used for staining.
  • There are 3 biotypes- gravis, intermedius and mitis.
  • The bacilli look green and metachromatic granules look bluish black by using albert stain.
  • Media enriched with blood serum or egg.
  • Hiss serum water:

  1. Liquid media containing serum- growth seen as turbidity and pellicle formation.
  2. Loefflers serum slope:
  3. Rapid growth 6-8 hours-small circular, white or creamy and glistening.
  • Tellurite blood agar:
  1. Potassium tellurite 0.04%- act as selective media
  2. Black colonies due to reduction of tellurite to tellurium.
  3. Colonies appears after 48 hrs.
  • Tinsdale agar
  1. Contains sheep’s blood, bovine serum, cystine and potassium tellurite is selective medium.
  2. Brown halo surrounding the colony is differentiating feature.
  • 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
  • Most common in children of 2-5 years.
  • Incubation period-2-5days
  • Mode of transmission- droplet spread
  • The diphtheria may be following clinical types:
  1. Faucial(commonest )
  2. Laryngeal
  3. Nasal
  4. Conjunctival
  5. Otitic
  6. Vulvovaginal
  7. Cutaneous mainly around mouth and nose
  • The toxin has both local as well as systemic effects.
  1. Toxin is phage mediated
  2. Toxin production is influenced by the concentration of iron in the medium.
  3. The optimum level of iron for toxin production is 0.1 mg per liter, while a concentration of 0.5 mg per litre inhibits the formation of toxin.
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.
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. 
  • Local
  1. The pseudomembrane may extend to the larynx which may lead to laryngeal obstruction, asphyxia and death.
  2. Isolated ocular palsies 
  • Systemic

  1. Diphtheritic myocarditis which may terminate in heart failure and death.
  2. Polyneuropathy and post diphtheritic paralysis of palatine and ciliary muscles
  3. Degenerative changes in adrenal, kidney and liver may occur.
  • The swabs are inoculated on the following culture media:

  1. Loefflers serum slope:
  2. Growth appears within6-8 hours.
  3. 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.
Virulence tests:
  • These tests demonstrate the production of exotoxin by bacteria isolated on culture.
  • Virulence testing may be done by: 
  1. In vivo: Guinea pigs and rabbits- by subcutaneous or intracutaneous.
  2. In vitro: Eleks gel precipitation test ( test for toxigenicity)and tissue culture tests
Schick test:
  1. Done to demonstrate circulating diptheria antitoxin.
  2. Skin test based on neutralization reaction 
    No Reaction No Reaction Immune
    No Reaction Positive
    Red flush of 1-5 cms diameter ,generally appears within 24-36 hrreaching its maximum develop-ment by 4-7 day.
    This fadesslowly
    Red flush but less circumscribed
    than positive fades by 4th day
    Pseudo positive
    Red flush equally in both arms less circumscribed
    Allergic type
    interpreted as
    Schick type
    Pseudo positive reaction Show positive reaction
    Combined reaction
  • 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).
  • Patients with allergies to penicillin G or erythromycin can use rifampin or clindamycin.
  • Diphtheria antitoxin is given intradermal

Exam Question
  • Positive Schick test indicates Susceptible to diphtheria
  • Schick test in diphtheria is done to know Subseceptibles in diptheria
  • Eleks test is done for diphtheria
  • Babes Ernest granules are seen in diphtheria
  • Post exposure prophylaxis in health care professionals is not indicated in infections with diphtheria
  • Skin test based on neutralization reaction in diphtheria is schik's test
  • Elek's Gel is a precipitation test for diphtheria 
  • Toxin is phage mediated in diphtheria
  • Toxin production is dependent upon critical concentration of iron in diphtheria
  • Incubation period of cynobacterium diphtheria is 2-5 days
  • Tellurite blood agar is used in growth of cynobacterium diphtheria
  • The most common ophthalmic effect of diphtheria is Isolated ocular palsies 
  • Commonest age group for diptheria is 2-5 Years
  • Erythematous reaction in both arms indicates Allergic type interpreted as Schick type
  • Diphtheria antitoxin is given intradermal
  • Chinese letter arrangement is seen in Corynebnacterium diphtheria 
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