Corynebacterium diphtheriae

Question 1

Disease

Answer 1

Diphtheria

Question 2

Important Properties

Answer 2

clubshaped gram-positive beaded rods (wider at one end), arranged in palisades or V- or L-shaped formations.

• beads are granules of
  highly polymerized polyphosphate (storage mechanism for high-energy phosphate bonds). - granules stain metachromatically
  (cell blue, granules red).

Question 3

Transmission

Answer 3

Humans -> natural host

• toxigenic & nontoxigenic organisms inside upper respiratory tract
• airborne droplets.
• infect skin at preexisting
  skin lesion.-> in tropics or persons poor skin hygiene.

Question 4

Pathogenesis

Answer 4

• invasiveness-> organism establish & maintain itself in throat.
• Diphtheria toxin inhibits protein synthesis by ADP-ribosylation of
  elongation factor-2 (EF-2).
• affects all eukaryotic
  cells regardless of tissue type, no effect on analogous factor in prokaryotic cells.
• toxin is single polypeptide with 2 functional
  domains.
• Binding (B) domain mediates binding of
  toxin to glycoprotein receptors on cell membrane.
• Active (A) domain possesses enzymatic activity cleaves nicotinamide from NAD & transfers remaining ADP-ribose to EF-2 inactivating it.
• DNA that codes for diphtheria toxin ->
  DNA of temperate bacteriophage= beta phage.
• lysogenic phase of viral growth, DNA of virus integrates into bacterial chromosome & toxin is synthesized.
• Cells not lysogenized by this phage dont produce exotoxin-> nonpathogenic.

Question 5

Pathogenesis: host response

Answer 5

(1) local inflammation in throat, with fibrinous
exudate-> tough, adherent, gray pseudomembrane.

(2) Antibody neutralizes exotoxin activity by
blocking interaction of B domain with receptors, preventing entry into cell.

Question 6

Pathogenesis: Schick’s
test

Answer 6

• assesses immune status
• intradermal injection of
  0.1 mL of purified standardized toxin.
• patient has no antitoxin, toxin cause inflammation at site 4 to 7 days later.
• no inflammation -> antitoxin present & patient immune.

Question 7

Clinical Findings

Answer 7

• thick, gray, adherent pseudomembrane over tonsils & throat.
• fever, sore throat, & cervical adenopathy.
• 3 complications:

(1) Extension of membrane into larynx & trachea-> airway obstruction.

(2) Myocarditis,arrhythmias circulatory collapse.

(3) Nerve weakness or paralysis (cranial
nerves).
- Paralysis soft palate &
pharynx muscles-> regurgitation of fluids through nose.
- Peripheral neuritis affecting muscles of extremities.

• Ulcerating skin lesions
  covered by gray membrane.
• indolent & don’t invade surrounding tissue.

Question 8

Laboratory Diagnosis

Answer 8

• isolating organism
  & demonstrating toxin production.
• treat with antitoxin-> cannot wait for lab results.
• Throat swab cultured on Loeffler’s medium (tellurite
  plate) & blood agar plate.
• Tellurite plate contains tellurium salt reduced to elemental tellurium within organism.
• Gray-black tellurium in colony->diagnostic criteria.
• after culture recovery, animal inoculation or
  antibody-based gel diffusion precipitin test performed->document toxin production.
• PCR assay for presence
  of toxin gene in organism isolated.
• Smears throat swab stained with Gram stain & methylene blue.
• diagnosis of diphtheria not made by smear;
  tapered, pleomorphic gram-positive rods can be suggestive.
• methylene blue stain-> metachromatic granules.

Question 9

Treatment

Answer 9

• antitoxin-> toxin binds rapidly & irreversibly to cells can’t be neutralized by antitoxin so antitoxin neutralizes unbound toxin in blood.
• antiserum made in horses-> hypersensitivity & anaphylaxis medications.
• Serum sickness->after administration of antiserum made in horses.
• penicillin G or erythromycin, but not substitute for antitoxin.
• Antibiotics inhibit organism growth-> reduce toxin production & decrease chronic carriers.

Question 10

Prevention

Answer 10

• immunization with diphtheria toxoid (combination of diphtheria toxoid, tetanus
  toxoid, & acellular pertussis vaccine[DTaP]).
• Diphtheria toxoid prepared by treating exotoxin with formaldehyde-> inactivates
  toxic effect but leaves antigenicity intact.
• Immunization->3 doses given at 2, 4, &
  6 months + boosters at 1, 6 years, & every 10 years.
• Immunization doesn’t prevent nasopharyngeal
  carriage of organism.