Diphtheria

Diphtheria

Write a short note on diphtheria.
Answer:

Diphtheria is caused by a slender, gram-positive rod, Corynebacterium diphtheriae.

• It is passed from person to person through aerosols or skin shedding.
• Diphtheria causes a range of illnesses: asymptomatic carriage; skin lesions in neglected wounds of combat troops in the tropics; and a life-threatening syndrome that includes the formation of a tough pharyngeal membrane and toxin-mediated damage to the heart, nerves, and other organs.

Diphtheria Pathogenesis

Diphtheria releases toxins and produces local and systemic effects.

Diphtheria Local Effects

The bacilli remain confined to the site of entry, where they multiply and start producing toxins. The toxin causes local necrotic changes along with superficial inflammatory reactions.

The necrosed epithelium, together with fibrinous exudate, leucocytes, erythrocytes, and bacteria, constitutes the pseudomembrane, which is a characteristic feature of diphtheritic infection.

The mechanical complications of diphtheria are due tothe pseudomembrane, whereas the systemic effects are due to toxins.

Diphtheria symptoms

Diphtheria Systemic Effects

Diphtheria toxin diffuses into the bloodstream and causes toxemia. The toxin has an affinity for cardiac muscle, adrenals, and nerve endings.

It acts systemically on the cells of these tissues. The bacilli themselves do not play any part in systemic effects because they neither penetrate into the tissues nor pass into the bloodstream, producing bacteremia.

Diphtheria Laboratory Investigations

Isolation of Organism

• Collection of specimens: Two swabs from the lesions (throat, nose, larynx, ear, conjunctiva, vagina, or skin) are collected. One swab is used for smear examination and the other for culture.
• Direct microscopy: Smears are stained with both Gram and Albert stains. Diphtheria bacilli show beaded slender, green rods in a typical Chinese letter pattern on Albert’s staining. In Gram staining, they resemble Gram-positive organisms.
• Culture: The swabs are inoculated on the following culture media:
  • Loeffl’s serum slope: Growth appears within 6-8 hours on this medium. Subculture from Loeffl’s serum slope is made on tellurite blood agar, and the plate is incubated at 37°C for 48 hours.
  • Tellurite blood agar: These plates have to be incubated at 37°C for at least 48 hours before declaring them as negative, as growth may sometimes be delayed.
  • Blood agar: It is useful for differentiating streptococcal or staphylococcal pharyngitis, which may simulate diphtheria.
• Colony morphology and staining: On Loeffl’s serum slope, the colonies are small, circular, white, or creamy. Diphtheria bacilli grow as black or gray-colored colonies on tellurite blood agar. Smears are prepared from the suspected growth on various media. These smears are stained with Albert and Gram stains to confirm the morphology of C. diphtheriae. Albert staining shows green bacilli with bluish-black metachromatic granules. Gram staining reveals Gram-positive bacilli.
• Biochemical reactions: Hiss serum water is used for testing the fermentation of carbohydrates.
• Elek’s gel precipitation test: This is an immuno-diffusion test. A rectangular strip of filter paper soaked in diphtheria antitoxin (1000 units per ml) is placed on the surface of a 20% horse serum agar plate while the medium is still fluid.

When the agar solidifies, the test strain is streaked at a right angle to the filter paper strip. The positive and negative controls are also put up. The plate is incubated at 37°C for 24 to 48 hours.

The toxin produced by the bacterial growth diffuses in the agar and produces a line of precipitation where it meets the antitoxin at optimum concentration.