Laboratory Diagnosis of Diphtheria

Tabarak Al-Rubyee

Laboratory Diagnosis of Diphtheria

Diphtheria is an upper respiratory tract illness caused by Corynebacterium diphtheriae.

The laboratory diagnosis of diphtheria is as follows:

1. The specimen used is a throat swab.

2. Under the microscope, it will show numerous pus cells and gram positive bacilli arranged in chinese letter patterns.

3. Albert’s stain will demonstrate metachromatic granules.

4. Culture:
Three media are used routinely for this purpose:

Sheep blood agar: It supports the growth of C.diphtheriae as well as all other common bacterial pathogens.

Loeffler serum slope: It is not a selective medium but gives growth of C. diptheria. This medium can also be used as a transport medium.

Blood tellurite agar: This is a selective medium which enhances the chances of isolation of C.diphtheriae.

5. Susceptibility to antimicrobial agents - Penicillin is the treatment of choice (Erythromycin can be used in those sensitive to penicillin. )

6. Virulence test – Must to see if the isolate is toxigenic.

• In vivo test - Guinea pigs: s/c or intra cutaneous
• In vitro test - Elek’s gel precipitation test