Diphtheria laboratory findings
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Overview
Laboratory Findings
A presumptive diagnosis is usually based on clinical features. Diagnosis is confirmed by isolating C. diphtheriae from culture of nasal or throat swabs or membrane tissue.
Culture and Staining
In order to accurately identify C. diphtheriae, a Gram stain is performed to show gram-positive, highly pleomorphic organisms with no particular arrangement (resembling chinese letters). Then, culture the organism on an erichment medium, namely Löffler's serum, to allow it to overgrow any other organisms present in the specimen. After that, use a selective plate known as tellurite agar which allows all Corynebacteria (including C. diphtheriae) to reduce tellurite to metallic tellurium producing brown colonies and, only in the case of C. diphtheriae, a black halo around the colonies allowing for easy differentation of the organism.
Elek's test
It's worth noting that a low concentration of iron is required in the medium for toxin production; as at high iron concentrations, iron molecules bind to a repressor which shuts down toxin production[1]. This is most appreciated when performing Elek's test for toxogenecity, in order to know if the organism is able to produce the diphtheria toxin or not.
PCR
PCR assays can also be performed on isolates, swabs, or membrane specimens to rapidly confirm the presence of the tox gene responsible for production of diphtheria toxin, but the test is available only in research or reference laboratories.
References
- ↑ Microbiology: A Human Perspective. Fourth edition. McGraw Hill