Corynebacterium diphtheriae

Corynebacterium diphtheriae
Gram stained Corynebacterium diphtheriae culture
Scientific classification
Kingdom:	Bacteria
Phylum:	Actinobacteria
Order:	Actinomycetales
Family:	Corynebacteriaceae
Genus:	Corynebacterium
Species:	C. diphtheriae
Binomial name
Corynebacterium diphtheriae Kruse, 1886

Corynebacterium diphtheriae is a pathogenic bacterium that causes diphtheria. It is also known as the Klebs-Löffler bacillus, because it was discovered in 1884 by German bacteriologists Edwin Klebs (1834 – 1912) and Friedrich Löffler (1852 – 1915).

Classification

Four subspecies are recognized: C. diphtheriae mitischodis, C. diphtheriae intermedius, C. diphtheriae gravis, and C. diphtheriae belfanti. The four subspecies differ slightly in their colonial morphology and biochemical properties such as the ability to metabolize certain nutrients, but all may be toxigenic (and therefore cause diphtheria) or non-toxigenic.

Diagnosis

In order to accurately identify C. diphtheriae, a Gram stain is performed to show gram-positive, highly pleomorphic organisms with no particular arrangement (classically resembling Chinese characters). Special stains like Alberts's stain and Ponder's stain are used to demonstrate the metachromatin (metachromacity is the phenomenon by which different parts of an organism can get stained in two or more different colours just by applying a single dye) granules formed by them in the polar regions (so they are also called as 'Polar Granules' along with few other names like Babes Ernst Granules, Volutin, etc). Then, culture the organism on an enrichment 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.

A low concentration of iron is required in the medium for toxin production. At high iron concentrations, iron molecules bind to an aporepressor on the beta bacteriophage, which carries the genes for the Tox gene, converting it to a repressor that 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.

Sensitivity

The bacterium is sensitive to the majority of antibiotics, such as the penicillins, ampicillin, cephalosporins, quinolones, chloramphenicol, tetracyclines, cefuroxime and trimethoprim.

See also

• Cutaneous diphtheria

References

External links

• CoryneRegNet - Database of Corynebacterial Transcription Factors and Regulatory Networks