Corynebacterium diphtheriae can grow on media with sheep blood with or without beta-hemolysis. Tinsdale agar (TIN) is used for the primary isolation and
identification of Corynebacterium diphtheriae. The medium differentiates between C. diphtheriae and diphtheroids found in the upper respiratory tract. This differentiation
was based on the ability of C. diphtheriae to produce black (or brown) colonies, surrounded by a brown/black halo. The dark halo is due to the production of H2S from cystine, interacting with the tellurite salt.
Corynebacteria are Gram-positive, catalase positive, non-spore-forming, non-motile, rod-shaped bacteria that are straight or slightly curved.
Metachromatic granules are usually present representing stored phosphate regions. Their size falls between 2-6 micrometers in length and 0.5 micrometers in diameter.
The bacteria group together in a characteristic way, which has been described as the form of a "V", "palisades", or "Chinese letters". They may also appear elliptical.
They are aerobic or facultatively anaerobic, chemoorganotrophs, with a 51–65% genomic G:C content. They are pleomorphic through their life cycle: they
come in various lengths and frequently have thickenings at either end, depending on the surrounding conditions.
The most notable human infection is diphtheria, caused by Corynebacterium diphtheriae. It is an acute and contagious infection characterized by pseudomembranes of dead epithelial cells,
white blood cells, red blood cells, and fibrin that form around the tonsils and back of the throat. It is an uncommon illness that tends to occur in unvaccinated individuals, especially school-aged children, those in developing countries, elderly,
neutropenic or immunocompromised patients. It can occasionally infect wounds, the vulva, the conjunctiva, and the middle ear. The virulent and toxigenic strains are lysogenic, and produce an exotoxin formed
by two polypeptide chains, which is itself produced when a bacterium is transformed by a gene from the ß prophage.
Four subspecies are recognized: C. diphtheriae mitis, 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. Unusually, the diphtheria toxin gene is actually encoded by a bacteriophage which is found in toxigenic strains, not on the bacterial chromosome itself.
Abbreviated from Wikipedia.