to iron, the aporepressor shuts down toxin production. Elek's test for toxigenicity is used to determine whether the organism is able to produce the diphtheria toxin. Identification To identify C. diphtheriae, a Gram stain is performed to show Gram-positive, highly pleomorphic organisms often looking like Chinese letters. Stains like Albert's stain and Ponder's stain are used to demonstrate the metachromatic granules formed in the polar regions. The granules are called polar granules, Babes Ernst granules or volutin granules. An enrichment medium, such as Löffler's medium, is used to preferentially grow C. diphtheriae. After that, a differential plate known as tellurite agar, allows all Corynebacteria (including C. diphtheriae) to reduce tellurite to metallic tellurium. The tellurite reduction is colorimetrically indicated by brown colonies for most Cornyebacterium species or by a black halo around the C. diphtheriae colonies. The organism produces catalase but not urease, which differentiates it from Corynebacterium ulcerans. C. diphtheriae does not produce pyrazinamidase which differentiates from Corynebacterium striatum and Corynebacterium jeikeium. Pathogenicity Corynebacterium diphtheriae is the bacterium that causes the disease called diphtheria. Bacteriophages introduce a gene into the bacterial cells that makes a strain toxigenic. The strains that are not infected with these viruses are harmless. C. diphtheriae is a rod-shaped, Gram-positive, nonspore-forming, and nonmotile bacterium. C. diphtheriae has shown to exclusively infect humans. It is believed that humans may be the reservoir for this pathogen. However, there has been extremely rare cases in which C. diphtheriae has been found in animals. These infections were only toxigenic in two dogs and two horses. The disease occurs primarily in tropical regions and developing countries. Immunocompromised individuals, poorly immunized adults, and unvaccinated children are at the greatest risk for contracting diphtheria. Mode of transmission is person-to-person contact via respiratory droplets (i.e., coughing or sneezing). Less commonly, it could also be passed by touching open sores or contaminated surfaces. During the typical course of disease, the body region most commonly affected is the upper respiratory system. A thick, gray coating accumulates in the nasopharyngeal region, making breathing and swallowing more difficult. The disease remains contagious for at least two weeks following disappearance of symptoms, but has been known to last for up to a month. The most common routes of entry for C. diphtheriae are the nose, tonsils, and throat. Individuals suffering from the disease may experience sore throat, weakness, fever, and swollen glands. This could cause even more dangerous