Corynebacterium is a genus of Gram-positive, aerobic or facultatively anaerobic, non-motile, non-sporulated, rod-shaped actinobacteria. Most do not cause disease, but are part of normal human skin flora. Corynebacteria are a diverse group found in a range of different ecological niches such as soil, vegetables, sewage, skin, and cheese smear. Some, such as Corynebacterium diphtheriae, are important pathogens while others, such as Corynebacterium glutamicum, are of immense industrial importance.^[1] The term diphtheroids is used to represent Corynebacteria that are non-pathogenic for example, excluding Corynebacterium diptheriae.

Some nondiphtheria species of Corynebacterium produce disease in specific animal species, and some of these are also human pathogens. Some species attack healthy hosts, and others attack immunosuppressed hosts. Some of their effects include granulomatous lymphadenitis, pneumonitis, pharyngitis, skin infections, and endocarditis. Endocarditis caused by Corynebacterium spp. is particularly seen in patients with indwelling intravascular devices.

Infection by diphtheroids tend to occur in elderly, neutropenic, or immunocompromised patients, and those who have indwelling prosthetic devices such as heart valves, neurologic shunts, or catheters.

Corynebacteria are common skin contaminants. They grow slowly, even on enriched media. The gram stain reveals irregular, rod shaped bacteria, that tend to arrange themselves in "Chinese Letter" and palisade arrangements. Metachromatic granules are usually present representing stored phosphate regions.

Some species of Corynebacterium have sequenced genomes that range in size from 2.5 - 3 Mbp. They can be found in many environments including soil, trees and skin. The non-diptheiroid Corynebecterium can also be found in human mucous membranes. Species of Corynebacterium have been used in the mass production of various amino acids including L-Glutamic Acid, a popular food additive that is made at a rate of 1.5 million tons/ year by Corynebacterium. The metabolic pathways of Corynebacterium have been further manipulated to produce L-Lysine and L-Threonine.