What is Corynebacterium?

Definition

Corynebacterium is a gram-positive, non-spore-forming rod with a
characteristic club-shaped appearance and worldwide distribution. C.
diphtheriae is a major human pathogen.

Natural Habitat and Features

Corynebacterium spp. are gram-positive, nonmotile, catalase-positive rods. Along with the Mycobacteria and Nocardia, they produce characteristic long-chain mycolic acids that can be used in their taxonomy. Their metabolism is varied, with both aerobic and facultatively anaerobic members of the genus. Those with anaerobic metabolism usually perform lactic acid fermentation. The bacteria are fastidious, and all strains require biotin and most require several other supplements. They are usually grown under an enriched carbon dioxide atmosphere and grow slowly, even on complex-enriched culture media.

The rods are pleiomorphic, some having club-shaped ends (the Greek word koryne means “club”), and often show incomplete separation during cell division. This has led some scientists to note their resemblance to Chinese characters. The incomplete separation is caused by a characteristic “snapping” cell division, which leads to their peculiar cell wall. The main wall constituent is commonly called mycolyl-AG-peptidoglycan and is made up of high-diaminopimelic-acid peptidoglycans, arabinoglactans, and mycolic acid, all connected through disaccharide linkages. During cell division, the plasma membrane divides normally, but the cell wall may only partially separate, forming V- and other odd-shaped assemblages of two or more cells.

The genomes of three species have been sequenced and contain a single circular chromosome of about 2.5 million base pairs with a high G-C content (53.5 percent). The taxonomy of Corynebacterium is based on genomic deoxyribonucleic acid (DNA), 16-s ribonucleic acid (RNA), and cell wall lipids. A major taxonomic realignment was made in the 1990’s. Some former Corynebacterium spp. have been moved to other related genera:C. acnes to Propionobacterium acnes and C. hemolyticum to Arcanobacterium hemolyticum. Other bacteria were added to Corynebacterium: For example, the JK bacterial group became C. jeikeium.

Many Corynebacterium spp. have industrial applications,
producing complex organic nutritional factors and medically important compounds.
They degrade hydrocarbons and age cheese. Arguably the most important of these
species is glutamicum, which is the primary source of the food
additive monosodium glutamate (MSG) and has been genetically
engineered to produce human epiderman growth factor, among other applications.
Corynebacterium spp. have a worldwide distribution, especially
in temperate areas, and are found in soils and water and in and on animals and
plants.

Pathogenicity and Clinical Significance

Diphtheriae is the most important corynebacterial pathogen of
humans and causes diphtheria. This disease is an upper respiratory infection
with a characteristic pseudomembrane that covers parts of the pharynx and adjacent
areas. Diphtheriae to epithelial cells at the site of infection,
causing them to produce the fibrin-based pseudomembrane. The toxin can also be
disseminated to many other areas of the body, leading to possible organ
failure.

Only those strains with an integrated lysogenic phage that carries the gene for the diphtheria toxin are able to produce the toxin. The disease severity is often a consequence of the strain of diphtheriae that causes the infection, because different strains grow at different rates and produce different amounts of diphtheria toxin. The toxin regulatory gene (DtxR), located on the bacterial chromosome, also affects toxin levels. Iron serves as the corepressor of DtxR’s product, so under normal iron concentrations, toxin production is greatly curtailed. Under iron starvation, toxin production is dramatically increased.

Diphtheriae also can cause cutaneous diphtheria, a skin infection, if it enters a break in the skin. In rare instances, it also can cause genital and eye infections.

Nonpathogenic Corynebacterium are often referred to as
diphtheroids, however, many of them can be opportunistic pathogens, especially in
the elderly, the immune compromised, and those with prosthetic devices.
Bovis and ulcerans have been isolated from
skin ulcers, and bovis and pyogenes have caused
systemic bacteremia. Corynebacteria that have been isolated from
other infections include xerosis, jeikeium,
striatum, and pseudodiphtheriticum. Many
other diphtheroids, found as commensal organisms on healthy persons, might become
pathogenic under the right circumstances.

Drug Susceptibility

Treatment of diphtheria is two-pronged. Diphtheria antitoxin, produced in horses, is used to neutralize the toxin;
antibiotics are used to kill the bacteria. The antibiotics
of choice are penicillin and erythromycin, administered for fourteen days.
Clindamycin, rifampin, and tetracycline can also be used. Antibiotic
susceptibility of the diphtheroids varies, but penicillins, erythromycin, and
rifampin are usually good choices. Penicillin resistance has been seen in some
nontoxigenic diphtheriae strains.

Bibliography

Burkovski, Andreas, ed. Corynebacteria: Genomics and Molecular Biology. Norfolk, England: Caister Academic Press, 2008. This book mainly focuses on C. glutamicim. Chapter 2, however, discusses the genomics of many Corynebacterium spp.

Guilfoile, Patrick G. Deadly Diseases and Epidemics: Diphtheria. New York: Chelsea House, 2009. This volume describes diphtheria in detail.

Krieg, Noel R., et al., eds. Bergey’s Manual of Systematic Bacteriology. 2d ed. New York: Springer, 2010. Volume 5 of this multivolume work describes Corynebacterium and its relatives in detail.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. This text outlines many common bacteria and describes their natural history, pathogenicity, and other characteristics.