Corynebacterium
Corynebacterium is a genus of gram-positive, non-spore-forming bacteria characterized by their club-shaped appearance and worldwide distribution. Notably, *Corynebacterium diphtheriae* is a significant human pathogen responsible for diphtheria, an upper respiratory infection that can lead to severe complications, including organ failure. These bacteria are nonmotile and catalase-positive, with a unique metabolism that includes aerobic and facultative anaerobic species. Corynebacteria are known for their complex cell wall structure, which contributes to their distinctive morphology and growth patterns, often resembling Chinese characters.
While *C. diphtheriae* is the primary concern due to its pathogenicity, many other species within the genus are nonpathogenic and exist as commensals in healthy individuals. However, some nonpathogenic strains can become opportunistic pathogens, particularly in vulnerable populations. Corynebacterium species also have industrial significance, being involved in the production of various organic compounds, including monosodium glutamate (MSG). Treatment of diphtheria typically involves a combination of diphtheria antitoxin to neutralize the toxin and antibiotics, such as penicillin or erythromycin, to eradicate the bacteria. Understanding the diverse roles and implications of Corynebacterium is essential for both public health and industrial applications.
Corynebacterium
- TRANSMISSION ROUTE: Direct contact, ingestion, inhalation
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 facultative anaerobic members of the genus. Those with anaerobic metabolism usually perform lactic acid fermentation. The bacteria are fastidious. All strains require biotin, and most require several other supplements. They are usually grown in an enriched carbon dioxide atmosphere and grow slowly, even on complex-enriched culture media.
The rods are pleomorphic, 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, arabinogalactans, 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 many 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 1990s. Some former Corynebacterium spp. were moved to other related genera: C. acnes to Propionibacterium acnes and C. haemolyticum to Arcanobacterium haemolyticum. 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 epidermal 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 can 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 older people, 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 in 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. Azithromycin, 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
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