Corynebacteria: Infection, Transmission & Diseases

Corynebacteria are gram-positive, rod-shaped bacteria. They are immotile and grow under both aerobic and anaerobic conditions. One of their species is responsible for diphtheria, among other diseases.

What are corynebacteria?

Corynebacteria are a genus of Gram-positive, rod-shaped bacteria that can grow facultatively anaerobically, meaning they can exist in the presence of oxygen, as well as in its absence. Their species are immobile and do not form spores. Moreover, they are catalase-positive, as well as oxidase-negative. Moreover, Corynebacteria grow only under demanding conditions, namely at 37 ° C and presence of 5% CO2. Corynebacteria have a great diversity of species. Some species are pathogenic to humans (such as the C. diphtheriae), other species are saprophytes, that is, they live on dying plant remains. Still others are non-pathogenic species, which occur in the normal flora on the skins and mucous membranes of humans. Characteristic for the corynebacteria is the club-shaped distension at one end, which gave them their name (gr. koryne = club). Another distinctive feature of corynebacteria is the presence of mycolic acids in the cell wall, which is also found in mycobacteria.

Occurrence, distribution and characteristics

Nonpathogenic species of corynebacteria occur primarily on the normal flora of the skin and mucosa of humans. However, pathogenic species are also widespread and found worldwide. The most common infectious disease caused by a Corynebacterium is diphtheria. Transmission is exclusively from person to person and can occur by droplet or smear infection. If a person becomes infected with a Corynebacterium, an initial infection is followed by local colonization of the pathogen. This can be followed by spread of the pathogen or, for example in the case of C. diphtheriae, by the formation of an exotoxin that inhibits protein synthesis. The incubation period ranges from 2 to 10 days. In general, corynebacteria are rarely the cause of an illness, especially since there is good vaccination protection in Germany. Exceptions are diphtheria, which is endemic in Russia, and Corynebacterium minutissimum. Corynebacteria are gram-positive rod bacteria. They have a certain pleomorphy, which means that they are able to change their shape depending on the conditions of the environment. They contain mycolic acid in their cell wall and are catalase positive but oxidase negative. Corynebacteria can be stained by Neisser stain to show yellow-brown bacteria with black-blue polar bodies.

Significance and function

There are numerous species of Corynebacteria that are found on the normal flora of the skin and mucosa. These include the C. minutissimum, C. xerosis, C. pseudotuberculosis, C. jeikeium, C. pseudodiphteriticum, and Corynebacterium bovis. Among these, some species are called facultative pathogens because they can cause disease under certain conditions, for example, weakening of the immune system. These species include C. minutissimum, causative agent of erythrasma, and C. jeikeium, possible causative agent of sepsis. The physiologically present corynebacteria break down the fats secreted by the sebaceous glands into fatty acids. These are subsequently responsible for the acidic environment of the skin and mucous membranes, which forms part of the protective acid mantle. This is a weakly acidic pH, which is found on the epidermis and thus forms a bactericidal effect on pathogens, resulting in inhibition of germ growth. Thus, corynebacteria form part of the innate, non-specific immune defense. In addition, the C. striatum is thought to be partly responsible for the typical underarm odor.

Diseases and complaints

Corynebacteria describe a genus of bacteria characterized by many species. The most important pathogenic species is C. diphtheriae. This is the causative agent of diphtheria. Humans are the only hosts of this bacterium and transmit the pathogen mostly by droplet infection. The C. diphtheriae then frequently enters the throat, less frequently skin wounds, and multiplies there. After multiplying, it produces the diphtheria toxin, which comes from the bacteriophages. Bacteriophages are viruses that infect bacteria. The diphtheria toxin acts by inhibiting protein synthesis.Even a dose of 100 – 150 ng per kg body weight is sufficient to kill a person. Initially, there is a local effect in the throat of the affected person. The epithelial cells of the mucosa are destroyed, bleeding and fibrin exudates. The latter form the characteristic fibrin coatings on the infected mucosa, which is called a pseudomembrane. Other bacteria, as well as cells and blood cells, become entangled in the pseudomembranes. Classic pharyngeal diphtheria is further characterized by fever, swelling of the lymph nodes, and soft palate paralysis. Feared complications include myocarditis, nerve and kidney damage if the toxin spreads systemically. In the past, the so-called laryngitis diphthericae was also a feared complication that quickly led to death by asphyxiation. It was characterized by a caesar neck (severe swelling of the lymph nodes) and a sweetish mouth odor. In addition to C. diphtheriae, other related species can cause diphtheria, including, for example, C. ulcerans, which can also affect animals. C. jeikeium is facultative pathogenic and can cause sepsis. In addition, C. minutissimum can cause erythrasma, a superficial, reddening dermatitis.