Corynebacterium: Characteristics, Taxonomy, Morphology, Culture

Corynebacterium is a genus of bacteria belonging to the Actinobacteria class, whose members are characterized by being Gram positive. They exhibit two or more structural forms during their life cycle (that is, they are pleomorphic). They are not mobile, not encapsulated and do not form spores.

Bacteria of the genus Corynebacterium can be present in soil, water, plants and animals. Some species are saprophytes, others are animal commensals, and others are pathogenic.

The pathogenic representatives are responsible for diseases such as diphtheria ( Corynebacterium diphtheria ) and caseous lymphadenitis ( C. pseudotuberculosis ). They can also cause nosocomial diseases.

A few species of this genus (eg C glutamicum and C feeiciens ) are important in biotechnology for the production of amino acids and other compounds.

General characteristics

Bacteria of the genus Corynebacterium belong to a group called the CMN group, which includes members of the families Corynebacteriaceae, Mycobacteriaceae, and Nocardiaceae.

All bacteria in this group share two common characteristics. One of these characteristics is the proportion of Guanine (G) and Cytosine (C) with respect to other nitrogenous bases. The other characteristic is the structure of the cell wall .

The genus is composed of pleomorphic Gram positive organisms. They are catalase positive, they do not form spores (they do not sporulate), nor are they resistant to acid-alcohol.

Generally, the Corynebacterium species are oxidative and facultative fermentative in their metabolism of carbohydrates or sugars (carbohydrates).

With regard to the content of G and C, it is high, and may be higher than 70%. The cell wall, for its part, is composed of peptidoglycan, arabinogalactan as well as mycolic acid.

All Corynebacterium are catalase positive, however, some of them are fermentative, others oxidative. Other species neither ferment nor oxidize.

Taxonomy

The genus Corynebacterium was erected by Lehmann and Neumann in 1896 to group the diphtheria-producing bacilli. It currently includes about 80 validly described species. More than half of these species are considered medically relevant.

The family Corynebacteriaceae, which includes the genera Corynebacterium and Turicella, is located taxonomically in the class Actinobacteria, order Actinomycetales. It belongs to the CMN group (Corynebacteriaceae, Mycobacteriaceae and Nocardiaceae). This group lacks taxonomic validity.

Some authors artificially divide the genus Corynebacterium  into two groups; diphtheria species on the one hand, and non-diphtheria corynebacteria (CND) on the other.

This division, based on the potential of the species to produce diphtheria, has no taxonomic validity. Among the CND there are both non-pathogenic species and species responsible for diseases, mainly of the nosocomial type.

Other methods of taxonomic identification

With respect to molecular taxonomy, the techniques applied to the characterization and identification of Gram positive bacilli have led to the description of new species of the genus Corynebacterium , especially human clinical samples.

The molecular methods used for the characterization of these bacteria include genetic analysis of 16S rRNA and rDNA sequencing, nucleic acid hybridization, among others.

Analysis of the presence and quantity of peptidoglycans, determination of mycolic acids, identification of menaquinone, analysis of cellular fatty acids, infrared spectroscopy, detection of the preformed enzymes glucosidase or aminopeptidase, among other analyzes, are also used.

Morphology

Bacteria of the genus Corynebacterium are pleomorphic (that is, they can present several different forms). They can be shaped like a coconut, a filamentous rod, a club or a whip handle. They can be straight or with curved ends.

Its length will be between 2 and 6 µm, while its diameter will be close to 0.5 µm.

Colonies can be in the form of palisades or Chinese characters. They are small, granular colonies, of variable color, yellowish white, gray or black. Its edges can be continuous, serrated or intermediate between these, depending on the culture medium.

In the cell wall they present peptidoglycan, arabinogalactan and mycolic acid. In addition to these, it also presents mesodiaminopimelic acid in the murein tetrapeptide.

A unique feature of the genus is the presence of conservative or fixed “insertion or deletion” sites (indels). Among these fixed inelines are the insertion of two amino acids into the enzyme phosphoribosiphosphate and the insertion of three amino acids into acetate kinase.

Corynebacterium culture 

Although Corinebacterium , in general, are not very demanding in relation to the culture media, some of them have very specific requirements for their growth. All need biotin and some also require thiamine and p-aminobenzoic acid.

Initial growth is slow but then improves rapidly. A widely used medium for the cultivation of species of this genus is Loeffler medium. This medium contains horse serum, meat infusion, dextrose and sodium chloride .

Loeffler’s medium is selective for  C. diphtheriae, if tellurite is added. Most CNDs, on the other hand, show better growth in media with sheep broth and blood, brain enriched with a lipid such as 0.1-1.0% Tween 80, or carbohydrate broth enriched with serum.

Pathogeny

Diphtheria, produced by Corynebacterium diphtheriae , is transmitted between humans through contaminated particles transferred during respiration. The bacterium produces a toxin that inhibits cellular protein synthesis.

It also destroys tissues and creates a pseudomembrane. The effects of the toxin include respiratory tract conditions, myocarditis, neuritis, and renal tubular necrosis. Diphtheria can cause death.

About 50 of the non-diphtheria corynebacteria have been associated with human or animal infections. The main human infections caused by CND are nosocomial in origin and affect people with weakened immune systems.

Among the disease-causing species most commonly isolated in humans are  C. striatum , C. jeikeium , C. urealyticum, and C. pseudodiphteriticum .

Among the diseases caused in humans by CNDs, infections of the urinary tract, skin, infections associated with prosthetic devices, osteomyelitis, septic arthritis, endocarditis, peritonitis, brain abscess, bacteremia, meningitis, premature labor and spontaneous abortions have been mentioned. . These nosocomial diseases have increased their occurrence in recent years.

Some species of  Corynebacterium affect animals. For example, C. pseudodiphteriticum produces pathologies such as: caseous lymphadenitis in sheep, cattle and other ruminants. It also causes abortions (in sheep) and folliculitis (in horses).

References

  1. C. Winn, S. Allen, WM Janda, EW Koneman, GW Procop, PC Schreckenberger, GL Woods (2008). Microbiological Diagnosis, Text and Color Atlas (6th ed.). Buenos Aires, Argentina. Editorial Médica Panamericana. 1696 pp.
  2. A. Von Graevenitz, K. Bernard (2006) Chapter 1.1.16. The Genus Corynebacterium — Medical. Prokaryotes.
  3. V. Ramana1, G. Vikram, PP Wali, K. Anand, M. Rao, SD. Rao, R. Mani, V. Sarada, R. Rao (2014). Non Diphtheritic Corynebacteria (NDC) and their clinical significance: clinical microbiologist’s perspective. American Journal of Epidemiology and Infectious Disease.
  4. A. Dorella, LGC Pacheco, SC Oliveira, A. Miyoshi, V. Azevedo (2006). Corynebacterium pseudotuberculosis: microbiology, biochemical properties, pathogenesis and molecular studies of virulence. Veterinary Research.
  5. M. Maheriya, GH Pathak, AV Chauhan, MK Mehariya, PC Agrawal (2014). Clinical and epidemiological profile of diphtheria in tertiary care Hospital Gujarat Medical Journal.
  6. On Wikipedia. Retrieved September 26, 2018 from https://en.wikipedia.org/wiki/Corynebacterium.
  7. C. Pascual, PA Lawson, JAE Farrow, MN Gimenez, MD Collins (1995). Phylogenetic analysis of the genus Corynebacteriurn based on 16s rRNA gene sequences. International Journal of Systematic Bacteriology.

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