CORYNEFORM BACTERIA
CORYNEBACTERIUM
INTRODUCTION TO CORYNEBACTERIUM
Corynebacterium are generally classified as Gram-positive (and non-acid-fast), nonmotile, nonsporulating aerobes, though some are facultatively anaerobic, (8). They are catalase positive and have a fermentative metabolism, with the ability to ferment carbohydrates to lactic acid. Members of the genus tend to be fastidious, growing slowly on enriched agar, and can be identified by the above features as well as their appearance on various selective and differential media. Optimum conditions for growth are 37°C and a neutral pH, however they are also capable of growing in temperate conditions between 15 °C and 40 °C. The majority of strains only grow adequately in the
presence of pantothenic acid and nicotinic acid, and a number of strains also require biotin, thiamine or pimelic acid, (9).
Corynebacterium cells are small (2-6µm in length) pleomorphic rods, with the characteristic of being irregular, club-shaped, or V-shaped. They may be arranged singly, in pairs (this gives rise to V-shapes), or in groups which can be described as looking like “palisades” (running parallel to one another) or “Chinese letters” , (9).
CELL MORPHOLOGY
The unusual cell arrangements are due to snapping division, which is a common feature among coryneform bacteria. This forms cell pairs which are described as V-shaped, but this type of division can occur multiple times to form larger groups of cells. When these rods appear to run parallel to one another, the formation is described as looking like palisades, and the groups are called “Chinese letters" when the arrangements are more irregular. In some species all three arrangements can be found, (9). Some Corynebacterium species have other cell morphologies, taking coccobacillus, filamentous,
Video 1: Cell growth and cell division in C. glutamicum
branching, or other irregular forms. It is for this reason that the cell shape of the genus is described as pleomorphic, (10). The most common form associated with the genus, however, is the club-shaped cell morphology.
METACHROMATIC GRANULES
The characteristic club-shaped cells of many Corynebacterium species are formed due to the presence of volutin granules (termed metachromatic granules because of their staining properties), which are large and make cells appear swollen at one end. These granules occur when a cell stores polyphosphate, which is used for phosphorous and energy storage. It is also thought to play a role in the stress response of an organism, and can be linked to pathogenicity (though some non-pathogenic species are known to store varying amounts of polyphosphate), (11).
The fact that metachromatic granules are more readily produced by pathogenic species can be used to
differentiate between Corynebacteria spp.
Fig 28: TEM of C. diphtheriae showing club shaped cells
Histochemical staining methods enable the identification between pathogenic (C. diphtheriae) and non-pathogenic Corynebacteria (C. glutamicum). The pathogenic strain contains volutin granules whereas the other does not.
Volutin granules have a set of specific characteristics by which they can be identified. They are typically electron-dense and can be seen to be rich in phosphorus when analysed via the use of X-ray microanalysis or transmission electron microscopy. They also display green UV fluorescence when stained with 4’6 – diamidino-2-pheylindole, (11).
HABITAT
Species are found in a wide range of habitats, (8), including soil, water, plants, and are even normal commensal bacteria on skin and mucous membranes in animals. Some species are opportunistic pathogens, and this includes C. diphtheriae, which is the type species of the genus, (10).
Ecology
