MYCOPLAMA GENERAL FEATURES / 1

Mycoplasma

Mycoplasma has been recognized as a causative organism for a number of diseases, as contaminants in tissue culture and as harmless saprophytes . The colonies consists of cells and fragments of various size and can be described as cocci ,filamentous ,disc and rosettes . They multiply by binary fision , by fragmentation of filaments and rings into cocci cells and a kind of sprouting. In liquid media they are found as very irregular and sometimes branched forms that pass through membrane filters.

Distribution:

The members of Mycoplasm group are parasitic bacteria . They do not kill host but they produce predominantly chronic infections and in this sense they are very successful parasites.

In animals they appear as harmless parasites . On serous epithelial of respiratory and genital tracks. They adhere tightly to the epithelial cells of the membrane. They do not excrete toxins but a weak metabolites like ammonium ions ,hydroxides like peroxides which may have toxic effects non cells .Infection by Mycoplasma may be symptom less in some animals while inflammation of respiratory system ( lungs ) occur in others.

In plants Mycoplasma cause yellow diseases . They predominantly localized in the phloem part of the vascular system and because of their morphological similarity to spirillum they have been collected in the genus Spiroplasma. Spiroplasma citri causes yellow disease in citrus trees . Similar forms have been demonstrated in other plants like maize and rice.

General Features of Mycoplasma:

Mycoplasmas are generally featured in the following ways .

1.Mycoplasmas are distinguished from other bacteria by the absence of cell wall . The lack of cell wall has 4 consequences:

  1. Mycoplasmas do not assume definite and clearly recognizable shape .instead they are highly pleomorphic organisms showing variations in size and shape.
  2. They are Gram-negative organisms.
  3. They are resistant to pencillin and cephalosporin that function by preventing cell wall formation.
  4. Mycoplasmas must be maintained on a special media that protect them from osmolysis. On the agar they produce characteristic colony with a “fried egg” appearance . The raised centre is nearly spherical mass of cells partly embedded in the agar and is surrounded by a thin film of surface growth. Colonies that develop on solid media are often small , microscopic ( 10-100m) typically nippled with a fried egg appearance . Mycoplasma pass through bacteria retaining filters and can be cultivated in vitro on non living media of rich composition as facultative or obligate anaerobe.

2.They have GC values from 23 -36% and a small genome size(molecular size of 0.5 x 109 ). Mycoplamas have been referred to as Pleuropneumonia like organism (PPLO).

Classification:

Mycoplasmas at present are placed in a taxonomic class mollicutes ( soft skin ) which consists of a single order Mycoplamatales.

Families of Mycoplasmatales are:

  1. Mycoplasmataceae: They require cholesterol for growth.Genus :Mycoplasma.
  2. Acholeplasmataceae: They do not require cholesterol for growth.
  3. Spiroplasmataceae/Asteroplasmataceae: Include plant pathogens.

Morphology and Structure:

Under phase contrast and dark field microscope Mycoplasma seem to have pleomorphic morphology characterized by small coccoid bodies, fine filaments some of which exhibit branching. Cell size may range from 0.3 -0.9m by electron microscope. They absence of cell walls. Their sole retaining structure is cytoplasmic membrane. The membrane is very thin, limp plastic and elatic. Hence they are extremely pleomorphic. Because of this fragility of the membrane they are susceptible to mechanical destruction and also osmotic rupture. Cell membrane is associated with substantial amount of polysaccharide.

Internal structures of Mycoplasma are similar to prokaryotes. They are:

  1. Nuclear structures are present but less evident in Mycoplasma.

Ribosomes are clearly visible within cells

  1. Mesosomes are conspicuously absent due to elasticity of the membrane.
  2. Enzyme systems and metabolic character of Mycoplasma differ from familiar bacteria but differences are not extreme.
  3. The fine structure of a typical Mycoplasma consists of a trilaminar membrane surrounding cytoplasm. The cytoplasmic membrane is typical of the unit membrane being composed of a light area about 5nm thick bound on either side by an electron dense protein layers. Each of about 3nm thick giving the membrane the overall thickness of 11nm.
  4. The nuclear material consists of an unbound fibrillar and granular region . The fibrillar strands are about 3nm thick and are not contained within the membrane. The chromosome is circular . The size is appoximately 100x106 daltons allowing for coding of more than a thousand cistrons.
  5. Ribosomes measure about 14nm in diameter and have a sediment coefficient of 70S.
  6. Within the cell , a dense region which occurs as protrusions are termed blebs. The ultra structure of blebs consists of a dense elliptical outer plate adjacent to the cytoplasmic membrane , a flat plate and fine threads connecting the flat plate to elliptical plate. Between the two blebs lies an intra bleb-region which is considered to be protein. Ribosomes are seen in actively dividing cells.
  7. Dense cytoplasmic bodies called elementary bodies that extrude from cells are noticed . these are considered to be sites of reproduction. Empty vesicles surrounded by trilaminar membranes are also observed.
  8. Motility has been observed in a few organisms and motility is by gliding . Electron micrographs of intact organisms fail to reveal any flagellar structure.

Reproduction :

Several members of mollicutes reproduce by binary fission. Very minute bodies called elementary bodies or minimal reproductive units are confined within large bodies of mature cells. The elementary bodies have bacteria -like structures and are considered the smallest living entities. They enlarge to form long filamentous or mycelial or chains of minute spheres like conidia but much smaller.

The conidia like bodies are liberated and each increase in size to become a large body several micrometers in diameter inside which new elementary bodies are found. These are released by rupture of the membrane of large body. The growth rate of Mycoplasma is moderately rapid , with a generation time of one to three hours depending upon the species and nutrient conditions.

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PASSWORD: mycoplasma