The Mycoplasma

The Mycoplasma

General Characteristics

· The mycoplasmas are essentially bacteria lacking a rigid cell wall during their entire life cycle, although they are also much smaller than bacteria.
· The first organism of this type was associated with pleuropneumonia of cattle, and was originally called the pleuropneumonia organism (PPO).
· Since that time, a number of organisms with similar morphological characteristics and cultural properties have been isolated. These are commonly referred to as pleuropneumonia-like organisms or PPLO.
· A certain group of mycoplasmas produce extremely tiny colonies on agar plates, and are called the T-strains.
· Some bacteria readily give rise spontaneously to variants that can replicate in the form of small filterable protoplasmic elements with defective or absent cell walls. These organisms, called L-forms, can also be formed by many species when cell wall synthesis is impaired by antibiotic treatment or high salt concentration.
· These organisms differ from mycoplasma in that they contain a rigid cell wall, at least at one stage of their life cycle and contain no sterols in their cytoplasmic membrane.
· These organisms are the smallest known free-living organisms.
· Because of the absence of cell walls, they do not stain with the Gram stain, and they are more pleomorphic and plastic than eubacteria.
· With Giemsa stain, they appear as tiny pleomorphic cocci, short rods, short spirals, and sometimes as hollow ring forms.
· Their diameter ranges from 0.15 u to 0.30 u.
· Most mycoplasmas require a rich medium containing a sterol and serum proteins for growth. Despite the lack of a cell wall, they do not require a medium of very high osmotic pressure.
· On solid media, they form minute, transparent colonies.
· When viewed under low-power magnification, the colony looks like a fried egg.
· The different strains vary in their growth rate and may take from two days to several weeks to form a colony.
Structure:
· The cell is enclosed by a limiting membrane which is more similar to that of animal cells than that of bacterial cells because of sterols present in the membrane.
· The cytoplasm contains ribosomes, but lacks mesosomes.
· There is no nuclear membrane.
· In some strains, amorphous material on the outer surface of the membrane suggests the existence of a capsule.
Metabolism :
· The parasitic mycoplasmas have truncated respiratory systems, lacking quinones and cytochromes.
· Another indication for the simplicity of the electron transport chain is the finding that the reduced nicotinamide adenine dinucleotide (NADH) oxidase activity is cytoplasmic.
· Complex electron transport chains are usually membrane bound, since they depend on the spatial organization of their components.
· Ruling out oxidative phosphorylation as an ATP-generating system leaves only two proven ways of ATP generation, both based on substrate level phosphorylation. The major source for ATP is the arginine dihydrolase pathway.

arginine deaminaseArginine + H2O --------------------------------> citrulline + NH3 ornithine carbamoyltransferaseCitrulline+inorganic orthophosphate------>ornithine+carbamoylPO
Carbamate kinaseCarbamoyl PO4 + ADP --------------------------> ATP + CO2 + NH3
Another mechanism for ATP generation is:
Phosphate acetyltransferaseAcetyl CoA + inorganic orthophosphate -----------------> acetyl PO4 + CoA
Acetate kinaseAcetyl PO4+ ADP ------------------------------------> Acetate + ATP

Multiplication:
· In the absence of a rigid cell wall, the pattern of replication is quite different from that of typical bacteria, whose division starts with the formation of a well-defined septum.
· Though the mechanism of division in mycoplasmas is controversial, sequential microscopic observation suggests that new elementary particles arise by fragmentation of filamentous cells containing several discrete DNA components.



Pathogenesis:
· M. pneumoniae is an extracellular pathogen that adheres to the respiratory epithelium by a specialized terminal protein attachment factor.
· This adherence protein interacts specifically with neuraminic acid residues on the epithelial cell surface.
· Ciliastasis occurs following attachment and then destruction of the superficial layer of epithelial cells. Destruction is due to release of hydrogen peroxide and superoxide anion.

Diseases:
The human diseases caused by mycoplasmas are shown in the table below.
Disease or symptom
Agent
Host
Primary atypical pneumonia
Mycoplasma pneumoniae
Man
Non-gonococcal urethritis (NGU)
Mycoplasma genitalium
Man
NGU
Ureaplasma urealyticum
Man
Stillbirth
Mycoplasma hominis
Man
Spontaneous abortion
Mycoplasma hominis
Man
Infertility
Mycoplasma hominis
Man

Laboratory Diagnosis :
The laboratory diagnosis of mycoplasma infection can be accomplished by:
1. Culturing the organism from sputum, mucous membrane swabbings or other specimens by direct inoculation into liquid or solid media containing serum, yeast extract and penicillin to inhibit contaminating bacteria. Colonies will become detectable in one to three weeks. They stain intensely with neutral red or tetrazolium or methylene blue. The organism can be presumptively identified by its hemabsorption or B-hemolysis of guinea pig red blood cells. It is conclusively identified by staining its colonies with homologous fluorescein-labelled antibody.
2. Quantitation of the patient antibody response to mycoplasma by complement fixation tests on acute and convalescent serum. Cold agglutinins to human O erythrocytes may also be

measuredChemotherapy:
Primary atypical pneumonia is usually selflimiting and does not require antibiotic treatment. However, if antibiotics are needed, the drug of choice is one of the macrolide antibiotics:
Azithromycin Clarithromycin Dirithromycin Erythromycin
Urogenital diseases may be treated with:
Metronidazole (except during the first trimester of pregnancy) Clindamycin

1. The mycoplasma is extremely small free-living bacteria which lack a cell wall and cytochromes.
2. Mycoplasma can be cultured on agar media but colonies take up to three weeks to develop.
3. Serological identification of mycoplasma disease relies upon the quantitation of cold agglutinins to human O erythrocytes or a complement fixation test or serum inhibition of mycoplasma growth.
4. Human diseases of mycoplasma etiology are primary atypical pneumonia, non-gonococcal urethritis, stillbirth, spontaneous abortion and infertility.

Best wishes
Prof. Dr. Fathi Awwad Mansour