Mycoplasmas

           Mycoplasmas are the smallest (0.2-0.3mm) free-living microorganisms, therefore they can pass through bacterial filters, and one of the most pleomorphic, since they are devoid of cell walls. They occur as granules and filaments of various sizes. The granules may range from 100-1000 mm in size, with coccoid, balloon,disc, ring or star forms. The filaments are slender of varying lengths and show branching. Multiplication is by binary fission, budding or chain of beads produced. A distinctive feature seen in some species is a bulbons enlargement with a differentiated tip structure, by means of which the organisms get attached to suitable host cells carrying neuraminic acid receptors. They may be responsible for the hemadsorption shown by some species. They do not possess spores, flagella or fimbria. Some species exhibit a gliding motility. Mycoplasmas are Gram negative, but better stained by Giemsa stain. The cells are bounded by a soft trilaminar unit membrane containing sterols. Because of their plasticity, they can pass through filters and often mistaken for viruses.

Mycoplasmas may be cultivated in fluid or solid media. They are generally facultative anaerobes, better growth aerobically. The growth temp range 22-41°C while parasitic species at 35-37°C and saprophytes at low temp. Media for cultivating mycoplasma are enriched with 20% serum and yeast extract. Penicillin and thallium acetate added as selective agents. The high concentration of serum is a source of cholesterol and other lipids.

Colonies appear after 2-6 day incubation with a “fried-egg” appearance, consisting of a central opaque granular area of growth extending into the depth of the medium, surrounded by a flat, translucent peripheral zone. Colonies cannot be picked with inoculating loops. Subculture is done by cutting out an agar block with colonies and rubbing it on fresh plates. Most mycoplasma colonies are hemolytic.

Most species utilize glucose, arginine as major sources of energy. Urea is not hydrolysed and non proteolytic, also they lack the ability to synthesize purines and pyrimidines. They are destroyed at 45°C in 15 minutes. They are relatively resistant to lysis by osmotic shock but                    sensitive to lysis by surface active agents. Resistant to penicillin, cephalosporin, lysozymes which act on bacterial cell walls, but sensitive to Tetracycline. Mycoplasmal surface antigens are glycolipids and proteins.

Antigenicity – M. pneumoniae possesses cell-membrane bound glycolipids and proteins which act as haptens. Glycolipids induce the production of antibodies that react in complement fixation test. Glycolipids with similar antigenic nature have been found in human brain, therefore the antibodies to  M. pneumoniae glycolipids may cross react with brain cells leading to brain damage. M. pneumoniae also possesses two major surface proteins , P1 protein mediates attachment.

Pathogenicity – Mycoplasmas are opportunistic infectious agents. Parasitic forms exhibit host specificity. They generally produce surface infections by adhering to the mucosa of the respiratory, gastrointestinal and genitourinary tracts. Mycoplasmas cause 2 types of diseases in humans - pneumonia and genital infections.

Primary Atypical Pneumonia or mycoplasmal pneumonia is caused by M. pneumoniae, usually a mild pneumonia. The disease is said to be atypical because the symptoms are different from those of classic pneumonia. Some patients have no signs or symptoms related to their respiratory tract, but fever and malaise. Patients often remain ambulatory, so the disease is sometimes called walking pneumonia. Transmission is by respiratory secretions in droplet form and onset of symptoms follows after an incubation period of 12-14 days.   Fever lasts 8-10 days and gradually declines along with the cough and chest pain. In some cases, alveoli(sac-like structures in the lungs where gaseous exchange occurs)decrease in size by inward swelling of the alveolar walls and alveoli do not fill with fluid.

Diagnosis can be made by isolating M. pneumoniae from sputum or from a nasopharyngeal  swab. This process takes 2-3 weeks due to the slow growth rate of the organisms. Serological tests such as direct immunofluorescence, ELISA or with DNA probes.

Erythromycin and tetracycline are the drugs of choice. No vaccine, prevention requires avoidance of contact with infected persons and their secretions.

Mycoplasmal genital infections  caused by M. hominis. They infect by fusing their cell membranes with the host cell membranes. They cause pelvic inflammatory disease in women and urethritis in men. Mycoplasmas on the cervix during pregnancy colonize the placenta and cause spontaneous abortions,premature births.

            Diagnosis is made by culture from urethral and viginal discharges and from placental surfaces.Tetracycline is effective.Transmission by sexual contact.