Mycoplasma

Mycoplasmas are bacteria that do not possess rigid cell walls for lack of a murein layer. These bacteria take on many different forms. They can only be rendered visible in their native state with phase contrast or dark field microscopy. Mycoplasmas can be grown on culture mediums with high osmotic pressure levels. M. pneumoniae frequently causes pneumonias that run atypical courses, especially in youths. Ten to twenty percent of pneumonias contracted outside of hospitals are caused by this pathogen. M. hominis and Ur-eaplasma urealyticum contribute to nonspecific infections of the urogenital tract. Infections caused by Mycoplasmataceae can be diagnosed by culture growth or antibody assays. The antibiotics of choice are tetracyclines and macrolides (macrolides not for M. hominis). Mycoplasmas show high levels of natural resistance to all betalactam antibiotics.     

Classification. Prokaryotes lacking cell walls are widespread among plants and animals as components of normal flora and as pathogens. Human pathogen species are found in the family Mycoplasmataceae, genera Mycoplasma and Ureaplasma. Infections of the respiratory organs are caused by the species M. pneumoniae. Infections of the urogenital tract are caused by the facultatively pathogenic species M. hominis and Ureaplasma urealyticum. Other species are part of the apathogenic normal flora.

Morphology and culture. The designation mycoplasma is a reference to the many different forms assumed by these pathogens. The most frequent basic shape is a coccoid cell with a diameter of 0.3-0.8 µm. Long, fungi like filaments also occur. Mycoplasmas are best observed in their native state using phase contrast or dark field microscopy. Staining causes them to disintegrate. In  contrast to all other bacteria, mycoplasmas possess no rigid cell wall. Flagellae, fimbriae, pili, and capsules are lacking as well. Due to their inherent plasticity, mycoplasmas usually slip through filters that hold back other bacteria. Since their cell wall contains no murein, mycoplasmas are completely insensitive to antibiotics that inhibit murein synthesis (e.g., betalactams). Mycoplasmas can be cultured on special isotonic nutrient mediums. After two to eight days, small colonies develop resembling sunny-side-up eggs and growing partially into the agar.

Pathogenesis and clinical pictures.

- Infections of the respiratory organs. The pathogen involved is M. pneumoniae. The organism is transmitted by aerosol droplets. The cells attach themselves to the epithelia of the trachea, bronchi, and bronchioles. The mechanisms that finally result in destruction of the epithelial cells are yet unknown. The infection develops into pneumonia with an inflammatory exudate in the lumens of the bronchi and bronchioles. The incubation period is 10-20 days. The infection manifests with fever, headache, and a persistent cough. The clinical pictures of the infection course is frequently atypical, i. e., the pneumonia cannot be confirmed by percussion and auscultation. A differential diagnosis must also consider viral pneumonias, ornithosis, and Q fever. Sequelae can set in during or shortly after the acute infection, including pericarditis, myocarditis, pancreatitis, arthritis, erythema nodosum, hemolytic anemias, polyneuritis, and others.

- Infections of the urogenital tract. These infections are caused by M. ho- minis and Ureaplasma urealyticum. These facultatively pathogenic species also occur in healthy persons as part of the mucosal flora, so that their etiological role when isolated is often a matter of controversy. U. urealyticum is considered responsible for 10-20% of cases of nongonococcal urethritis and prostatitis in men.

Diagnosis. These pathogens can be grown on special culture mediums. Com-mercially amplification tests are available for direct identification of M. pneumoniae. The CFT was formerly used to detect antibodies to M. pneumoniae; today this is done with IgM-specific ElAs. Antibody tests are of no diagnostic value in infections caused by M. hominis and U. urealyticum.

Therapy. The antibiotics of choice are tetracyclines and macrolides. M. hominis shows a natural resistance to macrolides, U. urealyticum to lincomycins. Concurrent partner treatment is recommended in urogenital infections.

Epidemiology and prevention. M. pneumoniae is found worldwide. Humans are the only source of infection. The pathogens are transmitted by droplet infection during close contact. Infections are frequently contracted in families, schools, homes for children, work camps, and military camps. Incidence is particularly high between the ages of five and 15 years. About 10-20 % of all pneumonias contracted outside hospitals are caused by this pathogen. M. hominis and U. urealyticum are transmitted either between sexual partners or from mother to neonate during birth. No specific prophylactic measures are available to protect against any of the mycoplasma infections.