Mycoplasma Pneumoniae
Mycoplasmas are the smallest free-living organisms. Their genome size is only 10-15% of that of usual bacteria, and they lack the genes necessary for cell wall synthesis. M. pneumoniae, is a common cause of acute upper and lower respiratory infections in children and young adults. The organism is transmitted by respiratory droplets. An estimated 2 million infections are caused by M. pneumoniae each year in the United States. Radiologically confirmed pneumonia is detected in 3% to 10% of cases. Estimates indicate that up to 40% of all community-acquired pneumonia is caused by M. pneumoniae. Pneumonia has a lengthy incubation period of 3 weeks, which can result in prolonged outbreaks. Macrolides and tetracyclines are the preferred antibiotics for treatment.
Disease onset is insidious. Fever, headache, and malaise occur 2 to 4 days before the onset of respiratory symptoms. Another common clinical syndrome caused by M. pneumoniae is acute or subacute tracheobronchitis. Some studies have shown a peak incidence of infection in the fall in temperate climates although sporadic cases may occur without seasonality.
Extrapulmonary manifestations occur rarely and include erythema nodosum, Stevens-Johnson syndrome, cardiac arrhythmias, pericarditis, myocarditis, coagulopathies, hemolytic anemia and central nervous system involvement. Vascular complications include Raynaud’s phenomenon, which may be associated with the production of cold agglutinins. There is accumulating evidence over the last few years that links M. pneumoniae with chronic asthma in some patients due to persistent infection of the lower respiratory tract.
The presence of cold agglutinins has been used as a screening test for Mycoplasma infection, but is neither sensitive nor specific. A cold agglutinin titer of greater than1 to 32 in the appropriate setting is considered indicative of infection. However, the sensitivity of the cold agglutinin test is low, approximately 50%. Specificity is also low since cold agglutinins are an acute phase reactant and may be seen in a variety of diseases including autoimmune disorders, dysproteinemia, and other atypical pneumonias including Legionella, CMV and EBV.
Mycoplasma can be cultured from respiratory tract specimens but must be specifically requested, since the organism does not grow on routine culture media. Detection usually requires one to two weeks, which limits clinical utility. For these reasons, serology and PCR are the preferred diagnostic tests.
M. pneumoniae serology to detect both IgG and IgM antibodies is both sensitive and specific. Sensitivities and specificities are both exceed 90%. Results are reported as negative or positive. Reference value is negative. The presence of IgM antibodies indicates recent infection. The presence of IgG antibody only may indicate recent or past infection. A paired serum sample drawn after 4 weeks may be required to confirm the diagnosis. Specimen requirement is one plain red top tube of blood.
Mycoplasma pneumoniae can be detected most quickly by polymerase chain reaction (PCR). It can be performed on a throat swab, saliva, sputum or bronchoalveolar lavage (BAL). Because M. pneumoniae is not part of normal human pharyngeal flora, its detection in nasopharyngeal or oropharyngeal specimens in persons with compatible clinical illness indicates that M. pneumoniae is the etiologic agent.The best practice is to simultaneously order mycoplamsa serology and PCR.
