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Mycoplasmas are small bacteria (0.3 to 0.8 μm in diameter and up to 150 μm in length) that are unique because they have no cell wall. Two genera, Mycoplasma and Ureaplasma (associated with genital infections), are of clinical importance to humans. M. pneumoniae is primarily a respiratory tract pathogen that involves the nasopharynx, throat, trachea, bronchi, bronchioles, and alveoli. Symptomatic infections attributable to this organism most commonly occur in children and young adults (ages 2 to 19 years). The organism has also been associated with infections of the cardiovascular, dermatologic, and central nervous systems. Other mycoplasmas may be present, especially in the upper respiratory tract; however, they are considered part of the normal microbial flora of the oral cavity and do not cause symptoms or disease.
Infections with M. pneumoniae occur worldwide throughout the year, especially in temperate climates. Epidemics occur at 4- to 6-year intervals in both civilian and military populations.
The infection often seems to be a sporadic, endemic illness in families or closed communities because it has a relatively long izncubation period (2 to 15 days), because its prolonged shedding in nasal secretions may result in additional infections during a protracted interval, and because asymptomatic infections are common.
M. pneumoniae infections may be diagnosed by cultivation of the organism on complex agar medium or by serologic methods. Because of the slow growth of the organism, partly reflecting fastidious nutrient requirements, culture techniques have been expensive, tedious, and somewhat unproductive. In the clinical microbiology laboratory at the Mayo Clinic, the rate of isolation of the organism has ranged from 0 to 43% from throat and sputum specimens in individual years.
The time interval between isolates, however, may be several months because of the sporadic occurrence of these infections. Moreover, the last five isolates of M. pneumoniae in our laboratory necessitated more than 20 days of incubation time before a report could be made. Perhaps genetic probes could produce more timely results with acceptable sensitivity and specificity relative to cultivation of the organism.
Indirect Immunofluorescence Test.
Titration of antibodies in acute and convalescent phase serum specimens is usually done with use of the complement fixation reaction. Currently, however, cells that contain M. pneumoniae antigenic substrate are commercially available (Zeus Technologies, Inc., Raritan, New Jersey), and these facilitate performance of the indirect immunofluorescence test
and separate measurement of antibodies of IgG and IgM classes. Of 14 paired (acute and convalescent phase) serum specimens, in each pair of which the presence of an M. pneumoniae infection was confirmed by a rising titer of complement fixation, 11 had fourfold or greater increases in titer, as measured by the indirect immunofluorescence test. One of the three paired serum specimens that did not have a diagnostic serologic increase in titer had IgM class antibody present in both acute and convalescent phase serum samples. Importantly, IgM antibodies, indicative of a current infection, were demonstrated in 13 of the 14 patients. In five patients, IgM antibodies were detected in the acute phase serum sample.
Twenty serum specimens submitted for cytomegalovirus serology were tested for antibodies to M. pneumoniae by the indirect immunofluorescence test. Seventeen of the 20 specimens (85%) had low levels of IgG antibody directed against the organism; of these, 15 had titers of 1:10 or less. None was positive for IgM antibodies to M. pneumoniae. Exposure to this organism is widespread—in one analysis, more than 90% of asymptomatic, apparently well, Mayo Clinic blood donors had IgG antibodies to M. pneumoniae, as detected by the indirect immunofluorescence test (Fig. 1).
The indirect immunofluorescence test for M. pneumoniae is sensitive and specific and can distinguish between current (IgM antibody) and past (IgG antibody) infections. Nonetheless, examination of both acute and convalescent phase (7- to 10-day) serum specimens is urged, because the IgM antibody may not be demonstrable early in some cases and a rising titer confirms the diagnosis of M. pneumoniae infection.
Clyde Jr, WA
Mycoplasma pneumoniae disease: clinical spectrum, pathophysiology, epidemiology, and control.