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Mycoplasma pneumoniae, Legionella species, and Chlamydia pneumoniae are often grouped together as the most important causes of “atypical” community-acquired pneumonia. (For a discussion of urogenital mycoplasmas, see Chap. 86.)


With a size of only 150–350 nm, mycoplasmas are the smallest free-living organisms. Genome sequence data from many different Mycoplasma species have helped define the minimal set of genes necessary for cellular life. Lacking a cell wall and bounded only by a plasma membrane, mycoplasmas colonize mucosal surfaces of the respiratory and urogenital tracts.


M. pneumoniae occurs worldwide with no seasonal pattern. Infection causes upper respiratory tract disease ∼20 times more frequently than pneumonia.

  • Infection is acquired by inhalation of aerosols, with an incubation period of 2–4 weeks.

  • M. pneumoniae accounts for ∼23% of cases of community-acquired pneumonia in adults.


The clinical presentation does not help distinguish M. pneumoniae pneumonia from pneumonia of any other bacterial etiology.

  • Acute M. pneumoniae infection manifests as a nonspecific upper-respiratory syndrome with pharyngitis, tracheobronchitis, and/or wheezing.

  • Pneumonia develops in 3–13% of infected pts. The most common presenting symptom is a nonproductive cough. Headache, malaise, chills, and fever are common.

  • On physical examination, ∼80% of pts have wheezes or rales.

  • Symptoms usually resolve in 2–3 weeks, and appropriate antimicrobial therapy significantly shortens the duration of clinical illness.

  • Infection uncommonly results in critical illness and rarely causes death.

  • Extrapulmonary manifestations of M. pneumoniae infection are relatively uncommon but include skin eruptions (e.g., erythema multiforme major, rashes), neurologic manifestations (e.g., encephalitis, Guillain-Barré syndrome, acute demyelinating encephalomyelitis), septic arthritis (particularly in pts with hypogammaglobulinemia), and hematologic manifestations (e.g., hemolytic anemia, coagulopathies).


Clinical findings, nonmicrobiologic laboratory tests, and CXR are not useful in distinguishing M. pneumoniae pneumonia from pneumonia of other etiologies.

  • Acute M. pneumoniae infection can be diagnosed by PCR analysis of respiratory tract secretions, which is 65–90% sensitive and 90–100% specific.

  • M. pneumoniae culture (which requires special medium) is not recommended for routine diagnosis because its sensitivity is ≤60% and growth of the organism can take weeks.

  • Serologic testing for IgM and IgG antibodies to M. pneumoniae requires acute- and convalescent-phase samples and is therefore less useful for diagnosis of active infections. Moreover, IgM antibodies to M. pneumoniae can persist for up to 1 year after acute infection.

  • Measurement of cold agglutinin titers is no longer recommended for the diagnosis of M. pneumoniae infection because the findings are nonspecific.


TREATMENT M. pneumoniae Infections

  • Antibiotic options include macrolides (azithromycin, 500 mg PO for 1 day followed by 250 mg for 4 days), tetracyclines (doxycycline, 100 mg PO bid for 10–14 days), and respiratory fluoroquinolones (levofloxacin, 500–750 mg PO qd for 10–14 days).


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