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15.1.4: Mycoplasma pneumoniae

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    42668
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    Mycoplasma
    Scanning electron micrograph of Mycoplasma. ("Mycoplasma" by AJC1 is licensed under CC BY-SA 2.0)

    Organism

    •  Mycoplasma pneumoniae is one of the smallest, free-living organisms (1 to 2 μm long and 0.1 to 0.2 μm wide)
      • cells cannot be seen with light microscopy
    • M. pneumoniae cells are pleomorphic (taking on many shapes), but often described as spindle-shaped
      • one end of the cell contains the "attachment organelle" where adhesins specific for respiratory tract epithelial cells are concentrated (Figure \(\PageIndex{1}\))
    • Although they lack a cell wall, mycoplasmas are evolutionarily most closely related to the Gram-positives and are in the Firmicutes phylum
      • require an osmotically stable environment (such as within host organism) to prevent lysis
    • In addition to small cell size, also have extremely small genome size (0.8 Mbp and 687 genes as opposed to the more typical 4-6 Mbp and 4000-5000 genes of bacteria)
    • Can be grown in culture, but colonies are so small a microscope is needed to see them clearly
    • Known to ferment glucose, but little else is know about metabolism

     

    clipboard_eb221920899062ab9e3f5d0f6aba41f5f.png

    Figure \(\PageIndex{1}\): Scanning electron micrograph of M. pneumoniae cell. Terminal attachment structures indicated by arrowheads. Bar = 200 nm (10.1128/JB.01865-14)

    Habitat

    • Exclusively found in human respiratory tract

    Source

    • Inhaled in aersols from infected individual

    Epidemiology

    • Transmission/outbreaks more common within groups often in close proximity (military bases, households, hospitals, and other congregate living situations)
    • Incubation period of 1-3 weeks
    • M. pneumoniae causes up to 40% or more of cases of community-acquired pneumonias and as many as 18% of cases requiring hospitalization in children (https://cmr.asm.org/content/17/4/697)
      • estimated 18,700 to 108,000 cases of pneumonia in hospitalized adults annually in US
    • Risk of hospitalization increases greatly after age 50
    • Mortality rate very low (~0.1%), but severe and fatal complications can occur

    Clinical Disease

    • M. pneumoniae infection is ordinarily mild, and as many as half of adult cases may be asymptomatic
    • Most common disease is pneumonia
    • Disease referred to by a couple different names
      • "walking pneumonia" because usually treated on outpatient basis
      • "primary atypical pneumonia" because of its unresponsiveness to antibiotics typically used to treat pneumonia
    • Symptoms: persistent dry cough, sore throat, hoarseness, fever, chills, body aches, earache, and general malaise
    • Can rarely cause fulminant (sudden with rapid progression) disease involving multiple body systems and organ failure or other extrapulminary infections such as septimcemia and encephalitis
    • Usually easily treated with tetracyclines, macrolides (both target ribosomes), or fluoroquinolones (target DNA replication)
      • antibiotics which target the bacterial cell wall are ineffective

    Primary Virulence Factors

    • Adhesins specific for ciliated cells of the respiratory epithelium contained within the attachment organelle allow tight association between M. pneumoniae and epithelial cells (Figure \(\PageIndex{2}\))
      • cilia movement is impaired and mucus removal from the respiratory tract is decreased
    • M. pneumoniae does not produce toxins, but prolonged colonization kills the epithelial cells

     

    Figure \(\PageIndex{2}\): Transmission electron micrograph of M. pneumoniae-infected hamster tracheal ring, demonstrating the close association of the attachment structure to the epithelium (arrow). (Copyright J. L. Jordan and D. C. Krause.)

    Additional Information:


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