15.1.4: Mycoplasma pneumoniae

 

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

 

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:

• https://cmr.asm.org/content/cmr/17/4/697.full.pdf
• https://www.cdc.gov/pneumonia/atypic...sma/index.html