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'''Mycoplasma pneumoniae''' is a very small bacterium, in the class Mollicutes. This class of organisms lack a peptidoglycan cell wall present on all other firmicute bacteria. Instead, it has a cell membrane which incorporates sterol compounds, similar to eukaryotic cells. It obtains these sterols from the host serum, allowing it to retain a simple structure. Lacking a cell wall, these organisms are resistant to the effects of penicillins and other beta-lactam antibiotics, which act by disrupting the bacterial cell wall.
''M. pneumoniae'' has one of the smallest genomes known, with 816 kilobase pairs (kbs). Its genome and proteome has been fully characterized. It uses some unique genetic code, making its code more similar to mitochondria than to other bacteria. Thus it is said that ''Mycoplasma pneumoniae'' has a degenerate genome. It lacks the cellular machinery for making many essential compounds, including new purines and pyrimidines. It also has no tri-carboxylic acid cycle and an incomplete electron transport chain. Because of this, it is an obligate parasite. No mycoplasma is found free-living.
''M. pneumoniae'' is spread through respiratory droplet transmission. Once attached to the mucosa of a host organism, ''M. pneumonia'' extracts nutrients, grows and reproduces by binary fission. Attachment sites include the upper and lower respiratory tract, causing pharyngitis, bronchitis and pneumonia. The infection caused by this bacterium is called atypical pneumonia because of its protracted course and lack of sputum production and wealth of extra-pulmonary symptoms. Chronic mycoplasma infections have been implicated in the pathogenesis of rheumatoid arthritis and other rheumatological diseases.
''M. pneumoniae'' infections can be differentiated from other types of pneumonia by the relatively slow progression of symptoms, a positive blood test for cold-hemagglutinins in 50-70% of patients after 10 days of infection, lack of bacteria in a gram stained sputum sample, and a lack of growth on blood agar.
Mycoplasma atypical pneumonia can be complicated by Stevens-Johnson syndrome, hemolytic anemia, encephalitis or Guillain-Barré syndrome.
Second generation macrolide antibiotics, doxycycline and second generation quinolones are effective treatments. Disease from mycoplasma is usually mild to moderate in severity.
M. pneumoniae was historically called "Eaton's agent"[1] due to the fact that it is grown on Eaton's agar.

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★ ''Mycoplasma pneumoniae'' genome