Mycoplasma pneumoniae

Epidemiology and Demographics
Each year an estimated 2 million cases and 100,000 pneumonia-related hospitalizations occur in the United States.

Trends

Unknown. However, with improved diagnostic testing, more cases may be identified.

Risk Factors
Persons of all ages are at risk but rarely children less than 5 years old. It is the leading cause of pneumonia in school-age children and young adults. Outbreaks can occur especially in crowded military and institutional (e.g., college) settings. Outbreaks in these settings can last several months.

Pathophysiology & Etiology
Etiologic Agent:

Mycoplasma pneumoniae, a small bacterium. 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.

Transmission:

Person-to-person transmission by contact with respiratory secretions. 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.

Diagnosis
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.

History and Symptoms
Majority with upper respiratory tract infections with fever, cough, malaise, and headache. May lead to tracheobronchitis with fever and nonproductive cough: radiologically confirmed pneumonia develops in 5-10% of cases; rare extrapulmonary syndromes, including cardiologic, neurologic, and dermatologic findings.

Risk Stratification and Prognosis
Persistent cough is common during convalescence; other sequelae are rare. Fatal cases are reported occasionally, primarily among the elderly and persons with sickle-cell disease.

Acknowledgements
The content on this page was first contributed by: C. Michael Gibson, M.S., M.D.