Mycoplasmas are a group of very small, wall-less organisms, of which Mycoplasma pneumoniae is the major pathogen.
Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 607.
M. pneumoniae causes “atypical” pneumonia.
Mycoplasmas are the smallest free-living organisms; many are as small as 0.3 μm in diameter. Their most striking feature is the absence of a cell wall.1
Consequently, mycoplasmas stain poorly with Gram stain, and antibiotics that inhibit cell wall (peptidoglycan) synthesis (e.g., penicillins and cephalosporins) are ineffective. Their outer surface is a flexible cell membrane; hence, these organisms can assume a variety of shapes. It is the only bacterial membrane that contains cholesterol, a sterol usually found in eukaryotic cell membranes.
Mycoplasmas can be grown in the laboratory on artificial media, but they have complex nutritional requirements, including several lipids. They grow slowly and require at least 1 week to form a visible colony. The colony frequently has a characteristic “fried-egg” shape, with a raised center and a thinner outer edge.
Pathogenesis & Epidemiology
M. pneumoniae, a pathogen only for humans, is transmitted by respiratory droplets. In the lungs, the organism is rod-shaped, with a tapered tip that contains specific proteins that serve as the point of attachment to the respiratory epithelium. The respiratory mucosa is not invaded, but ciliary motion is inhibited and necrosis of the epithelium occurs. The mechanism by which M. pneumoniae causes inflammation is uncertain. It does produce hydrogen peroxide, which contributes to the damage to the respiratory tract cells.
M. pneumoniae has only one serotype and is antigenically distinct from other species of Mycoplasma. Immunity is incomplete, and second episodes of disease can occur. During M. pneumoniae infection, autoantibodies are produced against red cells (cold agglutinins) and brain, lung, and liver cells. These antibodies may be involved in some of the extrapulmonary manifestations of infection.
M. pneumoniae infections occur worldwide, with an increased incidence in the winter. This organism is the most common cause of pneumonia in young adults and is responsible for outbreaks in groups with close contacts such as families, military personnel, and college students. It is estimated that only 10% of infected individuals actually get pneumonia. Mycoplasma pneumonia accounts for about 5% to 10% of all community-acquired pneumonia.