Page 208 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com mebooksfree.com C H A P T E R mebooksfree.com
23

Mycoplasmas

INTRODUCTIONmebooksfree.com
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mebooksfree.com mebooksfree.com Summaries of Organisms mebooksfree.com mebooksfree.com
CHAPTER C ONTENT S

Introduction
Mycoplasma pneumoniae
Practice Questions: USMLE & Course Examinations
Self-Assessment Questions

frequently has a characteristic “fried-egg” shape, with a
raised center and a thinner outer edge.
Mycoplasmas are a group of very small, wall-less organisms,
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mebooksfree.com mebooksfree.com mebooksfree.com Mycoplasma pneumoniae, a pathogen only for humans, is mebooksfree.com
of which Mycoplasma pneumoniae is the major pathogen.
Pathogenesis & Epidemiology
Additional information regarding the clinical aspects of
infections caused by the organisms in this chapter is pro-
transmitted by respiratory droplets. In the lungs, the
vided in Part IX entitled Infectious Diseases beginning on
organism is rod-shaped, with a tapered tip that contains
page 593.
specific proteins that serve as the point of attachment to the
respiratory epithelium. The respiratory mucosa is not
MYCOPLASMA PNEUMONIAE
invaded, but ciliary motion is inhibited and necrosis of the
epithelium occurs. The mechanism by which M. pneu-
Disease
hydrogen peroxide, which contributes to the damage to the
Mycoplasma pneumoniae causes “atypical” pneumonia.
respiratory tract cells.
Mycoplasma pneumoniae has only one serotype and is
Important Properties moniae causes inflammation is uncertain. It does produce
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antigenically distinct from other species of Mycoplasma.
Mycoplasmas are the smallest free-living organisms;
Immunity is incomplete, and second episodes of disease can
many are as small as 0.3 μm in diameter. Their most strik-
occur. During M. pneumoniae infection, autoantibodies are
1
ing feature is the absence of a cell wall.
Consequently, mycoplasmas stain poorly with Gram
lung, and liver cells. These antibodies may be involved in
stain, and antibiotics that inhibit cell wall (peptidoglycan)
some of the extrapulmonary manifestations of infection.
synthesis (e.g., penicillins and cephalosporins) are ineffec-
Mycoplasma pneumoniae infections occur worldwide,
tive. Their outer surface is a flexible cell membrane; hence
with an increased incidence in the winter. This organism is
these organisms can assume a variety of shapes. It is the only
the most common cause of pneumonia in young adults
bacterial membrane that contains cholesterol, a sterol usu-
and is responsible for outbreaks in groups with close contacts
ally found in eukaryotic cell membranes.
such as families, military personnel, and college students. It
Mycoplasmas can be grown in the laboratory on artifi-
is estimated that only 10% of infected individuals actually get
cial media, but they have complex nutritional require-
pneumonia. Mycoplasma pneumonia accounts for about 5%
ments, including several lipids. They grow slowly and
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require at least 1 week to form a visible colony. The colony
Clinical Findings
Mycoplasma pneumonia is the most common type of atypi-
1
Other types of bacteria, in the presence of penicillin, can exist in a wall-
cal pneumonia. It was formerly called primary atypical
less state called an “L form” but can resynthesize their cell walls when
pneumonia. (Atypical pneumonia is also caused by
penicillin is removed.
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