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Mycoplasma are bacteria that make up one genus
of a special class called Mollicutes. The class contains both pathogenic
and commensal species found in plants and animals. They are the
smallest known microorganisms (0.2 μm) able to
replicate in culture media. For instance, the Mycoplasmapneumoniae genome is
816,394 base pairs, approximately one sixth that of the common bacterium Escherichia
coli. Precursor substances (eg, peptides, native proteins,
cholesterol) are required for growth in vitro.
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Unlike other bacteria, Mycoplasma lack rigid cell
walls (Mollicute means soft skin); this feature precludes staining
with organic dyes (such as those used in the Gram stain procedure)
and renders the organisms insensitive to antibiotics that inhibit
wall synthesis (such as the penicillins and polymyxins). There are
16 Mycoplasma species found in humans. Disease
has been linked to four: M pneumoniae, M
hominis, M genitalium, and M fermentans. Ureaplasma constitute
another genus in the Mollicute class.
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The best-characterized human Mycoplasma disease
is respiratory tract infection due to M pneumoniae, a
prominent cause of the atypical pneumonia syndrome. M hominis,
another member of the genus, is associated with a variety of genitourinary
and perinatal conditions, including postpartum maternal sepsis,
neonatal skin infections and meningitis, and “sterile” pyelonephritis.
Problems associated with M genitalium include pelvic
inflammatory disease, salpingitis, and nongonococcal urethritis
in sexually active individuals.
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Infections with M pneumoniae occur throughout life,
but the respiratory disease syndrome, often called atypical pneumonia,
is most common in school children, adolescents, and young adults. Epidemiologic
studies have been conducted in various populations since the 1960s,
but many of those studies relied on serologic determinations, the
accuracy of which has been questioned as more advanced diagnostic techniques
have been developed. It is estimated that M pneumoniae is
responsible for about one fifth of all instances of pneumonia.1-3 Much more
frequent than atypical pneumonia are several nonpneumonic respiratory
syndromes; taken together, these conditions contribute about 6 episodes
per 100 population annually to the burden of human respiratory infections. The
prevalence of M pneumoniae disease varies greatly
from year to year, with larger clusters of infections occurring
in irregular 3- to 4-year cycles.4 Spread of infection
is facilitated by prolonged close contact, such as within household units,
childcare centers, college dormitories, and military barracks, and
by the cough that is associated with respiratory infection. The
incubation period can be as long as 3 to 4 weeks2,5 or
as short as 4 days,6 and asymptomatic shedding can
occur.1 Antimicrobial therapy may relieve symptoms
without eradicating the organism. Natural protective immunity is
apparently limited, because repeat episodes of infection and pneumonia
have been documented.
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M pneumoniae is transmitted from person to person
by large droplet particles. The organism is shed from the nose,
throat, and infected sputum of infected individuals. Upon reaching
the respiratory tract epithelium, it ...