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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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Epidemiology

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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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Pathophysiology

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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 attaches to ...

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