Mycoplasma Infections
Julia A. McMillan
Mycoplasma are bacteria that make up one genus of a special class called Mollicutes. 
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. 
MYCOPLASMA PNEUMONIAE
EPIDEMIOLOGY
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. It is estimated that M pneumoniae is responsible for about one fifth of all instances of pneumonia.1-3
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.
PATHOPHYSIOLOGY
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 the host cell using a specialized terminal tip structure. Attachment is thought to be dependent upon the P1 adhesin, a 170-kilodalton protein that is concentrated in the attachment tip. Damage to host cells is related to hydrogen peroxide and superoxide radicals interacting with host cell toxins leading to deterioration of epithelial cilia, cellular deterioration, and ultimately, exfoliation of epithelial cells.8
Infection of the lower respiratory tract by M pneumoniae provokes activation and migration of macrophages and formation of lymphocyte-laden alveolar fluid. 
CLINICAL FEATURES
The most frequent recognizable clinical presentation of M pneumoniae infection is tracheobronchitis, consisting of nonspecific systemic symptoms such as malaise, headache, low-grade fever, persistent cough, and increased respiratory secretions. Coughing may occur in paroxysms, is often worse at night, and may persist for several weeks if untreated. Patients with pneumonia present similarly but show evidence of progression to pulmonary involvement by physical examination or chest x-ray after as many as 7 days of generalized symptoms. Generally, pulmonary disease is not severe, leading to the term walking pneumonia. Untreated infections usually run their course in 1 to 3 weeks, but persistent coughing and malaise may extend several weeks into convalescence.
Other syndromes that may be seen are upper respiratory infections, pharyngitis, and wheezing illness, particularly in children with a history of asthma. Because these clinical findings are nonspecific, M pneumoniae disease tends to be underdiagnosed.
The nonspecificity and usually mild symptoms of M pneumoniae infection are often coupled with a paucity of physical findings. The patient’s complaints often appear greater than is suggested by examination. The pharynx can be erythematous, with or without associated lymphadenopathy. Palpation of the trachea may stimulate a coughing paroxysm. If pneumonia is present, scattered crepitations or wheezes can be heard on inspiration, usually over one of the lower lobes. Rarely is there sufficient pulmonary consolidation or pleural effusion to be recognized by physical examination. It should be emphasized that symptoms and signs evolve gradually over several days to 1 week or more, and repeated examinations may be necessary to suggest the correct diagnosis. Other findings may include otitis media, evidence of sinusitis, and skin rashes (maculopapular, occasionally vesicular, rarely erythema multiforme). Children and adults with conditions known to be associated with impaired antibody production (sickle cell disease, Down syndrome, hypogammaglobulinemia syndromes) are known to be susceptible to severe respiratory disease due to M pneumoniae.13-16
The signs and symptoms of M pneumoniae respiratory disease are similar to those caused by multiple respiratory viruses, including rhinovirus, adenovirus, influenza, parainfluenza, respiratory syncytial virus, human metapneumovirus, and coronaviruses. Infection due to Chlamydia pneumoniae and Bordetella pertussis may also mimic disease due to M pneumoniae. Streptococcus pneumoniae infection typically is associated with a greater degree of pulmonary consolidation and an increase in the white blood cell count than is seen with M pneumoniae pneumonia. Concomitant infection with M pneumoniae and virtually any of the viral or bacterial pathogens listed has been noted in a variety of surveys.
Blood leukocyte levels and differential counts are usually within normal limits, but elevation of white blood cell (WBC) count can be seen in association with pneumonia. Erythrocyte sedimentation rate may be elevated during the course of the disease. Subclinical hemolysis is occasionally evident. Children with sickle cell disease often show marked polymorphonuclear leukocytosis, which complicates the interpretation of their laboratory findings.
Chest x-rays can document and detail the occurrence of pneumonia. Usually, infiltration appears in a bronchopneumonic pattern involving segments of one of the lower lobes, though bilateral involvement may occur. Frank consolidation (filling of alveolar spaces) is uncommon, although subsegmental areas of atelectasis are frequent. Pleural effusions are usually unilateral, small, and transient when they occur.
EXTRAPULMONARY DISEASE
M pneumoniae infection has been associated with a variety of extrapulmonary sites, but uncertainty regarding definitive diagnostic testing has made it difficult to establish a causal role in every case. In the past, some nonrespiratory findings were thought to be manifestations of cross-reacting antibody and against human tissue and resulting inflammation, and there is evidence that this mechanism plays a role; but in recent years, identification of M pneumoniae in blood, cerebrospinal fluid (CSF), and synovial fluid using nucleic acid techniques has suggested that infection may also be involved outside the respiratory tract. Nonrespiratory conditions associated with M pneumoniae include hemolytic anemia, thought to be due to cross-reacting anti-I antibody; rashes, including Stevens-Johnson syndrome; a variety of neurological conditions; arthritis, primarily in patients with immune deficiencies; nephritis; cardiac involvement; and gastrointestinal disease, including hepatic dysfunction and pancreatitis. (See Table 273-1.) 
DIAGNOSIS
The diagnosis of M pneumoniae is suggested by the clinical picture, usual patient age (5–40 years), and exclusion of other entities. Helpful adjuncts are knowledge of other individuals in families, institutions, or communities with compatible respiratory infection. About 75% of adult patients with pneumonia due to M pneumoniae develop IgM antibodies against the I antigen on human erythrocytes (cold agglutinin antibody) within 7 to 10 days of initial symptoms. The titer of cold agglutinin antibody, as determined by serial dilution of the patient’s serum, correlates roughly with the severity of disease. A titer of at least 1:64 in a patient with pneumonia is highly suggestive of M pneumoniae infection1,48 Although this test is not specific (other bacterial and viral causes of pneumonia are infrequently associated with a positive test for cold agglutinins), a strong positive reaction coupled with a compatible clinical picture can be useful in choosing initial therapy. Cultures, not available in most diagnostic laboratories, may require 2 to 6 weeks to complete. Serodiagnostic testing for the complement-fixing (CF) antibody against M pneumoniae depends upon a fourfold or greater titer change between an acute specimen and a second specimen collected 2 to 3 weeks later. Thus, both culture and CF antibody tests offer only retrospective diagnosis. Commercially available methods for detecting IgM against M pneumoniae have been developed and correlate well with CF antibody testing and with polymerase chain reaction (PCR) detection.49-51 IgM antibody develops within 7 days of the onset of illness, thus diagnosis can be made using 1 serum sample early in the course of infection. The PCR assay detects M pneumoniae even when only a small number of organisms is present in respiratory secretions and other body fluids. This assay has allowed a more complete understanding of the role of M pneumoniae in non-respiratory tract illness. However, because asymptomatic shedding of M pneumoniae is known to persist for weeks following initial infection, the significance of a positive PCR in the absence of an associated antibody response is not known. From a practical viewpoint, diagnosis most often is based on clinical and epidemiologic features and response to empirical therapy. Severe or unusual presentations may require attempted laboratory diagnosis using antibody testing and PCR.
Table 273-1. Extrapulmonary Disease Reported Due to Mycoplasma pneumoniae Infection
