Atypical bacterial pneumonia is caused by atypical organisms that are not detectable on Gram stain and cannot be cultured using standard methods. The most common organisms are Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila. Atypical bacterial pneumonia generally is characterized by a symptom complex that includes headache, low-grade fever, cough, and malaise. Constitutional symptoms often predominate over respiratory findings. Although in most cases presentation can be in the milder spectrum of community-acquired pneumonia, some cases, especially if caused by L. pneumophila, may present as severe pneumonia, necessitating intensive care unit admission.
Other possible pathogens (although very rare) include other Chlamydophila species, other Legionella species, Coxiella burnetiid (Q fever), and respiratory viruses.
Knowledge of the diseases and clinical suspicion is important, but not sufficient for diagnosis. Only specific microbiological investigations allow a reliable diagnosis, which is useful for obligatory reporting and of great value for epidemiological purposes. A reliable microbiological diagnosis permits the tailoring of the type and duration of the treatment, even in cases where an adequate empiric, guideline-orientated treatment had been prescribed.
Chlamydophila (former Chlamydia) pneumoniae is an obligate intracellular parasite. According to the statistics, C. pneumoniae is the causative agent of about 5-15% of community-acquired pneumonia cases. The prevalence of the disease caused by this infectious agent tends to increase. Laboratory diagnostics using microbiological techniques is complicated. The comparison between the sensitivity of the microscopical method and PCR indicates that the frequency of pathogen detectability by microscopic examination is only 10-12% and by PCR at least 98% respectively. With meeting the requirements for culture technique, the sensitivity for chlamydia diagnostics is 60-80%, while PCR shows the sensitivity of at least 95-98%.
Mycoplasma pneumoniae is a single-celled Gram-negative microorganism without a cell wall. They are regarded as hardly cultivated microorganisms and considered to be superficial microorganisms of mucosa cells. Clinical, radiological, and laboratory data regarding M. pneumoniae-caused infections are inadequate to make an accurate diagnosis based on them. Therefore, laboratory diagnostics should include direct methods (first and foremost PCR) for detecting the microorganism in the biological material.
Legionella pneumophila is a Gram-negative bacterium, that proliferates in air conditioning systems, showerheads, humidifiers, and inhalers and causes legionellosis or Legionnaires’ disease. Diagnosis is based on the results of bacteriological inoculation and serological tests (indirect immunofluorescence). PCR-based diagnostics are used to confirm the diagnosis.
Prevalence data for patients admitted to the ICUs with community-acquired pneumonia: 17.8% of infections are due to Legionella pneumophila, and 2.7% and 2.2% to Mycoplasma and Chlamydophila, respectively. Legionellosis must therefore be considered in all patients admitted to the ICU with severe CAP. Mycoplasma pneumoniae and Chlamydophila pneumoniae causes infrequently severe illness in immunocompetent adults, but are more often diagnosed in children, the elderly, or immunocompromised hosts.