spread of RSV in pediatric patients. Health care-associated pneumonia due to methicillin-resistant S. aureus and multidrug-resistant Gram-negative bacilli, such as P. aeruginosa and Acinetobacter baumannii, is a concern for intubated patients. Because of their ability to survive within hospital water and air conditioning systems, the potential for outbreaks of pneumonia due to Legionella spp. is a constant threat.
Patients with chronic obstructive pulmonary disease brought on by frequent smoking develop bronchitis. S. pneumoniae, M. catarrhalis, H. influenzae, and P. aeruginosa are common causes of this type of infection. Patients with cystic fibrosis have chronic airway infections that are primarily responsible for their premature death. S. aureus and mucoid strains of P. aeruginosa are the most important agents of such chronic airway disease. Both of these patient populations have an increased risk of developing allergic bronchopulmonary aspergillosis. Patients with cavitary lung disease, frequently due to prior M. tuberculosis infection, are at increased risk for another type of infection, an aspergilloma or fungus ball caused by Aspergillus spp. This fungus grows in the form of a ball in the preformed cavity. A distinction between actual tissue invasion with this fungus and noninvasive disease is clinically difficult but is important.
The diagnosis of the etiology of lung infection in immunocompromised patients is one of the most daunting in clinical microbiology and infectious disease. It has been greatly facilitated by the use of the flexible bronchoscope, which provides a relatively noninvasive means to sample the airways and alveoli. Immunocompromised patients are typically at risk for essentially all recognized respiratory tract pathogens. However, a distinction must be made between different types of immunosuppression—defects in cell-mediated immunity, humoral immunity, and neutrophil number or function—because different types of immunosuppression predispose patients to infection with different pathogens. The most common comorbidity for lower respiratory tract infections is cigarette smoking, which causes impaired removal of pathogens due to defective mucociliary clearance. Although smoking results in a significantly increased rate of both bronchitis and pneumonia, smokers are not normally described as immunosuppressed.
In AIDS patients, Pneumocystis jirovecii, Cryptococcus neoformans, S. pneumoniae, and multidrug-resistant M. tuberculosis are all seen more frequently than in other patient populations. Solid-organ transplant recipients have a greatly increased risk for pneumonia with cytomegalovirus, herpes simplex virus, Legionella spp., P. jirovecii, and Nocardia spp. Prophylactic antibiotics are frequently taken by these patients to prevent pulmonary infections with P. jirovecii. Prophylactic therapies are not as widely used for other agents for a variety of reasons, including expense, questionable efficacy of the prophylactic measures, or the rarity with which the organism is encountered. Profoundly neutropenic patients, especially those in whom the duration of neutropenia is prolonged, not only have a risk of infection with routine bacteria but have a very high risk of invasive aspergillosis and other invasive fungal infections.
TABLE II SELECTED RESPIRATORY TRACT PATHOGENS
| ORGANISM | GENERAL CHARACTERISTICS | PATIENT POPULATION | DISEASE MANIFESTATION |
| Bacteria | |||
| Acinetobacter baumannii | Glucose-nonfermenting, Gram-negative bacillus | Hospitalized adults | Ventilator-associated pneumonia |
| Actinomyces spp. | Branching, Gram-positive bacilli, usually anaerobic | Adults with aspiration | Lung abscess |
| Bacillus anthracis | Spore-forming, Gram-positive bacillus | Victims of bioterrorism due to exposure to spores; woolsorters in endemic areas | Inhalation anthrax with widened mediastinum, high-grade bacteremia |
| Bordetella pertussis | Fastidious, Gram-negative bacillus | Children, adults | Whooping cough, chronic cough |
| Chlamydia trachomatis | Obligate intracellular bacterium; does not Gram stain | Neonatal | Conjunctivitis, pneumonia |
| Chlamydiophila pneumoniae | Obligate intracellular bacterium; does not Gram stain | Children, adults | Pneumonia, bronchitis |
| Chlamydiophila psittaci | Obligate intracellular bacterium; does not Gram stain | Children and adults with exposure to birds | Pneumonia, ornithosis (psittacosis) |
| Corynebacterium diphtheriae | Catalase-positive, Gram-positive, club-shaped bacillus | Unvaccinated adults and children, improperly vaccinated adults | Diphtheria |
| Enterobacter spp., Escherichia coli | Lactose-fermenting, Gram-negative bacilli | Hospitalized adults | Health care-associated pneumonia |
| Fusobacterium necrophorum | Anaerobic, Gram-negative bacillus | Adolescents, adults | Pharyngitis, Lemierre’s syndrome |
| Group A streptococci (Streptococcus pyogenes) | Catalase-negative, Gram-positive cocci in chains | Children >2 years, adults | Pharyngitis, pneumonia with empyema |
| Group B streptococci (Streptococcus agalactiae) | Catalase-negative, Gram-positive cocci in chains | Neonates | Pneumonia |
| Haemophilus influenzae | Pleomorphic, Gram-negative bacillus | Children; adults, especially with COPDa | Otitis media, conjunctivitis, epiglottitis, bronchitis, pneumonia |
| Klebsiella pneumoniae | Lactose-fermenting, Gram-negative bacillus | Adults | Community-acquired and health care-associated pneumonia |
| Legionella pneumophila | Poorly staining, fastidious, Gram-negative bacillus | Adults, especially immunocompromised | Pneumonia |
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