M. catarrhalis is being increasingly recognized as a cause of bacterial pneumonia, especially in older adults. It is the second most common bacterial cause of acute exacerbation of COPD in adults. Along with S. pneumoniae and H. influenzae, M. catarrhalis is one of the three most frequent causes of otitis media (infection of the middle ear) in children.
S. aureus is an important cause of secondary bacterial pneumonia in children and healthy adults after viral respiratory illnesses (e.g., measles in children and influenza in both children and adults). Staphylococcal pneumonia is associated with a high incidence of complications, such as lung abscess and empyema. Staphylococcal pneumonia occurring in association with right-sided staphylococcal endocarditis is a serious complication of intravenous drug abuse. It is also an important cause of nosocomial pneumonia (discussed later).
K. pneumoniae is the most frequent cause of gram-negative bacterial pneumonia. Klebsiella-related pneumonia frequently afflicts debilitated and malnourished individuals, particularly chronic alcoholics. Thick and gelatinous sputum is characteristic, because the organism produces an abundant viscid capsular polysaccharide, which the patient may have difficulty coughing up.
Although discussed here with community-acquired pathogens because of its association with infections in cystic fibrosis, P. aeruginosa infection is most commonly is seen in nosocomial settings (discussed later). Pseudomonas pneumonia also is common in individuals who are neutropenic, usually secondary to chemotherapy; in victims of extensive burns; and in patients requiring mechanical ventilation. P. aeruginosa has a propensity to invade blood vessels at the site of infection, with consequent extrapulmonary spread. Pseudomonas bacteremia is a fulminant disease, with death often occurring within a matter of days. Histologic examination reveals organisms invading the walls of necrotic blood vessels (Pseudomonas vasculitis), leading to secondary coagulative necrosis of the pulmonary parenchyma.
L. pneumophila is the agent of Legionnaire disease, an eponym for the epidemic and sporadic forms of pneumonia caused by this organism. Pontiac fever is a related self-limited upper-respiratory tract infection caused by L. pneumophila, without pneumonic symptoms. L. pneumophila flourishes in artificial aquatic environments, such as water-cooling towers and within the tubing system of domestic (potable) water supplies. The mode of transmission is thought to be either inhalation of aerosolized organisms or aspiration of contaminated drinking water. Legionella pneumonia is common in individuals with some predisposing condition such as cardiac, renal, immunologic, or hematologic disease. Organ transplant recipients are particularly susceptible. Legionella pneumonia may be quite severe, frequently requiring hospitalization and producing a fatality rate of 30% to 50% in immunosuppressed individuals. Rapid diagnosis is facilitated by demonstration of Legionella antigens in the urine or by a positive fluorescent antibody test on sputum samples; culture remains the standard diagnostic modality. PCR-based tests can be used on bronchial secretions in atypical cases.
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Mycoplasma infections are particularly common among children and young adults. They occur sporadically or as local epidemics in closed communities (schools, military camps, prisons). Tests for Mycoplasma antigens and polymerase chain reaction (PCR) testing for Mycoplasma DNA are available.
MORPHOLOGYBacterialpneumoniahastwopatternsofanatomicdistribution:lobularbronchopneumoniaandlobarpneumonia(