Pneumonia, Bacterial

DEFINITION
The fully developed inflammatory response to virulent bacteria in lung parenchyma characterized by exudation of cells and fluid into conduction airways and alveolar spaces

PATHOPHYSIOLOGY
Bacteria—enter the lower respiratory tract primarily by the inhalation or aspiration routes; enter less commonly by the hematogenous route; infections incite an overt inflammatory reaction.
Tracheobronchial tree and lungs—normally not continuously sterile
Oropharyngeal bacteria—frequently aspirated; may be present for an unknown interval in the normal tracheobronchial tree and lung; have the potential to cause or complicate respiratory infection; cloud interpretation of airway and lung cultures
Respiratory infection—development depends on the complex interplay of many factors: size, inoculation site, number of organisms and their virulence, and resistance of the host
Viral infections—alter bacterial colonization patterns; increase bacterial adherence to respiratory epithelium; reduce mucociliary clearance and phagocytosis; thus may allow resident bacteria to invade the lower respiratory tract
Exudative phase—inflammatory hyperemia; serous exudation of high protein fluid into interstitial and alveolar spaces
Leukocytic emigration phase—leukocytes infiltrate the airways and alveoli; consolidation, ischemia, tissue necrosis, and atelectasis owing to bronchial occlusion, obstructive bronchiolitis, and impaired collateral ventilation
Mortality—associated with severe hypoxemia (low arterial oxygen concentration) and sepsis