Moraxella Catarrhalis

Moraxella catarrhalis is being increasingly recognized as a cause of bacterial pneumonia, especially in the elderly. It is the second most common bacterial cause of acute exacerbation of

COPD. Along with S. pneumoniae and H. influenzae, M. catarrhalis constitutes one of the three most common causes of otitis media in children.

Staphylococcus Aureus

Staphylcocus aureus is an important cause of secondary bacterial pneumonia in children and healthy adults following 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. Intravenous drug abusers are at high risk of

developing staphylococcal pneumonia in association with endocarditis. It is also an important cause of nosocomial pneumonia, as will be discussed later.

Klebsiella Pneumoniae

Klebsiella pneumoniae is the most frequent cause of Gram-negative bacterial pneumonia. It commonly afflicts debilitated and malnourished people, 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.

Pseudomonas Aeruginosa

Although Pseudomonas aeruginosa most commonly causes nosocomial infections, it is mentioned here because of its

occurrence in cystic fibrosis patients. It is common in patients who are neutropenic and it has a propensity to invade blood vessels with consequent extrapulmonary spread. Pseudomonas

septicemia is a very fulminant disease.

Legionella Pneumophila

Legionella pneumophila is the agent of Legionnaires 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. This organism 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. It can be quite severe, frequently requiring hospitalization, and immunosuppressed patients may have fatality rates of up to 50%. 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 gold standard of diagnosis.

Morphology.

Bacterial pneumonia has two gross patterns of anatomic distribution: lobular bronchopneumonia and lobar pneumonia ( Fig. 15-32 ). Patchy consolidation of the lung is the dominant

characteristic of bronchopneumonia( Fig. 15-33 ). Lobar pneumoniais an acute bacterial infection resulting in fibrinosuppurative consolidation of a large portion of a lobe or of an

entire lobe ( Fig. 15-34 ). These anatomic but still classic categorizations are often difficult to apply in the individual case because patterns overlap. The patchy involvement may become

confluent, producing virtually total lobar consolidation; in contrast, effective antibiotic therapy for any form of pneumonia may limit involvement to a subtotal consolidation. Moreover, the

same organisms may produce bronchopneumonia

Figure 15-32Comparison of bronchopneumonia and lobar pneumonia.

Figure 15-33Bronchopneumonia. Gross section of lung showing patches of consolidation (arrows).

Figure 15-34Lobar pneumonia—gray hepatization, gross photograph. The lower lobe is uniformly consolidated.

Figure 15-35 A, Acute pneumonia. The congested septal capillaries and extensive neutrophil exudation into alveoli corresponds to early red hepatization. Fibrin nets have not yet formed.

B, Early organization of intra-alveolar exudate, seen in areas to be streaming through the pores of Kohn (arrow). C, Advanced organizing pneumonia (corresponding to gray hepatization),

featuring transformation of exudates to fibromyxoid masses richly infiltrated by macrophages and fibroblasts.

Figure 15-36Pyemic lung abscess in the center of section with complete destruction of underlying parenchyma within the focus of involvement.

Figure 15-37Laminated Histoplasma granuloma of the lung.

Figure 15-38Histoplasma capsulatum yeast forms fill phagocytes in a lymph node of a patient with disseminated histoplasmosis.

Figure 15-39Blastomycosis. A, Rounded budding yeasts, larger than neutrophils, are present. Note the characteristic thick wall and nuclei (not seen in other fungi). B, Silver stain.

Figure 15-40Coccidioidomycosis with intact spherules within multinucleated giant cells.

TABLE 15-9-- Causes of Pulmonary Infiltrates in Immunocompromised Hosts

Date: 2016-04-22; view: 776