Microbiology of pneumonia in the patient at risk.

Microorganisms causing pulmonary infections in high risk patients vary considerably with the predisposing illness (immunosuppression, alcoholism, or diabetes), the setting (nosocomial or community-acquired), and previous therapy (antibiotics, surgery, and inhalation therapy). Even in the immunocompromised patient, conventional bacteria are the most prevalent opportunistic pathogens, and gram-positive cocci such as staphylococci and gram-negative bacilli such as Escherichia coli cause most pneumonias. Fungi, viruses, and protozoa also cause pulmonary infections, but they vary in frequency from one institution to another. Diagnostic proof of the etiology of pulmonary infection is often difficult to obtain. The microbial flora of sputum is not definitive and must be confirmed by blood or pleural fluid culture, antigen or serologic response in body fluids, or morphologic presence in lung tissue. Resistance to antimicrobial therapy is increasing, especially among nosocomially acquired gram-negative bacilli and methicillin-resistant staphylococci. A potential for increased resistance exists in pneumococcal, viral, and fungal infection but is not yet apparent in pulmonary infections due to protozoal pathogens. Tests to predict antibiotic response such as serum bactericidal assay, repeated cultures, and serologic studies are helpful but correlate imperfectly with clinical outcome.