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Common bacterial agents of acute pneumonia include: Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, Pseudomonas aeruginosa, and members of the Enterobacteriaceae (Escherichia coli, Klebsiella species, and Enterobacter species) Clinical history, physical examination, and chest X-ray are usually adequate for the diagnosis of pneumonia, and antimicrobial treatment is typically based on these findings.
Culture of expectorated sputum is used by some for the evaluation of pneumonia, although controversy exists regarding this practice; both sensitivity and specificity of sputum cultures are generally regarded as poor (<50%). Specificity is improved by collecting expectorated purulent matter from the lower respiratory tract and avoiding mouth and oropharyngeal matter, thereby reducing contamination. Prior to culture, the specimen should be examined for the presence of WBCs (evidence of purulent matter) and a paucity of squamous cells (evidence of minimal contamination by oral matter).
Blood cultures should be performed to establish the definitive etiology of an associated pneumonia. However, only 20% to 30% of patients with bacterial pneumonia are bacteremic.
An aid in the diagnosis of lower respiratory bacterial infections including pneumonia
Organisms associated with lower respiratory tract infections are reported.
When culture of sputum is delayed, successful isolation of bacterial pathogens is less likely, due to the overgrowth of usual oropharyngeal flora.
Interpretation of lower respiratory cultures is often complicated by the presence of usual flora and the possibility of colonization, rather than infection. Results should always be evaluated in light of clinical information.
No growth or usual flora
Identification of probable pathogens
Chapter 2: Introduction to Microbiology, Part II: Guidelines for the Collection, Transport, Processing, Analysis and Reporting of cultures from Specific Specimen sources. In Color Atlas and Textbook of Diagnostic Microbiology. Sixth edition. Edited by E Koneman. Lippincott, Philadelphia, PA, 2006, pp 67-79