Acute: S. pneumoniae (GPDC) (20–43%), H. influenzae (GNCB) (21–35%), M. catarrhalis (GNDC) (2–10%), other streptococci (3–9%) (GPC), anaerobes (1–9%), viruses (4%) (adenovirus, influenza, parainfluenza), S. aureus (GPC) (1–8%).

Chronic (child): Viridans and anaerobic streptococci (GPC in chains) (23%), S. aureus (19%), S. pneumoniae, H. influenzae, M. catarrhalis, P. aeruginosa (GNR) in cystic fibrosis.

Chronic (adult): Coagulase-negative staphylococci (GPC) (36%), S. aureus (GPC) (25%), viridans streptococci (GPC in chains) (8%), corynebacteria (GPR) (5%), anaerobes (6%), including Bacteroides sp., Prevotella sp. (GNR), peptostreptococcus (GPC), Fusobacterium sp. (GNR).

Hospitalized with nasogastric tube or nasotracheal intubation: Enterobacteriaceae (GNR), Pseudomonas sp. (GNR).

Fungal: Zygomycetes (rhizopus), aspergillus, P. boydii , other dematiaceous mold.

Immunocompromised: P. aeruginosa (GNR), CMV, Aspergillus sp. and other filamentous fungi plus microsporidia, Cryptosporidium parvum, Acanthamoeba in HIV-infected patients.

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