Causative organism Estimated frequency
Gram-Positive Bacteria
Staphylococcus species (S. aureus, S. epidermidis)
Streptococcus species (S. pyogenes, S. pneumoniae)
35-40%
Gram-Negative Bacteria
Campylobacter jejuni
Klebsiella species
Haemophilus influenzae
Salmonella species
Escherichia coli (facultative anaerobe)
Pseudomonas aeruginosa
Acinetobacter and many others
25%
Anaerobes (Gram-positive and -negative)
Bacteroides species
Actinomyces species
Clostridium septicum among others
9-12%
(Often polymicrobial culture)
Mycobacteria
Mycobacterium tuberculosis
Atypical mycobacteria (e.g. Mycobacterium avium intracellulare)
9%
Fungal
Pneumocystis jiroveci
Aspergillus species
Candida species
Blastomyces dermatitidis
Coccidioides immitis
Approx 15%
Parasite
Echinococcus species, and others
Less than 1%
Polymicrobial culture 25- 30%
Table 1: Summary of causative organisms and estimated frequency.