Which microorganisms cause aspiration pneumonia?

Updated: Aug 15, 2018
  • Author: Justina Gamache, MD; Chief Editor: Guy W Soo Hoo, MD, MPH  more...
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Initial bacteriologic studies into the causative organisms revealed the anaerobic species to be the predominant pathogens in community-acquired aspiration pneumonia. However, subsequent studies revealed that Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and Enterobacteriaceae are the most common organisms. [1] In fact, in two studies of patients with acute, witnessed aspiration with protective specimen brush sampling and anaerobic culturing, no anaerobes were isolated. [6, 7] Moreover, hospital-acquired aspiration pneumonia is often caused by gram-negative organisms including Pseudomonas aeruginosa, particularly in intubated patients. [8]

While previous studies have suggested multidrug-resistant organisms, such as methicillin-resistant S aureus (MRSA), were more commonly found in those with healthcare-associated pneumonia (HCAP) versus community-acquired aspiration pneumonia (CAP), a 2014 meta-analysis of 24 studies found that patients previously classified as having HCAP (including nursing home–associated pneumonia) are not more likely to have multidrug-resistant infections than patients classified as having CAP. [9, 10] These studies demonstrated a limited role of anaerobic pathogens in both the community and nosocomial variants of the disease.

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