Aspiration pneumonia is bronchopneumonia that develops due to the entrance of foreign materials that enter the bronchial tree, usually oral or gastric contents (including food, saliva, or nasal secretions). Depending on the acidity of the aspirate, a chemical pneumonitis can develop, and bacterial pathogens (particularly anaerobic bacteria) may add to the inflammation.
Aspiration pneumonia is often caused by an incompetent swallowing mechanism, such as occurs in some forms of neurological disease (a common cause being strokes) or while a person is intoxicated. An iatrogenic cause is during general anaesthesia for an operation and patients are therefore instructed to be nil per os (NPO) (aka Nothing By Mouth) for at least four hours before surgery.
Whether aspiration pneumonia represents a true bacterial infection or a chemical inflammatory process remains the subject of significant controversy. Both causes may present with similar symptoms.
When bacteria are implicated, they are usually of the anaerobic oral flora: