Stroke propagates bacterial aspiration to pneumonia in a model of cerebral ischemia

BACKGROUND AND PURPOSE - Bacterial pneumonia is the most common cause of death in patients sustaining acute stroke and is believed to result from an increased aspiration. Recently, stroke-induced immunodeficiency was described in a mouse model of cerebral ischemia, which is primarily caused by overactivation of sympathetic nervous system. We tested if stroke-induced immunodeficiency increases the risk of pneumonia after aspiration in a newly developed model of poststroke pneumonia. METHODS - Experimental stroke in mice was induced by occlusion of the middle cerebral artery (MCAO) for 60 minutes. Aspiration pneumonia was induced by intranasal application of 20 μL of a defined suspension of Streptococcus pneumoniae in phosphate-buffered saline 4 or 14 days after MCAO. Treatment comprised moxifloxacin (100 mg/kg body weight, six times every 2 hours after operation) or propranolol (30 mg/kg body weight, immediately before as well as 4 and 8 hours after MCAO). Readout was lung histology and bacterial counts in lung and blood. RESULTS - Nasal inoculation of only 200 colony-forming units of S pneumoniae caused severe pneumonia and bacteremia after experimental stroke, whereas 200 000 colony-forming units are needed to induce comparable disease in sham animals. Aspiration pneumonia in stroke animals outlasted acute stroke state but was preventable by β- adrenoreceptor blockade. CONCLUSIONS - Experimental stroke propagates bacterial aspiration from harmless intranasal colonization to harmful pneumonia. Prevention of infections by β-adrenoreceptor blockade suggests that immunodepression by sympathetic hyperactivity is essential for progression of bacterial aspiration to pneumonia.