# Gut bacteria presence in the brain is increased after ischemic stroke in mice

## TL;DR

This study shows that gut bacteria can move to the brain after a stroke in mice, which may worsen recovery.

## Contribution

The study demonstrates for the first time that gut bacteria translocate to the brain after stroke and affect motor function.

## Key findings

- Gut bacteria were found in the ischemic brain hemisphere of mice after stroke.
- Bacterial translocation was not observed in germ-free mice.
- Sympathetic nervous system activation promotes bacterial migration to the brain.

## Abstract

Systemic infections are a common cause of complications and death after stroke. These infections can occur due to the breakdown of the gut epithelial barrier and the translocation of bacteria from the gut to peripheral systemic tissues. However, it remains unclear whether gut bacteria also translocate to the brain and contribute to stroke-induced neuronal damage. In this study, we observed a significant number of peptidoglycan- and lipopolysaccharide-positive bacteria in the ischemic hemisphere of mice subjected to either photothrombotic (PT) stroke or middle cerebral artery occlusion (MCAO). In contrast, no bacteria were observed in the ischemic brains of germ-free mice following MCAO. Absolute quantification via PCR also revealed increased bacteria in the ischemic hemisphere and blood of PT mice. Bacterial translocation to the brain is associated with the breakdown of the gut-epithelial and blood–brain barriers. Although inhibition of sympathetic tone reduces gut–epithelial barrier permeability, the bacterial load in the brain and functional deficits poststroke, it does not affect cerebral apoptosis, neuroinflammation or infarct volume. Collectively, these findings indicate that activation of the sympathetic nervous system after stroke promotes the migration of gut-derived bacteria into the ischemic brain, and this process worsens motor function in mice.