The key bacteria leave in the lock of the lysosome
Inside every cell there are waste-recycling stations: lysosomes. To digest the trash, they need an acidic environment, and the acid is pumped in by a molecular pump called V-ATPase. With age this pump works more and more weakly, and lysosomes stop keeping up. But the bacteria living in the gut have their own building material, peptidoglycan, and as it turns out, it can serve as a key that restarts the pump.