This research shows that different strains of bacteria called Desulfovibrio have very different effects on brain health. When scientists exposed worms to six different strains, they found that environmental strains actually protected against Parkinson’s-like symptoms, while strains from human patients and animals caused more damage. The worms strongly preferred to eat the environmental strains, suggesting they could sense which bacteria were harmful. This highlights how the type of bacteria in our gut matters just as much as the total amount.