Comment by ninetyninenine

They caused SAD in mice by introducing specific microbiota.

But does the system have hysteresis? Can the flip be switched the other way? Introduce antibiotics to wipe out all the microbiota and does the SAD improve?

There can be human trials for this given that antibiotics are approved low risk medicines. Clinical trials with antibiotics to clean out and reset the microbiome along with strict diets in attempt to prevent reintroduction of said microbiota.

It can even be DIY.