Eating more prebiotics in the form of unprocessed plants is much more effective at improving gut health than probiotic supplements or even probiotic foods. (Of course you can do both.)
The experiment looked to be the other way around. If you introduced new microbiota from patients with SAD this caused SAD in mice.
So introducing new microbiota won’t solve the problem. The theory that needs to be tested is that we know microbiota can switch SAD on, but does removal of said microbiota turn it off? It needs to be an antibiotic cleansing. It can even be DIY. People can try this on themselves. I think the unknown factor here though is whether or not a persons diet is introducing the SAD causing microbiota.
So if the choice is between unsanitary water or sterile and containing chlorine, etc., which has the effect of strengthening the microbiota so that the resulting population increases perceived wellness in the host organism’s sociopolitical fitness, its simple enough, we all stop eating and drinking for ultimate safety and maximum paperclips.
You have a back door as well for the gut that tends to open at least once a day. We know fecal matter is in the air in most bathrooms. It wouldn't surprise me if you got an infusion of community fecal biome right into your anus every time you used a restroom that isn't your own.
Its not like there is positive air flow keeping any microbes in the air out and away. If something lands on there it can probably colonize then spread to the inside slipping through the sphincter.
