* Anxiety and depression have many causes. If poor diet or internal ecology are wrecking your mood, adding probiotics could make a big improvement. But if you feel bad because people mistreat you, or the environment is cooking off, or other external causes then dietary changes are unlikely to help much. All this stuff gets lumped under the same name, but it's not really the same thing. It's like if they said all fevers were the same. On the bright side, it supports the general sense that if you feel crappy, checking your diet is an excellent first step.
Conversely, antibiotics -- which kill good as well as bad bacteria -- raise the risk of anxiety and depression, especially with more courses. This is a case where probiotics may help by replacing the lost microfauna. Regrettably there's no other option than doing it yourself, as the medical industry has not gotten far enough at figuring out how to fix a microbiome they broke while trying to kill a disease. But just consuming probiotics during and after a course of antibiotics can buffer some of the other fallout, like yeast infections. When you see complimentary studies like this (killing bacteria increases anxiety and depression, adding good bacteria lowers them) then the probability of a causal relationship is higher. It might not be the whole story, but it is very likely a valid piece of it. So watch for reports that match up like that on any topic.
I am also starting to see a confluence of evidence that eating certain types of live things, or a much wider range of things that were very recently alive, is good for humans. In some cases, the vital energy is stronger. In others, as with probiotics, the food actually contains symbiotes that become part of us and make us healthier. This may be part of why ultraprocessed foods are so bad: they're very very dead.