Interesting. But the more stuff like this I read, the more it seems unlikely to me that any one kind of solution will be effective in treating mood disorders.
The 'reason' why some people have anxiety is probably a complex interaction between genetics, epigenetics, diet, microbiome, environmental toxins, individual experiences, cultural expectations, and cognitive appraisals of self. Mental disorders, even more than physical disorders, require interdisciplinary approaches and solutions.
This is why some people do experience mood benefits from exercise or changing their diet; for other people, it won't be enough. Some people benefit from medication, others don't. CBT, which addresses cognitive appraisal, can be effective for some people, but isn't for many. So maybe changing a person's microbiome will help, maybe it won't. Mouse models lack the complexity of people 'living in the wild'.
My own anxiety, for example, has much more to do with cultural expectations that lead to discrimination and violence against persons like myself. Reframing how I view discrimination and violence (a CBT/ACT approach) does not eradicate discrimination and violence; but then, neither would changing my microbiome. One way to reduce anxiety in transgender people (who have very high rates of social anxiety) is to change cultural expectations and legal protections.
I think there's a real danger in looking for a single cause of any phenomenon. The focus on pharmacological solutions to mood disorders has a tendency to erode efforts to find solutions in other areas, for example; psychiatrists hand out pills and don't bother to explore a person's living situation, which may be toxic. In cases like that, a social worker might do more to alleviate the person's anxiety than a psychiatrist.
A lot of biologically minded people dismiss cognitive appraisal as a factor in their anxiety, even when they're obviously drawing incorrect conclusions from their own experiences. No doubt genetics is an important factor in their condition, but that doesn't mean that it's the only factor and that they can't ameliorate their condition by changing other contributory factors. You see the same tendency in people who push dietary solutions. There's a danger some people will treat the microbiome the same way.
Do you have a link to your source?
For forty-seven years I've put up with it now. I must stop Christmas from coming ... but how?