Here is a pretty comprehensive meta-analysis of literature around microbiome and depression. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7510518/ https://www.

Here is a pretty comprehensive meta-analysis of literature around microbiome and depression. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7510518/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7510518/ "This literature review established a strong link of microbes in the gastrointestinal tract, affecting how individuals think and how the gut-brain axis serves as an essential pathway in considering the management of several mental issues and psychiatric illnesses. Although promising, gut microbiome studies have a long way to go." So yes, you are right, the causal relationship is not definite. Still, it's an interesting lens which may one day prove to be clearer.