The gut microbiome consists mostly of bacteria and that is largely the portion of the microbiome we are focusing on (fungi and viruses exist but their function is largely unknown)
Communication pathways exist between the microbiota-gut-and brain.
Multiple mechanisms exist that allow gut microbiota to signal to the brain and control physiological processes.
These include release of gut peptides from enteroendocrine cells which activate receptors of the immune system and vagus terminals in the gut.
Studies indicate that these bacteria can manufacture and secrete essential neurochemicals including serotonin, dopamine, NE, GABA, and acetylcholine
Depression and anxiety have been linked to a less well diversified gut microbiome.
What can help diversify the gut microbiome? Diet, processed food, sugar, saturated fats, and red meat. Medication can also alter the gut microbiome, a good example is oral antibiotics used to treat an acute infection, sleep, exercise. Sounds a lot like a healthy lifestyle will get you the microbiome you need for optimal mental health.
However, if you want a treatment there have been several studies that looked at fecal transplant to treat psychiatric disorders. Fecal transplants are much easier these days and now there is a capsule version that you take orally. There is not enough data to recommend this as a practical treatment and if the patient goes back to eating a poor diet, sleeping poorly, not exercising then the gut microbiome will revert after the transplant.
What are the practical things you can do? Stop eating processed food, sugar, and red meat. Increase your fiber intake and select a diet like the Mediterranean diet or a plant based whole food diet that will provide those prebiotics. You could supplement with a probiotic but most of what you need can be had from a good diet alone and I think it’s far better to change the diet then to try using supplements to treat a poor diet. Fermented products like kimchi, kombucha and sauerkraut are good sources of live bacteria.
If you choose to take a probiotic make sure it’s a quality, 3rd party tested product.
Increase aerobic activity, I think if you goal is overall general health and you have limitted time, I think aerobic activity is a better bang for your buck.
The way I believe you get and keep a healthy gut microbiome is through lifestyle modification. Improving your diet, exercise, and sleep is a good place to start. If you want to supplement with food products like kimchi or kombucha, go for it. I do not believe there is enough evidence to support a probiotic supplement for psychiatric disorders at this point, but if you want to spend $30 or more per month on a product if it’s a quality one that’s fine. Remember you cannot supplement away a bad diet.
