It's become an accepted fact that the millions of bacteria that live inside and all over us have a big impact on our health, with connections to gastrointestinal conditions and relationships to digestion, obesity, and skin conditions being in a way, intuitive: All those bacteria are right there, so it's perhaps not that surprising that they affect bodily functions in their same region.

What's been harder to understand, historically, has been the apparent connection between our resident bacterial communities, especially those in the gut, and the human brain — notoriously protected from bacteria by the blood-brain barrier.

Yet there's more and more evidence that somehow, the bacterial colonies that live in our guts can affect and even somehow control how we feel.

Mark Lyte has been studying this relationship for approximately 30 years.

When he first started this work, "it was dismissed as a curiosity," according to a recent profile of research into the gut-brain connection in The New York Times, but over time, as Lyte and others continued to publish their research, they uncovered more and more compelling evidence that bacteria could not only affect but could perhaps even be a causal factor in mental disorders — something that implies they could also be eventually used to treat these same conditions.

Researchers have shown that the presence of certain bacteria can identify people that are more prone to depression and anxiety disorders. Other bacterial communities have even been connected to conditions like hyperactivity and autism. Just last September, as the Times reported, the National Institute of Mental Health awarded up to $1 million to four separate research projects looking at ways to better understand how these colonies of gut bacteria affect mental health.

This research is so fascinating because of its potential impact: if we find that abnormal bacterial colonies have a role in causing mental disorders, then perhaps if we alter these bacterial colonies, we can treat these mental conditions.

This is the promise and claim of the multi-billion-dollar and growing probiotics industry, but it should be very clearly noted that as of right now, we still have no idea how to effectively manipulate gut bacteria to treat mental conditions or many other issues; we're not even sure such treatments are possible. And it's worth noting that the gut-bacteria effective treatments that have proven useful for specific applications like a C. diff infection are vastly different from the probiotics being marketed at your local drugstore.

We're at a crucial moment

Despite the overhyping, the potential impact of our growing understanding of the weird relationship between our gut bacteria and our health is still huge. We have no idea how to treat some mental disorders and our treatments for many psychiatric conditions haven't gotten any more effective since the 1950s.

Through huge research initiatives, we're trying to understand the brain right now. At Smithsonian magazine's "The Future is Here" festival earlier this year, Dr. Thomas Insel, the Director of the National Institute of Mental Health, said: "We think about the 21st century as a century for chronic, non-communicable diseases ... most of all, brain disorders."

At the same time, a major project that's trying to categorize all the different bacterial communities in our bodies and the ways they affect our health could open up a whole new understanding of those "brain" disorders.

"We are, at least from the standpoint of DNA, more microbial than human," Insel told the Times. "That's a phenomenal insight and one that we have to take seriously when we think about human development."

Lyte's lab focuses on figuring out some of the ways these bacteria could affect our mental health. They've found that gut bacteria produce neurochemicals like dopamine and serotonin. We've known those were produced in the gut for a long time, but didn't know until recently it was the bacteria making them.

Other researchers have shown that providing mice with certain bacteria makes them less likely to give up when they are forced to swim after being dropped in a cylinder of water that they can't escape from. In other cases, mice given certain bacteria have become less anxious and stressed. In other fascinating work, mice with certain symptoms that resemble, in part, autism in humans have had those symptoms reverse after a bacterial transplant — something fascinating, if still incredibly far from being applicable to humans.

These potential treatments, called "psychobiotics," are being tested in humans too, for psychological conditions and neurological disorders.

But as transformative as the idea of bacterial manipulation might be, it's still a long way from being ready to be implemented. As Lyte told the Times:

"It's the Wild West out there ... You can go online and buy any amount of probiotics for any number of conditions now, and my paper is one of those cited. I never said go out and take probiotics ... We really need a lot more research done before we actually have people trying therapies out.''

Check out the full New York Times feature on gut bacteria and the brain here.