If your body is a ship and your brain is the captain, the billions of bacteria, parasites and microbes riding below deck have an unexpected sway over your course through life. In a story for Nature, Sara Reardon writes about the building evidence that the microbiome, the collection of wee beasties that live on and inside your body, can exert a powerful if subtle sway over your brain's behavior.

Though researchers have long known that we share our bodies with microscopic passengers, in the past few years new research has indicated just how important those organisms are to our lives. As this field becomes increasingly sophisticated, researchers are finding all sorts of unexpected links. According to Reardon, there is now “hard evidence linking conditions such as autism and depression to the gut’s microbial residents.”

Although correlations have been noted between the composition of the gut microbiome and behavioural conditions, especially autism, neuroscientists are only now starting to understand how gut bacteria may influence the brain.

It's still early to say much for certain, and the physical and chemical mechanisms by which these interactions play out are not known. But the realization that the microbial flora that inhabit our bodies can affect behavior certainly points towards intriguing new fields for research and possible therapies, much further down the line.

How little is known about the microbiome and its effects is a point well worth paying attention to. As Ed Yong wrote in an opinion piece for the New York Times recently, some people are already using the scientific sheen of this new field to peddle the idea that there is some better, more natural, healthier microbiome. (And it too can be yours for four easy payments of $29.99!)

We know very little about the microbiome, says Yong. But what we do know is that it's incredibly complex and can't be painted with broad strokes:

The microbiome is complex, varied, ever changing and context-dependent — qualities that are the enemies of easy categorization. “Healthy” microbes can easily turn rogue. Those in our guts are undoubtedly helpful, but if they cross the lining of the intestine and enter our bloodstream, they can trigger a debilitating immune response. The same microbes can be beneficial allies or dangerous threats, all for the difference of a few millimeters.

The growing understanding of the microbiome changes how scientists are thinking about the human body and its relatedness to the outside world, but beware the snake oil salesmen that so often accompany new ideas.