The bacteria in our gut have become very popular lately. Whether we are debunking probiotics, understanding antibiotics, deciding on our diet or trying to feel happier, the influence of these microbes seems to be everywhere in our daily lives.
A growing body of research justifies that attention. Recent studies have revealed not only the diversity of species living in our inner bacterial gardens but also the significant role they play in our physical and mental health. One recent study discerning the importance of the gut microbiome-brain axis—the scientific terminology for the connection between our bellies and brains—confirms the strong link between the two.
Researchers from the University of California, Los Angeles's David Geffen School of Medicine found an association between these microbes and sensory areas of the brain in people with irritable bowel syndrome (IBS), an intestinal disorder. And it's the first study to find this link in humans.
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Emeran Mayer and his colleagues collected behavioral information, stool samples and brain images from 29 adults with IBS and 23 healthy people. They used gene sequencing to identify the microbes present in the collection of stool samples and also determined the abundance and diversity of bacteria in each participant's fecal matter. The IBS patients fell into two groups, distinguished by their microbiomes. In one group, the collection of bacterial species was similar to that of the healthy patients. In the other group, the collection was more distinct.
Now for the behavior. The researchers wanted to know if the microbiomes of the IBS patients showed any association with psychological or emotional distress of any kind. They used an anxiety and depression scale and a health questionnaire to gather information, and they also asked patients about childhood trauma and other adversities they'd experienced before age 18. They gauged how stressed-out the patients were and checked what medications they'd been taking.
It turned out that the two IBS groups did not vary all that much in their behavior. IBS patients whose microbiomes were more distinct from healthy participants reported more emotional issues. But this IBS microbial community was not associated with heightened depression, stress or medication use.
However, there were surprising differences between the groups. Instead of manifesting in their emotions and mental health, these variations showed up in their brains. The images of certain sensory regions of the brain—the thalamus, the basal ganglia and the sensory motor cortex—showed differences for the IBS patients with the distinct microbiome, compared with the healthy subjects.
Mayer explains that the sensory differences seen among the bacterially distinct IBS patients could be connected to the food sensitivities that IBS patients often have. Individuals may complain of abdominal pain after eating or taking certain medications, for example. "I think all this is related to a fundamental change in the way that the brain processes any sensory [disturbance]," says Mayer.
The findings are the first to show an association between the gut microbiome of IBS patients and structural alterations in the brain. However, Mayer emphasizes that the association does not explain the cause.
"The study is exploratory," says Elyanne Ratcliffe, a pediatrician at the Farncombe Family Digestive Health Research Institute at Canada's McMaster University. "There are some plausible pathways, but it has to be worked out more rigorously."
Ratcliffe also cautions against zeroing in on any particular brain image connection. "If you look at tons of stuff, you're bound to find something," she says.
The direction of that pathway is also in question. If the brain and gut microbiome are connected, which determines which? Does the brain influence gut development, or does the gut influence brain development?
Mayer believes it's a two-way street. "Signals from the gut microbes shape the way the sensory system develops," he says. His theory, which he describes at length in his book, The Mind-Gut Connection, is that our microbial communities assemble early in life, when the brain is still very flexible. The microbiome of an infant's gut is created through the nutrition he or she receives from the mother, how that nutrition is delivered and other factors. It is possible, though not yet proved, that if the mother experienced stress during pregnancy, that might also influence the developing infant's microbial community.
Early-life trauma has also been found to shape the gut microbiome. "A lot of influences start during pregnancy and go on for the first three years of life," says Mayer. "That's the programming of the gut microbiome-brain axis." The established microbiome then influences the brain, and then the brain continues to influence the microbiome, in a lifelong loop.
Related: Antibodies in breast milk prime the baby's gut to handle Mom's invading microbes
The UCLA researchers, along with a host of other scientists worldwide, are continuing to study the link between the gut microbiome and brain development. Whether a problematic microbiome can be altered is another matter altogether. But, says Mayer, a diet that includes a diversity of microbes—namely, one consisting mainly of plants—is probably the best approach in the absence of further evidence.
Scientists do not yet understand the implications of the gut microbiome, and any interpretations of the ongoing research are still controversial and iffy. Studies continue to probe the influence of our inner flora on IBS, autism spectrum disorders and our general health (several were just presented at Digestive Disease Week). But none so far have offered concrete proof of these links. In addition, scientists are still trying to parse which bacterial species make for the ideal gut garden.
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