[DIGEST: IFL Science!, Science Alert, Psychology Today]

Next time you find yourself getting frustrated or overly irritable with your kids, spouse or boss, don’t blame hormones or your brain — blame your colon.

For years, animal studies have indicated a link between gut microbiome — the colony of bacteria and other microorganisms that live in your digestive tract — and overall health and behavior.

A recent study led by scientists at UCLA, however, has for the first time conclusively identified ways in which gut flora affects both mood and reaction in humans.

The paper, published in June in Psychosomatic Medicine: Journal of Behavioral Medicine, outlined an experiment using fecal samples from 40 healthy, normal-weight women between the ages of 18 and 55. Researchers divided the women into groups based on the makeup of their digestive microbiome — 33 had Bacteroides, a common bacteria found in people who eat a diet high in animal fats, and 7 had less-common Prevotella, more prevalent in those eating mostly high-fiber fare but also associated with chronic inflammation.

The researchers then studied the women’s brains as they looked at a series of images meant to provoke a range of emotions. Brain activity varied greatly depending on which gut bacteria resided in their intestines — women with Prevotella had less activity in the hippocampus, which is associated with memory, but greater connections between emotional and sensory regions in the brain, as well as higher levels of anxiety and irritability after looking at the images meant to provoke a negative response. Women with Bacteroides, however, were less likely to experience anxiety and irritability after looking at the negative images, plus had a thicker gray matter in areas of the brain that process complex information.

“To our knowledge,” reads the report, “this is the first report of behavioral and neurobiological differences related to microbial composition in healthy humans.”

Nutrition also plays a role in Prevotella and. Bacteroides colonization, but the research team noted that, for unknown reasons, changing from a gut flora of mostly one type to another is not achievable even with a controlled diet: “In a short term study using a…dietary intervention with either high fiber/low fat or high fat/low fiber, individuals failed to change from one ‘enterotype’ to another, despite rapid changes in individual bacterial species….” reads the report. “If an interaction between diet and the brain signatures described in the current study exists, it could be based on long term diet, or potentially on dietary factors in early life, while the brain is developing the cortico-limbic and sensory circuitry.”

The Psychosomatic Medicine study builds on a widely reported 2011 study on mice, which found that particularly anxious rodents fed Lactobacillus rhamnosus, a probiotic commonly found in yogurt, had about half as much of the stress hormone corticosterone in their blood than a control group after undergoing a stressful event. Researchers found that the rodents with the introduced Lactobacillus also had redistributed brain receptors, indicating that the bacteria had altered their brain chemistry. When researchers clipped the mice’s vagus nerve — the main line of communication between the gut and the brain — the differences disappeared.

"It's pretty convincing," Brett Finlay, a microbiologist at the University of British Columbia in Vancouver, Canada, then told Nature magazine. "These days our microbiota are being implicated in just about everything."