The latest evidence for this “gut-brain axis” comes from Javier Bravo at University College Cork. He fed mice with a probiotic bacterium called Lactobacillus rhamnosus, often found in yoghurts and dairy products. The bacterial menu changed the levels of signalling chemicals in the rodents’ brains, and reduced behaviours associated with stress, anxiety and depression.
Probiotic bacteria – those that benefit their host – are the subject of sweeping, hand-waving health claims. But beneath the breathless marketing hype, there is some intriguing underlying science. For example, some trials have found that probiotics can help to alleviate the mood symptoms that accompany irritable bowel or chronic fatigue syndrome. To that end, Bravo wanted to see if L. rhamnosus could influence the brains of normal, healthy animals.
Bravo found that his mice, after regularly eating Lactobacillus, were more likely to spend time in the exposed parts of a maze (a common test for anxiety symptoms) than those who ate bacteria-free meals. They were also less likely to drift motionlessly when plopped into water (a common test for depressive symptoms). And during stressful situations, they built up lower levels of stress hormones.
The bacteria also boosted the role of GABA, a restraining chemical that downplays the buzzing of excitable neurons. GABA works by docking with receptor proteins, and Bravo found that Lactobacillus increased the numbers of these receptors in parts of the brain associated with learning, memory and emotional control. The GABA system is involved in several stress-related mental conditions. For example, animals with depressive symptoms have lower levels of GABA receptors in the front of their brains, and one group of anti-anxiety drugs works by enhancing the effects of GABA receptors in humans.
It may seem odd that bacteria in an animal’s gut can control what happens in its brain, on the other side of the body. But the two organs have a direct line between them – the long, branching vagus nerve, which transmits information from the gut and other visceral organs to the brain. When Bravo severed the vagus nerve in his mice, Lactobacillus lost all of its influence. It changed neither the rodents’ behaviour nor their GABA receptor levels.
Bravo’s study is the latest in an accumulating body of evidence showing that gut bacteria are little backseat drivers for their hosts. Earlier this year, I wrote about work from Rochellys Diaz Heijtz at the Karolinska Institute, who showed that germ-free mice without any gut bacteria behave differently to mice with a normal complement. They were more active, less anxious and more likely to take risks. And when Heijtz transplanted the gut bugs from normal mice into sterile babies, the recipients behaved in the usual cautious way when they grew up. A few months later, a Canadian team led by Karen-Anne Neufeld found similar results.
These studies showed that gut bacteria can affect the way a mouse’s brain develops at a young age, and Bravo expands upon them in two important ways. First, he showed that inoculations of Lactobacillus can change the behaviour of normal, healthy mice, as opposed to artificially sterile ones. Second, he showed that this works in adults, rather than just in babies.
“We have no reason to expect that the same would not apply to humans,” says Cryan. “However, such clinical studies need to be carried out.” However, he cautions that not all probiotics do the same thing and he warns against overinterpreting the results of the study.
In a thorough survey of our skin microbiome, Elizabeth Grice identified species from at least 205 different genera. Your forearm has the richest community with an average of 44 species, while your nostril, ears and inguinal crease (between leg and groin) are the most stable habitats.
A team of international scientists led by Junjie Qin and Ruiqiang Li discovered that each of our bowels carries at least 160 bacterial species. Together, our collective guts have just under 3.3 million bacterial genes, more than 150 times as many as reside in our own genomes. They also showed that the gut microbiome of a health person looks very different to that of someone with a bowel condition like Crohn’s disease or ulcerative colitis.
Fat mice and humans have a less diverse milieu of gut bacteria, with a greater proportion of Firmicutes to Bacteroidetes in their bowels. This ratio increases if we eat high-fat diets and falls if we eat low-fat diets. Some species send out signals that make us hungrier, encourage us to eat more, and affect the way we store fat. And some of our immune genes help to moderate these signals.