The trillions of bacteria in and on our bodies can strengthen our health and contribute to disease, but exactly which microbes are key actors is elusive. Now a study involving thousands of people in Finland has identified a potential microbial culprit in some cases of depression.
The findings, which emerged from a study of how genetics and diet affect the microbiome, “are really solid evidence that this association can be of great clinical importance,” said Jack Gilbert, a microbial ecologist at the University of California, San Diego, who was not participates in the work.
Researchers are increasingly finding links between brain conditions and intestinal microbes. People with autism and mood disorders, for example, are deficient in certain key bacteria in their gut. Whether these microbial deficiencies actually help to cause the disorders is not clear, but the findings have led to the rush to use intestinal microbes and the substances they produce as possible treatments for various brain diseases. In fact, researchers recently reported in Limits in psychiatry that fecal transplants improve symptoms in two patients with depression.
Guillaume Merrick did not aim to detect germs that cause depression. A microbial bioinformatics researcher at the Baker Heart and Diabetes Institute, he and his colleagues analyzed data from a large study on health and lifestyle in Finland. Part of a 40-year effort to track the root causes of chronic diseases in Finns, the 2002 study assessed the genetic makeup of 6,000 participants, identified their gut microbes and collected extensive data on their diets, lifestyles, prescription drug use. and health. Researchers are monitoring the health of participants until 2018.
Merrick and his colleagues combed the evidence for how a person’s diet and genetics affect the microbiome. “There are very few studies that have been studied [all these factors] so detailed, ”says Gilbert. Two parts of the human genome appear to have a strong influence on which microbes are present in the gut, researchers said this week in Natural genetics. One contains the gene for the absorption of milk sugar lactose, and the other helps determine blood type. (Second study, also published today in Natural geneticsidentifies the same genetic loci by analyzing the relationship between the genomes and intestinal microbes of 7,700 people in the Netherlands.)
Merrick’s team is also investigating which genetic variants can affect the abundance of certain microbes – and which of these variants are associated with 46 common diseases. When it comes to depression, two bacteria that cause infections in hospitalized patients Morganella and Klebsiella, seems to play a causal role, researchers say. One of them, Morganellawas significantly increased in a microbial study of 181 people in the study who later developed depression.
“It’s really exciting,” said Jeroen Raes, a microbiologist at KU Leuven who was not involved in the study. The beauty of the job, he added, is that Merrick and colleagues have linked the increased levels of the bacterium to depressed patients.
Morganella is already involved in depression. As early as 2008, researchers investigating a possible link between depression and inflammation found that depressed people had a stronger immune response to the chemicals produced by Morganella and other gram-negative bacteria in the gut. Thus, the latest study appears to be “further evidence” that inflammation caused by intestinal microbes can affect mood, Gilbert said.
But this area is still in its infancy, says Gerard Clark, a microbiome researcher at University College Cork, because there are many forms of depression and many possible ways in which germs can affect the disease. The “holy grail” is to identify a missing germ that can be given as a supplement, he says. But it is less clear how Morganella can be eliminated from the gut to relieve symptoms. “It’s a little more challenging.”