Are your gut bugs making you moody?
The human body is teeming with microbes. In fact, there are more bacteria living on and in us than there are cells in our entire bodies: each of us is outnumbered 50 to one by these microbial passengers, which are collectively known as our microbiome. Now, scientists like Premsyl Bercik are finding that these bugs are not passive hitchhikers after all and, instead, are influencing how we feel; and there are lots of them doing it, as Chris Smith discovers...
Premsyl - For humans, it's approximately 1 or 2 kilograms of bacteria which we are carrying in our intestines. If you think about it, this is a large metabolic organ which is generating many molecules and some of these molecules were shown to be able to interact with the nervous system.
Chris - It's anxiety and depression that you are specifically studying here because you're trying to ask, "Can we induce or prevent anxiety or depression in animals based on what's living in their gut?"
Premsyl - Right. So up to now, most of the studies performed in animal models were using healthy animals. In this specific study, we took advantage of an established model of anxiety and depression which is based on early life stress. It's called the maternal separation where the newborn mice are separated from their mums for a period of 3 hours every day for 3 weeks. This manipulation changes their behaviour later on. What is very interesting that this also affects their reactivity to stress, that means the level of stress hormones, and it also changes the way how the intestine behaves. For example, motility, that means how the intestine moves, voluminal content or permeability, that means how we are absorbing certain nutrients or even bacteria products is altered.
Chris - What about the microbes themselves?
Premsyl - In the first part of our study, we actually measured the composition of bacteria and found that there is a difference between the healthy and maternally separated mice.
Chris - Is that just not a consequence of the animals are unhappy, they're unwell and therefore, they're eating less or they're eating different foods, or do you think that the microbes changing is having an additional effect on the mood of these animals?
Premsyl - Well, I think this is the crucial point of this study. We performed similar experiments in mice without bacteria. Mice which live in sterile environment and like this, we were able to test the effect of presence of bacteria. What we found in mice without bacteria was that they have abnormal stress hormone levels. Their gut function is still abnormal. However, they behave normally. That means they don't show any signs of anxiety or depression.
Chris - Are the microbes that are in the animals normally exacerbating the symptoms then? How do you explain the fact that they don't behave like animals that do have bugs in them?
Premsyl - Well, we think what is happening is that the different function of the gut creates different habitat for bacteria and these bacteria then start producing different molecules which in turn can alter the behaviour of the host.
Chris - It's not for example that an animal that's not feeling very happy just eats different food.
Premsyl - Well, these mice we all fed by the same food and we have - in previous experiments, we have measured the consumption of food. So, it cannot be attributed just to change in their diet or eating habits. But we have identified actually several molecules in these altered bacteria which are known to affect behaviour. So, we think that we have at least some possible candidates which are responsible for this abnormal behaviour.
Chris - Now, the obvious question is, how does this relate to humans?
Premsyl - I think this is the one million dollar question. So far, there are no data in humans to show a cause-effect relationship between bacteria and depression or anxiety. However, there are some hints that would suggest that bacteria may be involved. Two recent studies suggested that microbial profiles differ between patients with depression and healthy controls. And finally, a study from the UCLA has demonstrated using brain imaging that mixture of probiotics, that means these healthy bacteria can alter brain connectivity patterns in healthy volunteers.