Researchers from McMaster University (Canada) had a clear question in mind when they conducted their recent experiment: if a mouse had its gut microbiota altered by antibiotics in early life, what would happen to its brain?
The question might have seemed a non-sequitur—why would something that changes the gut have any effect on the brain?
Yet the group of researchers, led by John Bienenstock and Sophie Leclercq, found that exposure to low-dose antibiotics did affect the mouse brains—and not just a little. The antibiotic-exposed mice showed an altered blood-brain barrier and a spike in specific immune-signalling molecules (cytokines) in the frontal cortex. Most importantly, however, the antibiotics changed mouse behaviour: the young mice acted differently in social situations and when faced with difficult tasks. They were also more aggressive than the mice with no alterations in their gut microbiota.
For starters, they have shown that some of these effects depend on the nervous system. The role of the immune system, however, is not yet clear
“We used really low dose penicillin—a pediatric dose in a mouse, which is miniscule—and showed significant effects,” John Bienenstock explains in an interview with GMFH editors. “There were all sorts of effects in terms of social behaviour, social interaction, social avoidance.”
But the researchers took the idea even further—could these effects on the brains of the mice be counteracted by adding different microbes? The answer was yes. Experimental mice that were given the probiotic bacteria Lactobacillus rhamnosus JB-1 in addition to the antibiotics showed fewer of the changes in brain biology and behaviour. The experiment showed not only that changes in gut microbiota could affect the brain, but also that the specific type of bacteria mattered for the end result.
Bienenstock, along with his colleagues Paul Forsythe and Wolfgang Kunze, have been working on the gut-brain axis for many years—using germ-free mice as well as normal mice in their models, trying to zero in on how bacteria manage to influence the two-way communication between gut and brain.
Bienenstock says that although researchers can change the bacterial inputs and measure what happens to mice, the real mystery is the chain of events leading from the gut to the brain changes. He says, “All we know is that something happens, [like a change in behaviour]. So there’s a huge black box still.”
For starters, they’ve shown some of these effects depend on the nervous system; but the role of the immune system is not yet clear. Describing a landmark 2011 paper from his group that showed how L. rhamnosus JB-1 regulated emotional behaviour and aspects of central nervous system function in mice, Bienenstock says, “If you gave L. rhamnosus to the mice you could show all these effects—structural and functional—and then if you cut the vagus nerve those complicated events did not occur. So at least in that bug, most of the effects were related to nervous function. But we don’t know whether that works in the absence of the immune system or how much the immune system is indirectly or directly associated.” That is, the mechanisms need to be clarified.
Bienenstock emphasizes that although the mouse models are important for investigating mechanism, they’re not the end goal. As a medical doctor (an internist by training), Bienenstock never forgets that, long-term, the research could have serious implications for those living with mental or behavioural problems.
“We have to do all this stuff as much as we can in the human because that’s clearly a big area of ignorance of the moment in terms of microbial change and microbiome changes,” he says.
Putting all the pieces together in humans, though, is a massive challenge. “Experimentally, there are clues as to how some of these bacteria can cause these changes. We’re beginning to understand that it may be bits of the bacteria… it may be stuff that the bacteria makes in situ in the gut, or that it’s fermentation products themselves like short-chain fatty acids,” he says. “But we don’t know what they do when they’re all tucked in together.”
Leclercq S, Mian FM, Stanisz AM, et al. Low-dose penicillin in early life induces long-term changes in murine gut microbiota, brain cytokines and behavior. Nature Communications. 2017; 8. doi:10.1038/ncomms15062