Microbes Affect Brain Development

Incredibly, your personality may have been forged as a baby by gut microbes.

The following is adapted from The Psychobiotic Revolution:

“I don’t dislike babies, though I think very young ones rather disgusting.” —Queen Victoria

Newborn babies can’t talk, and most of what happens to them won’t be remembered. That’s probably for the best. Birth is traumatic. But it’s also an important time for brain development. Your brain is simply popping with activity. You are sprouting neural branches everywhere while simultaneously pruning them. Challenges to the brain at this early stage can profoundly alter your development.

The reason you don’t remember anything before you were six months old is partly related to the insulation coating the wiring of your brain. Neurons are brain cells that act a little bit like wires, in that they conduct electrical signals. Like wires, they need to be insulated or they can short-circuit. That insulation is provided by layers of fatty myelin. Before myelin is wrapped around the neurons, signals are slow and so flooded with crosstalk that the signals get washed out.

It’s unlikely that you can either lay down memories or retrieve them until your brain is myelinated. Before then, it’s unlikely that anything like normal thinking can occur. A lot of that myelination happens in the womb, but the process isn’t completed by the time you’re born. At delivery, your brain is still a work in progress.

The myelination proceeds from the center of your brain to the outer layers over a period of several years. It’s like an oil stain slowly spreading out from the core brainstem.

Early memory is surreal

As a consequence of this continuing construction work, people can’t remember anything that happened to them before the age of three, although there may be exceptions. Salvador Dali said he could remember living in the womb, but Dali also had a famously active imagination. It’s not that you don’t have a memory at that age; you do. It just isn’t being laid down with much permanence. Newborns can remember certain things like colors and simple shapes for up to 24 hours, but not much longer than that.

As you get older, the duration of your memories increases, but by the end of your first week, you’ve likely already forgotten what it was like to be born. And then, around the age of seven, you start to forget your earliest memories. This is called infantile amnesia, and it explains a lot of fogginess about your past.

An easy target for disruption is your gut microbiota. That’s important because your microbiota affects how your brain develops. That’s right, beyond tutoring your immune system and affecting your mood, your microbiota can change how your brain gets wired up. If that doesn’t make you appreciate the power of microbes, nothing will.

It all started with germ-free mice

The first studies on how microbes affect brain development were done on germ-free mice. In 2004, Nobuyuki Sudo performed a landmark study showing that mice raised in a sterile environment had an abnormal stress response. Oddly, they seemed less anxious than ordinary mice. That sounds pleasant enough – until you consider that tiny prey animals are actually better off with a little anxiety to keep them on their toes. Importantly, their stress response could be normalized by simply feeding them bacteria, specifically bifidobacteria.

Further studies by Roman Stilling in the labs of John Cryan and Ted Dinan at University College Cork showed that germ-free mice have an abnormal amygdala, a part of the brain that responds to danger, and is linked to anxiety, depression and autism. These alterations may have a long-term effect on behavior and attitude.

A study by Elaine Hsiao in 2013 showed that a mouse model of autism was affected by gut bacteria. In particular, a single species of bacteria, Bacteroides fragilis, was able to reverse some of the behavioral problems in the mice, indicating an improvement in brain development. The exciting hope is that psychobiotics may help with human autism as well, but more work is needed. Mice are not men, and B. fragilis can sometime be pathogenic in humans. That’s because there are two strains of B. fragilis that are similar, but one leads to a good gut and the other leads to colitis. Interestingly, like Cain and Able, the more closely related microbes are, the more ferociously they tend to fight for dominance.

Memory by microbe

As well as the amygdala, studies in the Cryan and Dinan lab have shown that a germ-free mouse may have an unusual hippocampus, a seahorse-shaped part of the brain that is involved with both memory and emotion. Some of the changes noted were dramatic. Somehow, the microbiota controls the growth and development of important memory and anxiety centers of the brain.

If you restore the gut microbiota of the germ-free mice before they are weaned, you may normalize these changes. After weaning, though, the changes to the hippocampus seem to get set in stone. That suggests that your pre-toddler microbiota has a window of opportunity to make permanent changes to the way you remember and process stress.

It’s complicated

It goes both ways: stress can also affect your microbiota. At this early age, that can have far-reaching knock-on effects. This story is nothing if not convoluted.

It makes sense that early exposure to microbes would lead to a change in your immune system. Newborns are so tiny and helpless that it seems almost cruel to expose them to such a nasty, germy world. But for most of us, it is a well-protected period where your parents – especially mom – can guide your development by seeding you with good bacteria and ensuring that your immune system will accommodate them.

But the effect on the brain itself seems more surprising. Why are bacteria needed to develop a properly functioning brain? It is as if microbes are trying to socialize us, to make us want to explore our world and, perhaps, to meet up with other people with something exciting to share: new microbes.

References

Stilling, Roman M., Feargal J. Ryan, Alan E. Hoban, Fergus Shanahan, Gerard Clarke, Marcus J. Claesson, Timothy G. Dinan, and John F. Cryan. “Microbes & Neurodevelopment--Absence of Microbiota during Early Life Increases Activity-Related Transcriptional Pathways in the Amygdala.” Brain, Behavior, and Immunity 50 (November 2015): 209–20. doi:10.1016/j.bbi.2015.07.009.

Hsiao, Elaine Y., Sara W. McBride, Sophia Hsien, Gil Sharon, Embriette R. Hyde, Tyler McCue, Julian A. Codelli, et al. “Microbiota Modulate Behavioral and Physiological Abnormalities Associated with Neurodevelopmental Disorders.” Cell 155, no. 7 (December 19, 2013): 1451–63. doi:10.1016/j.cell.2013.11.024.

Ogbonnaya, Ebere S., Gerard Clarke, Fergus Shanahan, Timothy G. Dinan, John F. Cryan, and Olivia F. O’Leary. “Adult Hippocampal Neurogenesis Is Regulated by the Microbiome.” Biological Psychiatry 78, no. 4 (August 15, 2015): e7-9. doi:10.1016/j.biopsych.2014.12.023.

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This article is adapted from the bestselling book I wrote with the top two scientists in the gut-brain field: John F. Cryan and Ted Dinan. They are simply brilliant, and are changing the world one psyche at a time! From National Geographic.