The Reach of Probiotics

The real breakthrough in treating psychiatric disorders may hinge on cultivating the “downstairs brain."

By Hara Estroff Marano, published on July 8, 2015 - last reviewed on June 10, 2016

Your unconscious is alive and active, although not quite the way Freud envisioned it. It’s at least as much in your gut as in your head, and its primary protagonists are the intestinal bacteria known as probiotics, organisms that foster health—and especially mental health.

More than 100 trillion bacteria inhabit the gut; collectively they weigh as much as the brain. Over the past decade, researchers have been exploring their contribution to  well-being  in the gut and beyond. Not only does the microbiome enable normal brain development and social interaction, gut bacteria produce an array of bioactive compounds that influence behavior from anxiety to pain perception. 

“To have a healthy brain and mind, you need a healthy gut,” neuroscientist John Cryan told a recent symposium. Cultivating the right bacteria, accumulating evidence suggests, may prevent and even reverse symptoms of disorders ranging from autism to stress reactivity.

Cryan, of University College Cork in Ireland, believes that the microbiome fosters neural development and exerts its greatest effect on the brain early in life. He and colleagues have recently found that animals born by cesarean section, and not exposed to the microbes accompanying vaginal birth, later had more symptoms of anxiety and depression than animals born naturally.

Other researchers have shown that in mice born with autism-like symptoms, the composition of the gut biome is markedly different from that in normal mice. Treating the animals with the probiotic Bacteroides fragilis, a common species of gut bacteria, reversed many, but not all,  of the deficits, notably in communication, repetitive behaviors, and anxiety. 

In other studies, Cryan has shown that animals raised in a germ-free environment have an exaggerated stress response, as indicated by high stress hormone levels. They are also deficient in BDNF, a nerve growth protein known to abet the survival of existing neurons and stimulate the creation of new ones. But when the animals are given everyday gut bacteria, the stress response normalizes. 

Even in adult animals, the gut biome regulates nerve growth in the hippocampus, a region of the brain important for learning and memory and highly susceptible to damage by stress, Cryan recently reported in Biological Psychiatry.

Clinical research is gathering steam, too. Taking cues from animal studies, Timothy Dinan, also of University College Cork, recently gave healthy human volunteers Bifidobacterium longum and recorded reductions in stress levels and  improvements in cognition. He aims to identify the specific bacteria—he calls them psychobiotics—that curb depression, anxiety, and other  mental ills. 

From birth on, maintaining the right composition of what scientists call the “downstairs brain” may be essential to the function of the “upstairs brain.”

Illustration by Mark McGinnis
  • Bifidobacterium infantis is one type of beneficial bacteria that normally inhabits the gut and shapes brain processes. When given to animals, the probiotic attenuates depressive symptoms.
  • Lactobacillus rhamnosus JB-1 has been shown to reduce anxiety- and depression-related behaviors in animals. In combination with L. helveticus, it reduces memory dysfunction.
  • Mycobacterium vaccae given to rats reduces anxiety and improves performance on a complex maze task.
  • Bifidobacterium longum normalizes anxiety-like behavior. Combined with other strains of Bifidobacterium and Lactobacillus, it stimulates memory formation.
  • Bacteroides fragilis given to young animals reversed some deficits in sociability and repetitive behaviors seen in neurodevelopmental disorders such as autism and schizophrenia.
  • Lactobacillus helveticus ROO52 reduces anxiety-related behavior and alleviates memory dysfunction in animals.

The Broad Reach of Probiotics

  • Anxiety
  • Stress Response
  • Gut Integrity
  • Depression
  • Autism Spectrum Disorder
  • Obesity
  • Immune Function
  • Neural Development
  • Cognitive Function
  • Chronic Pain

How the Gut and the Brain Talk 

The brain and the gut communicate in both directions, directly through the vagus nerve running between the brain and digestive tract and indirectly through various body processes. 

Gut bacteria:

  • are affected in number and metabolic activity by stress. Conversely, a lack of critical gut bacteria upsets the hypothalamic-pituitary axis, leading to overproduction of stress hormones
  • hack the brain’s reward system, producing food cravings and avoidance.
  • alter expression of genes and production of fatty acids, including those involved in production of the nerve-growth factor BDNF.
  • produce neurotransmitters such as GABA, norepinephrine, dopamine, and the precursor to serotonin.
  • affect levels of appetite-regulating hormones, including leptin and ghrelin.
  • influence immune function, producing pro- and anti-inflammatory substances. 
  • act through digestion of high-fiber foods to create short-chain fatty acids, which are neuroactive and influence energy metabolism.