Our Well-being and Our Microbes

Micro-organisms

Micro-organisms

The following blog, about the microbiota in our intestines, is a simplification of the New York Times article, Can the Bacteria in Your Gut Explain Your Mood? by Peter Andrey Smith, June 23, 1915.

Bacteria in the gut produce vitamins and break down our food; their presence or absence has been linked to obesity, inflammatory bowel disease and the toxic side effects of prescription drugs. The Human Microbiome Project catalogs the micro-organisms living in our body. Appreciation for the influence of such organisms has grown rapidly.

Mark Lyte studies how gut microbes communicate with the nervous system using some of the same neurochemicals that relay messages in the brain. Our digestive tubes contain vast quantities of what biologists call gut microbiota. The genetic material of these trillions of microbes, as well as others living elsewhere in and on the body, is collectively known as the microbiome, and makes up a sort of organ whose functions have only begun to reveal themselves to science.

  • Biologists now believe that much of what makes us human depends on microbial activity. The two million unique bacterial genes found in each human microbiome can make the 23,000 genes in our cells seem paltry, almost negligible, by comparison.
  • ‘‘This has enormous implications for the sense of self,’’ Tom Insel, the director of the National Institute of Mental Health, said. ‘‘We are, at least from the standpoint of DNA, more microbial than human.”
  • Micro-organisms in our gut secrete a profound number of chemicals. Researchers have found that among those are the same substances used by our neurons to communicate and regulate mood, like dopamine, serotonin and gamma-aminobutyric acid (GABA). These, in turn, appear to play a function in intestinal disorders, which coincide with high levels of major depression and anxiety.
  • A group in Norway examined feces from 55 people and found certain bacteria were more likely to be associated with depressive patients.



  • Anxiety, depression and several pediatric disorders, including autism and hyperactivity, have been linked with gastrointestinal abnormalities. We might one day use microbes to diagnose neurodevelopmental disorders, treat mental illnesses, and perhaps even fix them in the brain.
  • Lactobacilli are one of the organisms babies ingest as they pass through the birth canal. Mice stressed during pregnancy pass on lowered levels of this to their pups. It is known to release immense quantities of GABA; as an inhibitory neurotransmitter, GABA calms nervous activity, which explains why the most common anti-anxiety drugs, like Valium and Xanax, work by targeting GABA receptors.
  • Micro-organisms in the gut tickle a sensory nerve ending in the fingerlike protrusion lining the intestine and carry that electrical impulse up the vagus nerve and into the deep-brain structures thought to be responsible for emotions like anxiety. These potentially mind-altering microbes are called ‘‘psychobiotics.’’
  • Much of our supply of neurochemicals — an estimated 50 percent of the dopamine, for example, and a majority of the serotonin — originate in the intestine, where these chemical signals regulate appetite, feelings of fullness, and digestion.
  • Lyte suspects stress directly affects the bacterial bugs that cause infections.
  • Lyte showed, by inducing stress in mice, that introducing a pathological bacterium into the gut will cause a change in behavior.
  • Lyte also showed how stressful conditions for newborn cattle worsened deadly E. coli infections. In another experiment, he fed mice lean ground hamburger that appeared to improve memory and learning — a conceptual proof that by changing diet he could change gut microbes and change behavior.
  • In 2011 Lyte proposed probiotic bacteria could be tailored to treat specific psychological diseases.
  • Sarkis Mazmanian and colleagues found mice exhibiting abnormal communication and repetitive behaviors, like obsessively burying marbles, were mollified when given a strain of the bacterium Bacteroides fragilis. Microbes don’t just affect the permeability of the barrier around the brain but also influence the intestinal lining, which normally prevents certain bacteria from leaking out and others from getting in. When the intestinal barrier was compromised in his model, normally ‘‘beneficial’’ bacteria and the toxins they produce seeped into the bloodstream and raised the possibility they could slip past the blood-brain barrier.
  • Mazmanian’s results offered a provisional explanation for why restrictive diets and antibacterial treatments seem to help some children with autism: Altering the microbial composition might be changing the permeability of the intestine. ‘‘Is a disease like autism really a disease of the brain or maybe a disease of the gut or some other aspect of physiology?’’ For any disease in which such a link could be proved, he saw a future in drugs derived from these small molecules found inside microbes. Disorders once thought to be conditions of the brain might be symptoms of microbial disruptions.

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