An increasing amount of evidence is coming to light suggesting that human moods, emotions and perceptions can be influenced by the type and number of microscopic life forms inhabiting our gut, according to an article in Scientific American magazine. Scientists say a time may even come when we treat mental illness and depression with probiotic supplements, and that the bacteria, viruses and fungi that inhabit our gut -- making up what is called the gut's microbiome -- may be an indicator of our ability to handle stress and what diseases we will be vulnerable to over the course of our lives.

It has been observed in the animal kindgom that some microorganisms can influence behavior in their hosts. The single-celled protozoan Toxoplasmosis gondii has been shown to make rats less frightened of cats and perhaps even cause them to be drawn to the smell of cats' urine. This makes the rat more likely to be eaten by a cat, whose digestive tract is a necessary stop in the organism's reproductive cycle.

Some scientists, like Jaroslav Flegr, who was profiled in the Atlantic in March of this year, have theorized that Toxo can have a similar effect in humans, producing the "crazy cat lady" effect, wherein people become inured to the stench of cat wastes.  Flegr, who is himself infected with Toxo, has shown that people carrying the disease suffer from slower reaction times and impaired judgment.  Men who are infected with Toxo were shown to be more introverted, suspicious of other people's motives and less concerned with others' perceptions of them.  Infected women, on the other hand, were more "outgoing, trusting, image-conscious, and rule-abiding than uninfected women."

The fungus Cordyceps invades the brains of ants and other insects and fills them with an irresistible urge to climb to the highest place they can find.  Once they're in position, they die and a mushroom sprouts from their head, enabling the fungus to spread its spores over a much greater area than if its host were at ground level.

But can microbes living in our bodies influence the normal everyday behaviors of average people?  The answer may well be yes.

We know that the health and variety or a person's enteric flora play a role in the body's susceptibility to diseases like irritable bowel syndrome (IBS) and stomach ulcers.  Evidence is showing, however, that our emotional and mental health and well-being could be inextricably linked with the contents of our guts.

In a Japanese study, tests with rats have shown that rats bred without a normal microbiome of enteric organisms were less able to handle stress or to return to an unstressed state when unpleasant stimulus was removed.  Within a few weeks of being given probiotic supplements, however, these rats handled stress as well as rats who were born with regular enteric flora.

Humans are more complex than rats, of course, but in a 2011 French study, women test subjects who consumed a daily portion of fermented dairy products (like yogurt) that contained the bacteria Lactobacillus and B. longum, two bacteria commonly found in normal enteric flora, showed greater decreases in signs of psychological and emotional stress than women who ate a non-fermented dairy product or those who took nothing.  According to Scientific American, when researchers "scanned each woman’s brain, they found that compared with the two control groups, the participants given probiotics had significantly less resting-state activity — the brain’s firing patterns when thinking about nothing in particular — as well as a dampened response in their arousal networks, which includes the amygdala, in response to emotional faces."

The magazine conceded that, ultimately, to read too much into the gut-brain connection could prove to be too simplistic, as it does not take into account the chemical and hormonal influences of other organs in the system. Other factors, including the environment and genetics, can affect the type and number of organisms living in the gut as well, with genetics taking a particular role given that the people with the most similar combinations of organisms in their micobiome are identical twins.

The relationship between gut flora and brain function may go beyond mood and our ability to handle stress, however.  In 2005, the Journal of Medical Microbiology published a study saying that the enteric flora of children with autism is markedly different from that of their healthier siblings, opening a previously unexplored avenue of autism research.

As far back as 1930, dermatologists John Stokes and Donald Pillsbury put forward the “gut-brain-skin axis” hypothesis connecting stress and skin irritations like acne.  Stokes and Pillsbury theorized that emotional states alter the gut's assortment of microbes, increasing the digestive tract's permeability and triggering skin inflammation.  The doctors prescribed doses of  Lactobacillus acidophilus, a microbe commonly added to milk products, a practice that has recently found new supporting evidence.  Studies have shown that "Lactobacillus soothes skin inflamed by stress and restores normal hair growth in mice."

John Bienenstock of the Brain-Body Institute at McMaster University in Ontario told Scientific American that we're only at the beginning of this line of inquiry.  He theorizes that bacteria on our skin may communicate with bacteria in our gut to influence our behavior.  “Could we have some microbial ointment that improves health and well-being?" he asked, "The mind boggles at the possibilities.”

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