Essay: Forming the human connections between bacteria in the gut to mental health and behavior

Bacteria from your stomach tells you what to feel. That’s not a sentence you hear every day. A groundbreaking study correlating gut microbes to mental health and human behavior has opened up a whole new world in the approaches of current pharmacological methods.

Gut microbes and humans have always been in a symbiotic relationship as these bacteria have been influencing our allergies, metabolism, and digestion since the beginning of time. These bacteria affecting our health is not a new idea. But recent studies have shown that these bacteria not only affect our body, but our brain as well. Researchers in the University College of Cork in Ireland have been investigating the way that this microbiome can help to govern the way people feel. John Cryan, from this investigation, observed the connection through experiments done with mice. Mice lacking gut bacteria were discovered to have a diminished fear response, while mice with the gut bacteria had no change. This resulted in the framing of the field of “psychobiotics,” referring to the ingestion of live organisms, that produce beneficial effects in the patients with mental illnesses.

Figure 1: Mice used in the experiment to determine the connection between gut bacteria and behavior

This experimentation with mice fueled many more biologists to not only continue the research of the brain-gut connection, but to advance it even further. Emeran Mayer, a researcher in UCLA University, was the first to test this hypothesis on humans by administering probiotic yogurt to groups of healthy women for 4 weeks. Through the usage of functional magnetic resonance scans, or the fMRI, they were able to identify that these women ingesting the probiotics had a lessened responses to images with an adverse connotation.

Though my AP Psych teacher never let me forget that correlation does not necessarily mean causation, these results still validate the connection between the gut microbiome and the human brain. The future of this research holds to whether these bacteria influence brain activity or if the reverse is true and changes in the brain affect the type of gut microbes that reside in your body. Not only that, but scientists are looking at effects that other factors have such as environmental, lifestyle, and dietary changes on the gut microbiome connection to the brain.

There were many fields investigated with the discovery of the gut microbiota connection to the brain. People were interested to understand if  these connections could lead to advances in some already pressing mental health problems with no cure present, such as autism, anxiety, or depression. Specifically, in the field of autism, it is known that not only do these patients suffer from severe disabilities mentally, learning wise and socially, but physically, they suffer from intestinal problems as well. Some of these include excessive food allergies, extra sensitivity to glucose, and digestion issues. Researchers were intrigued by the potential relationship between bacteria in the gut and its connections to autism.  Particularly, Sarkis Mazmanian, a biologist from CIT University, who conducted an experiment on mice that exhibit symptoms similar to those of autism in humans. By injecting a species of this gut bacteria, named bacteroides fragilis, into the rodent, they presented signs of improved social behaviors, less anxiousness, and decreased repetitions. This was immediately seen as a potential breakthrough in understanding how these microbes can specifically affect the brain, and other pressing neurological disorders. These results can lead to the potentiality that gut bacteria can be used as a feasible treatment or cure to these diseases.

Figure 2: Researchers and graduate students work in the labs of the University of Chicago to test the gut-brain connection

In addition to this, researchers have also found evidence that suggest gut bacteria might have the ability to influence depression and anxiety disorders. Stephen Collins, a researcher from McMaster University, did a study on mice to test the hypothesis of whether bacteria affects these mental health disorders. He took a strain of gut bacteria from mice that are prone to anxiety like behaviors, only to inject it into mice that were inclined to be calmer and healthier. The calm mice started showing signs of this anxiety-like behavior, which suggests that these 2 strains of bacteria did directly influence the mental disorder of anxiety in these healthy mice. Many researchers also believe that these gut microbiota can create the body’s chemical messengers called neurotransmitters. These neurotransmitters: serotonin, GABA, and dopamine all help to regulate mood and emotion. This connection leads to the possibility that the micro bacteria in our stomach can be used as a supplement for antidepressants as they increase the levels of these hormones in the body. These bacteria could also be beneficial by helping to regulate the way our body systems metabolize the neurotransmitters, affecting the amount that circulates through our blood and into the brain. More studies are being done, to test this hypothesis on humans as well.

One important understanding that scientists have identified is the vagus nerve. This is a paired nerve that helps to link the brain to other parts of the body, or in this case, the stomach, explaining how this connection exists. Not until recently, has the connection between the microbiota in the gut and mental health/behaviors ever been explored. In addition to the vagus nerve, the gut is connected to the body’s immune system, which in itself influences mood.

Figure 3: This figure outlines the communication between the brain and the gut through the vagus nerve and the immune system.

All things considered, the gut microbiota seems to have a direct link to the brain and its influences on our behaviors and emotions. As researchers continue to dig up the truth behind this connection, there are many inquiries into how it could be manipulated to help treat patients with mental disorders. Some say it could be used a replacement for the drug Prozac now, and some say it could be used to further understand the connections between these diseases and how they could be used to synthesize cures. This connection could be the change we need in current medicine to treat diseases as microbes have been living in the human gut for years.  Having been adapted to the experience of modifying our behaviors and secreting neurotransmitters to affect our mood, they become a more reliable source for treatment than recent pharmacological approaches.