Gut Microbiota, the Immune System and Disease

by Shane Bross, MBS 2018,
Geisinger Commonwealth School of Medicine
Mentor: Jennifer Boardman, PhD

Every person on this planet has a gut that is home to trillions of microorganisms. Though it has been established that multiple species of bacteria make our gut their home, the reasoning behind why we have these microscopic friends may not be so clear. Exploring the relationship between ourselves and our intestinal occupants sheds some light on how the gut microorganisms influence multiple components that contribute to a healthy life. More importantly, these little guys in our guts significantly impact our immune system, the breakdown of which can lead to different types of diseases.

By focusing on how these organisms relate to one another and to their physical surroundings (our gut), as well as considering the fact that humans and microbes evolved together, researchers have been able to establish how various forces allow such diverse populations of microbes to thrive in the human intestine. Additional studies have produced findings that reveal how our miniscule microbes shape intestinal immune responses by affecting the development of immune-specific tissues. Researchers have used mouse models as a comparative tool to explore how immune-system function can be impacted by the presence of bacteria in the gut. The bacteria in our gut have also been investigated in regard to their potential influence on specific genes are responsible for shaping our immune system.

Shortly after we are born, our environment, experiences and the foods we eat all contribute to forming our own unique gut bacterial populations -- our gut microbiota. These tiny tenants are with us because, a long time ago and throughout the course of human evolution, they gave us advantages that helped us survive. As it turns out, we also help them by giving them a place to stay and food to eat. Think of this as an “I scratch your back, you scratch my back” kind of relationship. Essentially, we evolved with our microbiota because we need them and they need us.

So how exactly do they impact our health? We mentioned this before: they can shape our immune system. Normal proportions of different kinds of gut bacteria allow for proper immune-system regulation. When there is a change or shift in the number of certain gut microbiota, it causes a change in immune-system regulation, often resulting in inflammation. Think of this situation like a teeter totter: when there are healthy amounts of gut microbiota on each end of the teeter totter, there is a healthy balance (regulated) and a properly functioning immune system. When there is more of a certain kind of bacteria on one side of the teeter totter, the teeter totter shifts to that side, creating an imbalanced (improperly regulated), ill-functioning immune system, resulting in inflammation and diseases related to the intestines, thyroid, pancreas and even nervous system.

Our gut microbiota can also impact our immune system by creating certain types of products. The food we eat is broken down by our gut microbiota into certain products that impact the immune system. One of these products calls upon cells that help modulate the immune system (the same way a recruiter selects qualified candidates for an important job). The other product helps to guide the behaviors of different types of bacterial populations that are part of immune-related processes (the way a team leader guides and coordinates the actions of the team members). One study found that mice that were born and raised in one environment had immune systems that functioned significantly differently from mice raised in another environment. The mice without gut microbiota had compromised immune systems, whereas the mice that had an established gut microbiota population were healthy with a properly functioning immune system.

So why is this important? By going over what we have discussed, we should now understand the importance and significance of the role our gut microorganisms play in our immune system and overall health. Building upon this discussion and the gravity of these findings, it is my personal opinion that perhaps there should be additional effort invested in incorporating teachings focused on the ecology and evolutionary aspects of human health and disease. By incorporating these topics in the healthcare-related curricula, students and patients would benefit. Students could better understand how a person’s internal landscape and microbiota are unique, facilitating a more patient-centered approach. That being said, future questions surrounding these findings include investigations into potential therapies -- or even cures -- for immune-system-related diseases using metabolic products produced from gut microbiota. Also, identifying certain genetic variations that influence our relationship with the microbiota could potentially provide more effective therapies tailored for the individual patient.