The human body is a complex ecosystem, home to trillions of microscopic organisms like bacteria, fungi, and viruses. These microbes are found on our skin and within our digestive tract. The field of host-microbe interactions studies the communication between our bodies (the host) and these microorganisms. This dynamic relationship involves the entire collection of microbes, known as the microbiome, which influences our health and is involved in many bodily functions.
The Spectrum of Microbial Relationships
The relationships between a host and its microbes exist across a spectrum, categorized by the outcome for each participant. In a mutualistic relationship, both the host and the microbe benefit. A well-known example is the gut bacteria that help break down complex food components for us, while we provide them with a stable, nutrient-rich environment.
Another interaction is commensalism, where one organism benefits while the other is unaffected. Many bacteria on human skin are considered commensal. They consume dead skin cells and oils for survival but, under normal circumstances, provide no advantage or disadvantage to the person they inhabit.
The final category is parasitism, a relationship where the microbe, or pathogen, benefits at the expense of the host. This is the dynamic we associate with getting sick, such as when a virus hijacks our cellular machinery to replicate and cause illness.
How the Body Manages Microbes
The immune system acts as a manager of the body’s microbial communities, tasked with more than just eliminating invaders. A primary function is to distinguish between beneficial microbes, harmful pathogens, and neutral residents. This recognition is mediated by specialized proteins on the surface of immune cells called Pattern Recognition Receptors (PRRs), which identify molecules on microbes known as Pathogen-Associated Molecular Patterns (PAMPs).
When a pathogen is detected, the immune system launches an inflammatory response, recruiting immune cells to the site of infection. This process involves the release of signaling molecules like cytokines and chemokines, which orchestrate the body’s defensive actions to neutralize the threat.
Conversely, the immune system must learn to coexist with beneficial microbes. This process, known as immune tolerance, is active in the gut, where the immune system is trained to recognize resident microbes as allies. This selective tolerance prevents chronic inflammation and allows beneficial microbes to function without interference.
Beneficial Partnerships in Action
Mutualistic partnerships with gut microbes provide several advantages, such as the digestion of dietary fibers. Human digestive enzymes cannot break down many complex carbohydrates in plants. Gut bacteria possess the enzymes to ferment these fibers, releasing energy and valuable compounds.
Through their metabolic activities, these microbes produce nutrients the human body cannot synthesize on its own. For instance, gut bacteria are a primary source of Vitamin K, which is needed for blood clotting, and also produce several B vitamins. These vitamins are absorbed through the intestinal wall and contribute to overall health.
A healthy and diverse microbiome also offers protection by preventing pathogenic organisms from taking hold, a concept called “colonization resistance.” The sheer number of beneficial microbes occupies physical space on the intestinal lining and consumes available nutrients. This competition leaves very little for invading pathogens to use for their own growth, making it more difficult for them to establish an infection.
When Interactions Lead to Disease
Interactions with microbes can shift to become harmful, leading to disease. The most direct path is pathogenesis, where an invading microbe causes damage to the host. In this parasitic relationship, the pathogen infects tissues and replicates, often producing toxins that disrupt cellular functions and cause illness. An example is the bacterium Streptococcus pyogenes, which causes strep throat.
Disease can also arise from an imbalance within the microbial community, a state known as dysbiosis. This is a disruption of the normal composition of the microbiome, not an infection from an outside pathogen. Factors like a course of antibiotics or a significant change in diet can trigger this imbalance.
In a state of dysbiosis, potentially harmful resident microbes that are normally kept in check can overgrow, or beneficial microbes can be depleted. This shift leads to a dysfunctional microbial community that no longer provides its protective and metabolic benefits.
This imbalance has been linked to a range of chronic health issues, including inflammatory bowel disease and allergies. The altered microbiome can contribute to chronic inflammation and improper immune responses.