Our bodies host a vast microbiota, a world of microscopic organisms. While some microbes are commonly associated with illness, many others coexist with us in a balanced relationship, offering benefits without causing harm. These beneficial residents, known as commensal microbes, are found throughout the human body, playing important roles in maintaining overall well-being.
Defining Commensalism
Commensalism describes a biological interaction where one organism benefits, and the other is neither helped nor harmed. This contrasts with mutualism, a relationship where both organisms derive a benefit, and parasitism, where one organism benefits at the expense of the other. In commensal relationships, the “commensal” typically gains advantages like shelter, food, or transportation, while the “host” remains largely unaffected. For instance, certain birds may build nests in trees, gaining shelter without harming the tree. Another example involves golden jackals following tigers to feed on their leftover scraps; the jackals benefit from an easy meal, and the tigers are not impacted. These interactions highlight how one party can thrive without negatively affecting the other, a principle that applies to the microbial world.
Commensals in the Human Body
The human body provides diverse environments for commensal microbes, with distinct communities residing in different areas, particularly on surfaces exposed to the external environment.
On the skin, Staphylococcus epidermidis is a common commensal that helps protect against harmful bacteria by competing for resources and space. This bacterium also contributes to skin barrier integrity by producing substances preventing skin dehydration.
The gut harbors an exceptionally diverse community, with Bacteroides species being among the most abundant. These bacteria play a role in processing complex molecules and are generally beneficial.
In the mouth, Streptococcus salivarius is an early colonizer, establishing itself shortly after birth and remaining prominent. This microbe contributes to a balanced oral microbiota and can even interfere with the growth of certain respiratory pathogens.
The urogenital tract, particularly in women, is typically home to Lactobacillus species. These lactobacilli help maintain an acidic environment, which discourages the growth of many harmful bacteria and contributes to urogenital health.
While these microbes are normally commensal, their status can change. If conditions within the body are altered, such as a weakened immune system or a shift in their location, some commensals can become opportunistic pathogens and cause disease.
Unseen Contributions to Health
Commensal microbes contribute to our health in ways that are not always immediately apparent. One significant contribution is competitive exclusion, where commensals occupy available space and consume resources, thereby preventing pathogenic microbes from establishing themselves. This “first come, first served” strategy limits the ability of invaders to colonize the body.
The continuous presence of commensal microbes also plays a part in the development and maintenance of our immune system. They help “train” the immune system to distinguish between harmless entities and potential threats, contributing to a state of immune tolerance. This interaction helps prevent the immune system from overreacting to benign substances while remaining ready to respond to actual pathogens.
Some commensals subtly influence their local environment, making it less hospitable for undesirable microbes. They can alter factors like pH levels or oxygen availability, which creates conditions that are unfavorable for the growth of many pathogens. This environmental modification serves as another layer of defense, indirectly supporting our health by maintaining a balanced microbial ecosystem.
Factors Influencing Commensal Communities
The composition and stability of commensal microbial communities are shaped by various factors, beginning from early life. The initial acquisition of microbes occurs during birth, with vaginally born infants exposed to maternal bacteria, while those born via C-section are colonized by environmental microbes. Infant feeding patterns, such as breastfeeding versus formula feeding, also influence the early development of the gut microbiota.
Throughout life, several factors can disrupt the delicate balance of these microbial communities. Diet plays a significant role, with different dietary patterns influencing the diversity and types of microbes present. For instance, a diverse diet generally supports a healthy, varied microbiome.
Antibiotic use is another major disruptor, as these medications can eliminate beneficial bacteria alongside the target pathogens, leading to imbalances. Hygiene practices, environmental exposures, and even psychological and physical stress can also impact the composition of commensal communities. An imbalance in these communities is known as dysbiosis, which can involve a loss of beneficial organisms, an increase in potentially harmful ones, or a reduction in overall microbial diversity.