The human body is an ecosystem, home to trillions of microorganisms. This collection of microbes, primarily bacteria, is a normal and necessary aspect of human biology, not a sign of illness. These resident bacteria, known as host bacteria, have co-evolved with humans over millennia, forming intricate connections that influence health. The term “host bacteria” defines the relationship between these microscopic organisms and the larger organism they inhabit.
Types of Host-Bacteria Interactions
The interactions between a host and its resident bacteria can be categorized into three main types, defined by the outcome for both organisms. These relationships are not always fixed and can shift based on changes within the host’s body or the environment. This framework helps explain how different bacteria can support health or contribute to disease.
The most beneficial of these relationships is mutualism, where both the host and the bacteria gain advantages. A classic example is the relationship between humans and certain gut bacteria. The bacteria receive a nutrient-rich environment and, in return, they help digest food components that the human body cannot break down on its own.
Another form of interaction is commensalism, where one organism benefits, and the other is unaffected. The commensal bacterium gains a place to live and access to nutrients without causing harm or providing any significant benefit to the host. Many bacteria living on human skin are considered commensals, as they feed on dead skin cells and secretions with a neutral effect.
The third category is parasitism, a relationship where the bacterium, a pathogen, benefits at the expense of the host. These are the bacteria associated with illness and disease. For instance, Streptococcus pyogenes can colonize the throat and cause strep throat, deriving nutrients while causing inflammation and pain for its human host.
The Human Microbiome
The vast communities of microorganisms living in and on the human body are collectively known as the human microbiome. This “hidden organ” is composed of bacteria, viruses, fungi, and archaea, with bacteria being the most numerous members. The genetic material of these microbes vastly outnumbers the genes in the human genome, providing an expanded set of functional capabilities that humans rely on.
The gut microbiome is the largest and most complex of these communities, containing trillions of microbial cells. The majority of these are anaerobic bacteria, thriving in low-oxygen environments like the large intestine. The oral microbiome is the second-largest and most diverse microbial community, as the mouth provides many unique surfaces for bacteria to colonize, from teeth to the soft tissues of the gums and tongue.
The skin also harbors its own complex ecosystem of bacteria, known as the skin microbiome. These communities vary significantly depending on the location, influenced by factors like moisture, temperature, and oil content. The bacteria on the oily skin of the face are different from those in the moist environment of the armpit or the dry surfaces of the forearm.
Functions of Beneficial Bacteria
Bacteria in a mutualistic or commensal relationship with their host perform a wide range of functions for human health. Their metabolic activities and interactions with human cells contribute to nutrition, immune function, and protection against disease. Key functions include:
- Nutrient metabolism: Gut microbes possess enzymes to ferment complex carbohydrates like dietary fiber into compounds the body can absorb and use for energy. This process also produces beneficial byproducts like short-chain fatty acids (SCFAs), which are a primary energy source for the cells lining the colon.
- Immune system regulation: Early-life exposure to a diverse range of microbes helps “train” the immune system to differentiate between harmless resident bacteria and harmful invading pathogens. This education is important for preventing inappropriate immune responses, and a healthy microbiome helps maintain a balanced immune state.
- Vitamin synthesis: Gut bacteria synthesize several vitamins that are important for human health, including vitamin K for blood clotting and various B vitamins like B12 and folate. The body can then absorb and utilize these vitamins, supplementing what is obtained from diet.
- Protection against pathogens: A healthy microbial community provides a strong defense through a process known as colonization resistance. The established beneficial microbes outcompete newcomers for space and nutrients, making it difficult for invading pathogens to cause infection. Some also produce natural antimicrobial compounds that inhibit harmful species.
Disruption of the Host-Bacteria Balance
The balanced relationship between a host and its resident bacteria can be disturbed, leading to a state known as dysbiosis. Dysbiosis is an imbalance in the composition and function of the microbial community, often involving a loss of beneficial bacteria or an overgrowth of harmful organisms. This disruption can compromise the beneficial functions the microbiome provides.
One of the most common causes of dysbiosis is the use of antibiotics. While effective at killing pathogenic bacteria, they are often not specific and can eliminate large numbers of beneficial microbes as well. This can create an opportunity for antibiotic-resistant or harmful bacteria to proliferate in the absence of their competitors.
Diet and lifestyle factors also play a role in shaping the microbiome. Diets low in fiber and high in sugar and processed foods can starve beneficial bacteria that rely on complex carbohydrates. Chronic stress, both physical and psychological, has also been shown to negatively impact the balance of gut bacteria and can weaken the immune system.
An imbalanced microbiome is associated with a range of negative health consequences. It can lead to gastrointestinal issues such as inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). Beyond the gut, dysbiosis has been linked to an increased risk of infections, metabolic disorders like obesity and diabetes, and effects on mood through the gut-brain axis.