The human respiratory system is a habitat for a vast community of microorganisms known as the commensal respiratory flora. This community is comprised of bacteria and fungi that reside on the surfaces of our airways. The term “commensal” describes a relationship where these microbes benefit from living in our respiratory tract, while their human host is not significantly harmed or helped.
These resident microorganisms are a normal part of the body’s ecosystem. They have co-evolved with humans, adapting to life on the mucosal surfaces that line our breathing passages. This relationship means that while they derive nutrients and shelter from our bodies, they do not cause illness in healthy individuals.
Composition of the Respiratory Microbiome
The respiratory system is divided into two main sections, the upper and lower tracts, each hosting a distinct microbial population. The upper respiratory tract, which includes the nose, sinuses, and pharynx (the back of the throat), is densely populated with a wide variety of microbes. This area is a primary interface with the outside world and is constantly exposed to microorganisms from the air we breathe, contributing to its diverse composition.
Common inhabitants of the upper tract include genera like Staphylococcus and Streptococcus. For instance, Staphylococcus epidermidis is a frequent resident of the skin inside the nostrils, and various non-harmful Streptococcus species are found in the throat. Other bacteria such as Prevotella, Haemophilus, and Neisseria species are also considered normal members of this community.
Conversely, the lower respiratory tract, encompassing the bronchi and the alveoli within the lungs, was once believed to be sterile. Modern research has shown that it contains a much smaller and less diverse microbial population than the upper tract. The microbes found here are often transient, arriving from the upper airways through micro-aspiration, the inhalation of tiny droplets from the throat.
The Protective Role of Commensal Flora
The community of microbes in our airways provides a protective barrier against potential invaders, partly through a process called competitive exclusion. The resident harmless bacteria occupy physical space on the mucosal surfaces and consume available nutrients. This makes it more difficult for incoming, potentially harmful pathogens to establish a foothold and multiply.
Some members of the commensal flora can also produce their own antimicrobial substances, which can directly inhibit or kill pathogenic bacteria. For example, certain strains of non-pathogenic Streptococcus in the throat can interfere with the growth of Streptococcus pyogenes, the bacterium responsible for strep throat. This active defense helps maintain a healthy balance within the microbial community.
The constant presence of this flora also plays a part in educating the local immune system. The interaction between the commensal microbes and immune cells in the respiratory lining helps train the system to distinguish between harmless residents and dangerous pathogens. This “education” ensures the immune system remains prepared to fight off invaders while tolerating neutral organisms, preventing unnecessary inflammation.
Factors That Disrupt Respiratory Balance
The balance of the respiratory microbiome, sometimes called dysbiosis, can be disturbed by several factors. The use of broad-spectrum antibiotics is a significant disruptor. These medications can wipe out large portions of the beneficial commensal flora, leaving the respiratory tract vulnerable to opportunistic pathogens.
Viral infections, such as influenza or the common cold, can also alter the respiratory environment. These viruses can damage the cells lining the airways, making it easier for secondary bacterial infections to take hold. The initial viral illness can allow bacteria, including members of the normal flora, to overgrow and cause conditions like bronchitis or pneumonia.
Environmental exposures play a role in shaping the respiratory microbiome. Inhaling pollutants from sources like cigarette smoke or poor air quality can introduce irritants into the lungs. This can cause inflammation and shift the composition of the microbial community, potentially favoring the growth of less desirable bacteria. An individual’s age and the state of their immune system also influence the stability of this microbial ecosystem.
The Link to Respiratory Conditions
A disruption in the normal respiratory flora is increasingly being linked to a variety of respiratory health issues. When the protective commensal microbes are diminished, a person’s susceptibility to acute infections can increase. This allows pathogens to more easily colonize the airways, leading to infections like sinusitis, pneumonia, or bronchitis.
For chronic conditions, research suggests a strong association between an imbalanced microbiome and the severity of diseases like asthma and Chronic Obstructive Pulmonary Disease (COPD). In asthma, for example, certain microbial profiles have been associated with increased inflammation and more severe symptoms. The absence of specific beneficial bacteria may impair the normal development of immune tolerance in the airways.
Similarly, in individuals with COPD, studies have noted differences in the lung microbiome compared to healthy individuals, often with a lower diversity of bacteria. These changes are believed to contribute to the chronic inflammation that characterizes the disease. For many of these chronic conditions, the relationship is still being studied to determine if the microbial imbalance is a cause or a consequence of the disease.