by Mechas
With few well-delivered words, author Annie Dillard distills the essence of our lives: "How we spend our days is, of course, how we spend our lives." In this seemingly effortless string of words, she captures a simple truth: our lives are stitched from numerous activities, many of which go largely unnoticed. One of these is breathing, which is essential but only occasionally acknowledged.
Pause and take a breath. Breathing moves air in and out of the lungs where gas is exchanged, eliminating carbon dioxide, the side product of metabolism, via exhalation and bringing in oxygen for cellular energy production. Normal breathing occurs unconsciously (fortunately) and is regulated by a part of the nervous system that controls other vital bodily functions like heart rate and digestion.
This process of inhalation also exposes us to a continual flow of air, particles, and microbes. We breathe in and out an estimated 22,000 times a day, taking in about 11,000 liters of air each day. How do we avoid being overwhelmed by the inhalation of microbes, some of which could be potentially harmful? Thankfully, we have evolved an immune system and mechanisms like coughing that protect us against harmful agents. On rare occasions, microbes breach our defenses and cause disease, as occurred with the SARS-COV-2 virus during the recent COVID-19 pandemic and as happens with pathogens like Mycobacterium tuberculosis.
The lung microbiome. Image credit: Mechas Zambrano
Until recently, lungs were believed to be sterile. Sequencing technologies that are sensitive enough to probe environments with low amounts of microbes like the lungs have, however, recently debunked the notion of the lung as a sterile site. This should not have come as a surprise, given that microbes are found in almost all environments studied, including the air we breathe. In fact, the lungs in healthy individuals contain a distinctive lung microbiome made of bacteria, fungi and viruses. This microbiome originates mainly from microbes found in the upper respiratory tract, which includes body sites heavily exposed to the external environment like the nose and mouth. However, the lungs have fewer microbes, thanks to mechanisms we use for clearance and protection from microbial intruders, and the communities differ in composition from those found in the upper respiratory tract.
What is of course more interesting is that these lung microbiomes are also important for host health and for airway mucosal immunity. Alteration, or dysbiosis, of these communities is therefore associated with the development and progression of various respiratory diseases. Despite the importance of these microbes to our health, disentangling their effect on the host or, perhaps more difficult still, deciphering how to preserve those that are beneficial remains a conundrum.
The quality of the air we breathe became of paramount concern during the recent COVID-19 pandemic, forcing us to gear up to protect ourselves and others from infection. As often occurs, this heightened awareness tends to wear off with time and we revert to basal pre-pandemic levels of protection. We have again become oblivious to the ongoing influx of microbes. We are even less aware of the constant interactions taking place within our airways, necessary to fend off intruders, and those occurring among incoming microbes and our resident airway microbiomes.
Take a minute to appreciate the act of breathing. Both our defenses and resident microbiomes industriously work to ensure healthy, albeit not necessarily sterile, airways fit to capture oxygen while resisting the continual encroachment of inhaled microbes.
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