Episodic microaspirations are the rule rather than the exception, which was previously understood as an anatomical flaw. But according to recent study data, enriching the lungs with pharyngeal bacteria may actually have a concrete benefit.
Aspiration is a common and feared complication in diverse clinical settings - from anaesthesia induction, to foreign body asphyxia in children, to the high prevalence of aspiration pneumonia in the elderly. So it seems puzzling why nature has made it comparatively easy for such events to occur. With every bite of food and every sip of liquid, only a small piece of tissue (and an ingenious coordination of muscles) prevents stomach or throat contents from taking the wrong path.
A mishap of evolution? Should this precarious proximity of trachea and oesophagus put us at a disadvantage compared to animals like dolphins and whales, which have completely separate entrances for air and food? Exciting findings from a study recently published in the American Journal of Respiratory and Critical Care Medicine suggest otherwise.1,2
Subclinical aspiration of throat contents is a ubiquitous occurrence, even in healthy, asymptomatic people. Recent studies of the microbiome have confirmed that the lungs of healthy humans and mice are continuously exposed to the bacteria of the pharyngeal flora, albeit in small amounts, but correlating well with the host's basal immune response.
Although the existence of a dynamic low-biomass lung microbiome in healthy individuals and its importance are receiving increasing attention, the immunological and clinical implications of this constant microbial bombardment remain poorly understood. The authors of the current study found that a single aspiration event with oral commensal bacteria is rapidly cleared from the lower respiratory tract in otherwise healthy mice, but triggers a sustained Th17 response that consecutively reduces susceptibility to S. pneumoniae (Th17: IL17-producing T helper cells).
They used a novel murine model of microbial aspiration with human oral commensal bacteria. After simulating subclinical microaspirations with non-pathogenic bacteria, they recorded subsequent changes in lung microbiome composition, host immune response and susceptibility to infectious attack. As expected, aspiration of human commensals (Prevotella melaninogenica, Veillonella parvula and Streptococcus mitis) triggered a predictable change in the lung microbiota that regressed after five days. Remarkably, these experimental animals were protected from subsequent infection with S. pneumoniae for up to two weeks after a single aspiration.
So the fact that we don't have a separate blowhole on our heads is probably a good thing after all. Incidentally, whales and dolphins have a higher risk of pneumonia, which is a common cause of death in them.
These observations open up exciting questions and spur further work in the future. Among other things, the situation in humans and the possible effects on other pathogens need to be investigated. "The amount of unknowns about what keeps our lungs healthy and how we can try to mimic that feels limitless," comments a colleague of the researchers who works at the New York University Langone Medical Center.3
"These data suggest an immunoprotective role for episodic microaspiration of oral microbes in regulating lung immunophenotype and attenuating host susceptibility to infection with lower respiratory tract pathogens," the authors conclude.
Sources:
1. Aogáin, M. M., Baker, J. M. & Dickson, R. P. On Bugs and Blowholes: Why Is Aspiration the Rule, Not the Exception? American Journal of Respiratory and Critical Care Medicine (2021) doi:10.1164/rccm.202011-4257ED.
2. Wu, B. G. et al. Episodic Aspiration with Oral Commensals Induces a MyD88-dependent, Pulmonary T-Helper Cell Type 17 Response that Mitigates Susceptibility to Streptococcus pneumoniae. American Journal of Respiratory and Critical Care Medicine (2021) doi:10.1164/rccm.202005-1596OC.
3. shari brosnahan, md (@ShariBrosnahan) / Twitter. Twitter https://twitter.com/ShariBrosnahan