They traveled a huge distance, evaded a protective barrier and found themselves in a strange and unwelcoming land.

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They may be linked to dangerous diseases and are under constant threat of being expelled from their adopted home. Their contribution to the greater community is only beginning to be understood. And every day, more of them arrive.

They’re bacteria living in human lungs.

New research pinpoints just how they get there, and it opens the door to more work on what happens to them — and our bodies — as a result.

Writing in the journal mBio, researchers from the University of Michigan Medical School and VA Ann Arbor Healthcare System offer microbiome-based evidence that most of the bacteria in the lungs of healthy people got there by way of microaspiration.

In other words, they ride in on tiny droplets of saliva that made it from the microbe-filled mouth to the lungs. That means they avoided the movable tissue barrier, called the epiglottis, which keeps most saliva from getting into the lower respiratory tract.

By studying the DNA of these bacteria throughout the lungs of healthy volunteers, the researchers confirmed that the population of microbes in the lungs closely resembles the population found in the mouth. And by studying the microbes’ distribution within the airways, the researchers could determine their most likely entry route.

They found that many of the immigrant microbes make their home near the main carina, the spot at the end of the trachea where the airway branches off to the left and right lungs. This spot in the lungs is a “landing pad” where aspirated saliva — because of gravity and our upright posture — is likely to collide with the airway. But some bacteria manage to make it all the way to the deepest reaches of the pulmonary system and reside in the tiny air sacs called alveoli.

Wherever the bacteria land, the researchers found, they join a community made up mostly of other recent immigrants. Few microbes are thought to be long-term residents of healthy lungs. Unlike the gut, healthy lungs are an inhospitable environment for bacteria, with little nutrition and constant surveillance by the immune system.

“This is the most comprehensive topographic survey of the healthy lung microbiome to date. It adds to the evidence that healthy lungs are like an island whose population is determined by the balance of immigration and elimination of species: who moves in and who moves out,” says Robert P. Dickson, M.D., the first author of the new study. “The microbiome of the lung plays by a different ecologic rulebook than the gut microbiome, and this study helps clarify what those rules are.”

Inspired by classic models of ecology, Dickson and his colleagues propose an “adapted island model” of the lung microbiome, in which the competing pressures of microbial immigration and elimination determine the lung’s ecosystem.