Healthy lungs are home to bacterial communities, just as healthy intestines and skin are.
That was one finding of new research that also showed that while patients with cystic fibrosis may be hampered by the bacterial communities that live in their lungs, it is the composition of these colonies, rather than their mere presence, that causes problems.
"For many years, people have considered the lung to be a relatively sterile organ," said study researcher Dr. David Cornfield, a professor of pediatrics at the Stanford University School of Medicine. "This provides some of the first and most definitive data that there is a forest of flora that exist in the lung, that are present even in the healthy lung."
The finding also suggests that probiotics could help in the treatment of cystic fibrosis, a genetic disorder that results from receiving copies of a recessive gene from both parents. The disorder results in thicker, stickier mucus in the lungs and other organs, which allows certain bacteria to overgrow.
This can lead to infections and blockages, and eventually breathing difficulties, nutritional deficiencies, endocrine disorders such as diabetes, and infertility in men, among other problems. Patients tend not to live past age 40.
In the new study, researchers examined the sputum from 16 cystic fibrosis patients and nine healthy control patients, and surveyed the organisms present using gene sequencing. By examining the genes, rather than growing the bacteria in lab dishes, the researchers were able to get a better handle on the diversity of bacteria present.
While each person studied had a unique bacterial community in their lungs, the cystic fibrosis patients had communities that were more similar to each other than to those of the healthy people. The patients' communities were also less diverse.
"This suggests there's probably some benefit to having bacteria in the lungs, to some balanced degree," Cornfield said.
"The traditional paradigm, that all bacteria in the lungs are bad, is probably not correct."
Instead, he said the findings support the idea of thinking of the bacteria in the lungs as a "rain forest."
"We know that in a rain forest, if species are lost, there develops an imbalance, and the ecosystem becomes much more fragile," Cornfield said.
While the lungs have been thought of as sterile, this is not the first study to find bacteria in healthy lungs, although the idea of communities is still somewhat controversial.
"Anything that comes into contact with the world is not going to be sterile," said Katrine Whiteson, a postdoctoral fellow at San Diego State University who studies microbe communities in the human body.
Whiteson said one issue not addressed by this study, but that is being tackled elsewhere, is the question of whether bacteria remain in the lungs over the long term, or whether they are "tourists" — with some strains leaving a person's lungs and new ones entering.
The new research, Cornfield said, could lead to better treatments for cystic fibrosis patients. He said long-term antibiotics, a treatment given to many patients today, might become more effective if combined with probiotics to enhance bacterial diversity in patients lungs, although much more research is necessary to see if that is the case.
"That would be a paradigm shift of virtually epic proportions in the cystic fibrosis and even the lung biology community," Cornfield said.
The study appears online Sept. 26 in the journal Science Translational Medicine.
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