What Are Gut Bacteria Doing in Critically Ill Lungs?

New discovery could change ICU care

Researchers at the University of Michigan Medical School have discovered that bacteria that normally live in the gut can be detected in the lungs of critically ill patients. The more severe the patients’ illness, the more their usual lung bacteria were outnumbered by the misplaced gut bacteria. The new findings were published in Nature Microbiology.

“Our results suggest that in our past attempts to find treatments for sepsis and ARDS [acute respiratory distress syndrome], we may have been overlooking a major part of the story,” said lead author Robert P. Dickson, MD. “Virtually all of our attempts to treat these critical illnesses have been aimed at fixing the disordered inflammation and tissue injury we can see in our patients. But our study raises the possibility that this inflammation and injury may actually be downstream consequences of an upstream source: disordered bacterial communities in the gut and lung.”

Dickson and his colleagues believe that patients with these conditions may be stuck in a vicious cycle caused by dysbiosis––an out-of-whack microbiome. Based on their findings, they suggest that the insult that caused the patient’s illness triggers a “chicken and the egg” feedback loop.

Changes in the microbiome lead to inflammation as the body’s immune system tries to fend off what it sees as invaders, and that inflammation, in turn, injures delicate lung tissue. The combination of inflammation and damaged tissue changes the environment within the lung, allowing microbes that don’t normally grow there to invade the tissue or to “bloom” if they were already present in low levels.

Improving the survival of critically ill patients, therefore, would require breaking that cycle––which means figuring out how to keep the microbiome relatively normal, the researchers say.

One possible explanation is that in patients with critical illnesses, the walls of the intestines get “leaky,” and bacteria escape and travel upward into the lungs. Another potential explanation is that small numbers of these gut bacteria were present in the lungs all along, but couldn’t grow for lack of the proper environmental conditions.

Dickson notes that the new findings help explain something that critical care teams have known for decades: that the gut microbiome is somehow linked to a person’s chances of surviving a critical illness. Animal studies have shown since the 1950s that pretreatment of the gut with antibiotics before trauma or other critical illness can protect against lung injury and death.

“In the long run, we need to start thinking of the microbiome as an organ that can fail in critically ill patients,” Dickson said. “We’re studying how it gets disordered, how it impacts other organs, and how we can fix it. The importance of the microbiome in the ICU [intensive care unit] has been clear for decades, but with these new tools we’re finally able to ask and answer the right questions.”

Source: University of Michigan Health System; July 18, 2016.