When we’re born, our lungs are thought to be sterile. But from the moment we take our first breath, our pristine lungs are exposed to all the bugs that are in the air. It has become clear in the last 10 years that the lungs rapidly acquire a population of many different microorganisms (mostly bacteria and viruses) that colonise the lungs and remain with us for the rest of our lives. This population of bugs is called the lung microbiome.
We now know more about the lung microbiome thanks to genetics. In the past, identifying the types of bugs present in the lungs depended on being able to grow them in a laboratory, and for many types of bug this was difficult. The big change that happened recently is our ability to recognise both the different bug species, and their relative abundance, by using DNA sequencing. This can be done either from a sample taken from the lungs or from sputum (the mucus we cough up when we have an infection).
Is the lung microbiome a good or a bad thing?
We all know that bacteria in the lungs can be harmful. When harmful bacteria multiply, they cause pneumonia which, despite the existence of antibiotics, can still be deadly. However, it seems that the lung microbiome usually exists in a balanced state, such that harmful types of bugs do not increase in number sufficiently to cause pneumonia. In fact, it’s possible that the very presence of such a diverse range of bugs in the lungs is one of the reasons it’s quite difficult for harmful bugs to multiply and cause disease.
There are a number of reasons why the lung microbiome is important. It can alter when we have underlying chest diseases, especially those in which there are marked structural changes in the lungs. A good example is cystic fibrosis. In this disorder, a fault in a gene changes a protein that regulates the movement of salt in and out of cells. It results in sticky sputum and infections, which in turn can cause structural damage to the airways – a condition called bronchiectasis.
This makes it easier for some kinds of bacteria – especially Pseudomonas – to grow in the lungs, and it becomes very difficult to get rid of these bacteria. The continuing inflammation in the lungs leads to more damage and the chest disease worsens. Treatment is therefore aimed at trying to eradicate these damaging bacteria, but it’s very difficult to eradicate infection once it has become established.
The difficulties of treating infection in patients with cystic fibrosis have been known for a long time. Recently, attention has been focused on trying to identify if changes in the lung microbiome are present in other more common chest diseases, such as asthma or chronic obstructive pulmonary disease (COPD). Recent work has suggested that the bugs grown from airways in patients with these conditions can be different from bugs grown in healthy people. This raises the intriguing possibility that the different bugs present may, in part, be responsible for either the development of these diseases, or for altering the severity of disease once it develops.
If it does prove to be the case that the different types of bugs in our lungs are in part responsible for development of chest disease, it follows that altering the lung microbiome might be a useful treatment. Directly altering the lung microbiome by administering “friendly” bacteria into the gut or lungs has mostly been tested in animals to date. There are, however, examples of this approach working in other kinds of disease, particularly for conditions affecting the gut, for example for the treatment of irritable bowel syndrome.
Altering the lung microbiome
I think the most important question about the lung microbiome is whether or not altering it could be helpful in treating or preventing chest disease. A good example is asthma. It’s well recognised that asthma seems to be more common in countries where early childhood exposure to infection is low. It has also been suggested that this increase in risk is due to lack of exposure to bugs in early life (an idea called the hygiene hypothesis.
Some work has suggested that asthma risk could be reduced by encouraging early exposure to bugs, for example by spending more time in dirtier environments such as farms. Whether or not this approach works in the long term remains unclear, but if it does, it seems likely that changes in the lung microbiome may in part be responsible for the beneficial effects.