- Scientists reviewed evidence that links the community of microorganisms living in the lungs — the lung microbiome — with lung cancer.
- They conclude that there are opportunities to use the microbiome to diagnose lung cancer early and improve the survival chances of those with the condition.
- However, many uncertainties remain about how particular bacteria contribute to lung cancer.
- Unlike the gut microbiome, the lung microbiome is difficult to sample and study.
Experts used to think that healthy lung tissue is a sterile, microbe-free environment. However, recent advances in genome sequencing technology have shown that this is not the case.
The latest research suggests that the community of bacteria, fungi, and viruses that live in the lungs, known as the lung microbiome, has links with several respiratory conditions.
These conditions include chronic obstructive lung disease (COPD), asthma, and cystic fibrosis.
In addition, the microbiomes in the gut, mouth, and lungs may interact to cause lung disease.
Researchers led by the Shanghai Jiao Tong University School of Medicine in China reviewed existing knowledge about the links between microbiomes and lung cancer and how this could help with the diagnosis and treatment of the condition in the future.
They report their findings in the journal npj Precision Oncology.
Diagnosis comes too late
Lung cancer is the leading cause of cancer-related deaths worldwide. This is mainly because individuals have few symptoms in the early stages, which means that around 75% of cases are advanced when doctors diagnose them.
Treatment options for advanced lung cancer are limited, and the prognosis is poor.
In the United States alone, around 130,000 people are likely to die from the condition in 2022.
“Therefore, early detection and improved treatments for lung cancer are becoming increasingly urgent,” the authors of the review state.
They report that in healthy lungs, Propionibacterium, Streptococcus, Haemophilus, and Veillonella bacteria coexist with fungi such as Aspergillus, Penicillium, and Candida.
By contrast, Pseudomonas, Streptococcus, Staphylococcus, Veillonella, and Moraxella bacteria are often associated with lung cancer.
“However, there is no widely accepted consensus definition for healthy or harmful lung microbiota,” said the researchers.
A 2019 pilot study found associations between the lung microbiota and the participants’ chances of survival.
“We have investigated further in a large study, showing the lung microbiome as a potential predictive biomarker for lung cancer treatment outcomes,” said senior author Prof. Jiyoung Ahn, Ph.D., who is a cancer molecular epidemiologist at New York University School of Medicine.
She told Medical News Today that the paper is currently undergoing peer review prior to publication.
“I think the microbiome is a potential biomarker for cancer treatment outcomes, which helps patients and clinicians make informed decisions, but we still need rigorous studies to test the hypothesis,” said Prof. Ahn.
Linked communities
The communities of microbes that live in the lungs, mouth, and gut appear to interact.
This may happen directly through the dispersion of mucus and through respiratory and digestive processes, say the researchers, or it may happen indirectly via immune factors and metabolic products in the bloodstream.
Research suggests that these modes of communication may allow “unfriendly” gut microbes to play a role in lung conditions, such as COPD, asthma, cystic fibrosis, and lung cancer.
In addition, a 2018 study found that people with lung cancer had more oral bacteria in their lungs than the participants in the control group. The study authors associate the presence of the bacteria with increased activity in cancer signaling pathways.
How these interactions work remains elusive.
The authors of the present review conclude,
“Understanding the relationship between human microbiota, especially gut microbiota, and lung cancer, may open a new window for the diagnosis and treatment of lung cancer.”
However, possible treatments, which include probiotics, special diets, and fecal microbiota transplants from healthy individuals, remain at a very early stage of development.
Moreover, the authors note that taking direct samples from the lungs involves technical and ethical challenges. This is not the case with the gut microbiota, which is relatively easy to study using fecal samples.
The researchers write that, as a result, most studies of the lung microbiota rely on nasal secretions, saliva, sputum, and a diagnostic test that involves flushing fluid through a small part of the lung. Experts call this bronchoalveolar lavage.
Possible mechanisms
The authors propose several ways in which bacteria could cause lung cancer.
For example, damage to the mucous lining of the lung that is not repaired quickly could upset the balance of microbes and provoke inflammation, potentially leading to cancer over time.
Older research in mice suggests that bacteria in the lungs could trigger inflammation, which in turn may lead to the development of lung cancer.
Alternatively, toxins or metabolites that bacteria produce could directly damage human DNA.
On the plus side, the researchers write that there is “inestimable” potential to enhance different treatments for lung cancer, including radiation therapy, chemotherapy, surgery, and immunotherapy.
For example, probiotics or fecal microbiota transfers could optimize the gut microbiota to enhance the efficacy of chemotherapy and minimize side effects.
There are also hints that the gut microbiome helps determine the success of immune therapy.
In a 2019 study, participants with a higher diversity of bacterial species in their gut microbiota responded better to immunotherapy for a type of advanced lung cancer.
By contrast, a 2017 study found that antibiotics greatly reduced the efficacy of immune therapy.
The review authors point out, however, that many more studies and clinical trials will be needed to determine whether modulating the gut microbiota can improve responses to treatment for lung cancer.
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