The bacteria living on and in the human body, collectively described as the microbiota, are ubiquitous and are estimated to outnumber human cells 10-fold. Given the vast bacterial communities within the human body and the variety of processes they are involved in, bacteria have substantial influences on health and disease cause and development. Microbes are residents in a number of body sites, including the oral and nasal cavities, which are connected to the lung via the throat.
Lung cancer is considered a terminal illness with a five-year survival rate of about 11%. It is the most common cause of cancer-related deaths in North America and worldwide. Although tobacco is the primary cause, 53% of lung cancer cases worldwide are not attributable to smoking, suggesting other factors may independently increase risk or modify the effects of smoking on risk. In particular, chronic inflammation may initiate and promote cancer formation.
The abundance of salivary bacteria is different between lung cancer and healthy people. Capnocytophaga (Capnocytophaga canimorsus is a bacterium that lives as a commensal in the dog mouth and causes severe infections in humans), Selenomonas, and Veillonella are found to be more abundant in both SCC and AC patients, whereas Neisseria is less abundant in both SCC and AC patients than than in healthy individuals. Bacterium-produced toxins can disturb the cell cycle, resulting in altered cell growth. This is caused by alterations to the genes that control normal cell division and programmed cell death. It is well known that gut bacteria is involved in the development of inflammatory bowel disease due to the disruption of immune defense mechanism. Similarly, salivary microbiota may also affect lung cells by inducing long-term immune response.
Periodontal disease (PD) is an oral bacterial infection leading to periodontal inflammation and loss of teeth. Studies showed an association between periodontitis and combined lung and bronchus cancer, particularly with non-small cell lung cancer that includes adenocarcinoma (AC) and squamous cell carcinoma (SCC). The gingival damage and chronic bacterial infections do not resolve with quitting and former smokers have a 30% increased risk of getting lung cancer.
Periodontal disease serves as a source of inflammation of other organs. A recent study detected elevated Fusobacterium nucleatum, an invasive periodontal bacterium, in colorectal tumors compared to normal tissues taken from the same patients.