In 2012, over 300.000 new cases of patients suffering from the oral cavity and lip cancer were diagnosed worldwide. Smoking and the use of alcohol are the main risk factors of oral cancer explaining nearly 80% of all cases. In addition to human papillomavirus (HPV), oral bacteria have recently been implicated in the pathogenesis of oral cancer, particularly squamous cell carcinoma. Periodontal disease (gum disease) may be causally related to oral cancer through inducing chronic inflammation. The oral bacterial microbiota consists of 500-700 species and specific oral bacteria such as Porphyromonas gingivalis and Fusobacterium nucleatum, have been shown to have oral carcinogenic potiential in vitro and in animal studies. Furthermore, it is well established that certain oral bacteria express the enzyme alcohol dehydrogenase which catalyses the production of acetaldehyde, the first metabolite of ethanol metabolism and a grade I carcinogen associating with the use of alcoholic beverages and/or smoking. Assessing the oral microbiota may therefore carry potential as a diagnostic and prognostic marker in oral cancer (Guerrero-Preston et al., 2016).
Describing oral bacterial colonization and how oral microorganisms interact with each other and the host are most likely the key determinants for understanding disease etiology and progression. Future work should strive for methodological standardization including acquisition, storage and handling of samples. It is also important to recognize that the oral cavity and the head and neck region represent areas with various tissue specific functions and diseases. Case-control studies implementing full metagenomics or metatranscriptomics analyses should be undertaken in an attempt to explain the role of the oral microbiota in oral cancer. Large-scale, longitudinal studies with follow-up are needed preferably commencing with potentially malignant oral lesions, which precede actual carcinoma in most cases of oral cancer.
It is paramount to include the known major determinants of oral disease risk in the analyses assessing the contribution of oral microbiota to disease development. Tooth decay, periodontitis, oral hygiene in general, diet, smoking, and alcohol consumption have all been reported to affect oral microbiota composition. It is intriguing to hypothesize that altered microbiota may be one of the mechanisms by which these risk factors lead to carcinoma.
Guerrero-Preston R, et al. 16S rRNA amplicon sequencing identifies microbiota associated with oral cancer, human papillomavirus infection and surgical treatment. Oncotarget 2016;7:51320-34.