Oral Bacteria and Cancer

Over a number of years, epidemiological studies established several well-defined risk factors for cancer, including age, heredity, diet, tobacco use, chronic viral infections, and inflammation. Paradoxically, the success of these studies left little room for incorporation of any new factors or causative agents, and, consequently, the idea that a bacterial infection could contribute to cancer was generally disregarded. However, landmark studies in the early 1990s established Helicobacter pylori as a causative agent of gastric cancers, resulting in a paradigm shift regarding the relationship between microbial agents and cancers [1]. Indeed, in 1994, H. pylori became the first bacterial species to be officially recognized by the World Health Organization as a definite cause of cancer in humans. Since then, there has been a growing body of evidence supporting an association between specific microorganisms, including those in the oral cavity, and various types of cancers. The oral cavity is inhabited by complex multispecies communities that usually exist in a balanced immunoinflammatory state with the host [2]. Certain species, such as Porphyromonas gingivalis, can disrupt this equilibrium, resulting in a dysbiotic host– microbiota interaction. Subsequently, other community constituents, such as Fusobacterium nucleatum, can become opportunistically pathogenic, and the combined effect of a dysbiotic microbial community along with a dysregulated immune response ultimately causes periodontal disease [2]. These well-studied periodontal organisms have now emerged as the focal point for the developing association between oral bacteria and cancer. Perhaps the most likely carcinogenic link with oral bacteria is with oral squamous cell carcinoma (OSCC), one of the most common cancers worldwide. OSCC surfaces have been reported to harbor significantly higher levels of Porphyromonas and Fusobacterium compared with contiguous healthy mucosa [3]. Moreover, immunohistochemistry with P. gingivalis antibodies revealed higher levels of detection and intensity of staining in gingival carcinomas compared with healthy gingival tissue, although only a small number of cases were examined [4]. A striking association has also been demonstrated between P. gingivalis infection and pancreatic cancer. In a prospective cohort study of over 400 cases and controls, a .2-fold increase in risk of pancreatic cancer was observed among those with high levels of antibodies to P. gingivalis, after adjusting for known risk factors [5]. Similarly, in the extensive National Health and Nutrition Examination Survey III, orodigestive cancer mortality was found to be related to the levels of P. gingivalis antibodies, independent of periodontal disease [6]. Several recent studies have shown a strong association between F. nucleatum and colorectal cancer (CRC) [7–10]. F. nucleatum was found to be one of the more abundant species within and around CRC neoplasms, and levels of F. nucleatum correlated with the presence of lymph node metastases.