|Year : 2020 | Volume
| Issue : 1 | Page : 16-19
Oral microbiome: Tracing the microbial kingdom in oral cancer
A Kshitij, K Ankita, P Bharat, T Shalini, R Vishal, S Anand
Department of Head and Neck Surgical Oncology, HCG Cancer Centre, Bengaluru, India
|Date of Web Publication||22-Oct-2020|
Dr. K Ankita
Department of Head and Neck Surgical Oncology, HCG Cancer Centre, Bangalore
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Kshitij A, Ankita K, Bharat P, Shalini T, Vishal R, Anand S. Oral microbiome: Tracing the microbial kingdom in oral cancer. J Precis Oncol 2020;1:16-9
| Introduction|| |
The oral cavity is often considered a mirror of systemic diseases. Various micro-organisms harmoniously co-exist in the human body. Oral cavity houses a lot of these micro-organisms. The balance between micro-organisms helps maintain and enhance individual health.
The oral cavity harbours about 700 - 1000 different species of micro-organism (1–3). These species are in a symbiotic relationship with the human body. However, some can turn hostile, resulting in damage to the host (4). The symbiotes are responsible for playing a critical role in immune response, digestion and metabolism of the carcinogens (5–7). The hostile can lead to local diseases like dental caries, periodontitis, chronic inflammation (8) and systemic illnesses (9–11).
The temperature in the oral cavity provides an ideal environment for the maintenance of the microbial species to flourish. These organisms are in continuous interaction with the saliva. The saliva retains a stable pH of 6.5 to 7.5, hydrates the bacteria and nourishes them by providing essential nutrients. The National Institute of Health in a study has identified 770 different spices of micro-organisms in the oral cavity (12).
Tobacco and alcohol consumption remains the leading cause of Oral squamous cell carcinoma (OSCC)(13). However, many patients develop oral cancers in the absence of these risk factors. This understanding has led to greater interest in other possible etiological factor and interplay in OSCC development.
H. Pylori, a gram-negative bacillus, is now a recognised carcinogen for gastric carcinoma (14). Fusobacterium nucleatum, an oral bacterium with an association to periodontal disease, has been implicated in the development of colorectal cancer (15,16). Studies have implicated the role of poor oral hygiene and pathological loss of tooth in the development of OSCC. These critical developments suggest a plausible role of oral micro-bacteria in the development of oral cancer (17,18).
The oral cavity can be easily examined and under direct vision, but yet most oral cancers are diagnosed at advanced stages. Genomic level detection of the bacterial population having a link to oral cancers could help in early detection. We may be able to glean information from the oral microbiome and trace the subtle changes in tumourigenesis before the disease becomes clinically apparent or symptomatic.
This review intends to summarise our current understanding of the oral microbiome and highlights possible avenues in this regard for future research.
| The Bacterial Influence on the Pathogenesis of Cancer|| |
Multiple mechanisms underlying microbiome induced carcinogenesis have been elaborated in the literature (19–24). Oral microbiome metabolises alcohol to acetaldehyde (19), and acetaldehyde which is a known carcinogen (20). Chronic inflammation releases nitrosamine a carcinogen (21), causes DNA methylation (22) and influence the pathogenesis of cancers by altering cell proliferation patterns, cytoskeletal rearrangements, activation of NF-κB, and inhibition of cellular apoptosis (23). Additionally, the production of toxic metabolites interferes with cell signalling (24) eventually leading to cancer.
The microbiome also indirectly promotes metastasis by producing an acidic environment (25,26). Studies have shown that in-vitro cultivated tumour cells in an acidic environment, when injected in vivo, enhances lung metastasis (27–29). Low pH-induced up-regulation of the proteolytic enzymes MMP-2 and MMP-9 and the angiogenic factors VEGF and IL8 in vitro, all of which are known to be involved in the metastatic process (29).
| Methods of Analysis|| |
The age-old method for detecting/analysing microbiome is to culture them. Culturing is time-consuming, and many microbes cannot be cultured ex-vivo. The advantage of culture is the availability of metabolites and toxins for analysis which is absent in sequencing methodologies. Toxic metabolites also influence the pathogenesis of cancer (24), and hence analysing them also remains crucial.
The 16S ribosomal RNA sequencing technique sequences the hypervariable 16S rRNA region and classifies bacteria to the genus level (30). Metagenomic shotgun sequencing technique has been used to identify the whole genome sequence of the microbiome. It is expensive, but generates a large amount of data, requiring further analysis (31). IS - pro sequences the interspace of the 16S-23S region and can identify bacterial species, again this is expensive and proprietary technology. This state of the art technology is rapid and can detect many organisms as compared to culturing (32).
Metabolomics identifies metabolic products of microbial and host metabolism in bodily fluids and faeces (33). Meta-transcriptomics is the sequencing of transcribed bacterial RNA content in the sample. It is not standardised and has several limitations like difficulty in the storage of the RNA sample, limited RNA extraction from the bacterial cells, cross-contamination of bacterial and host RNA, limited database availability, which makes the entire process less practical and challenging (34).
The Human Oral Microbe Identification Microarray uses probes and can detect 300 to 400 of previously sequenced bacterial species and cannot detect new undiscovered species (35).
| Oral Dysbiosis|| |
The oral microbiome plays a critical role in maintaining good oral health. Dysbiosis is defined as the breakdown in the balance between the “protective” versus “harmful” bacteria which leads to poor oral hygiene and plausibly oral cancer (8, 36, 37).
A balance in the oral ecosystem maintains the microbiome population in proper numbers. Oral viruses that act as bacteriophages maintaining the microbial balance (38,39). Saliva provides the proteins and other nutrients, thereby supporting the microbiome (40). Constant shedding of the epithelial cells, immunoglobulins, agglutinins, lysozymes and histatins in the saliva prevent microbial injury (41).
Habits have been implicated in oral dysbiosis, and their role needs a mention.
Alcohol consumption disturbs bacterial -salivary interactions (42–45). Its cytotoxic effects (46), impact the host defence system (47,48) leading to changes in the oral microbiome. Animal studies have shown that addition of 20% ethanol to diet leads to an increase in Streptococcus mutans. Alcohol consumption also decreases the lactobacillus counts in the mouth, a bacteria associated with good oral health. Studies have shown a dose-dependent relationship between alcohol and poor oral health.
Alcohol is also implicated in oral microbiome changes which develop poor oral health and periodontitis, ultimately leading to oral cancer. Additionally, alcohol directly produces carcinogenic activity as the microbiome metabolise alcohol to acetaldehyde.
Potential mechanisms for smoking-induced dysbiosis include acidification of saliva, antibacterial effects, impairing immunity, influencing bacterial adherence to the mucosa and oral hypoxia. A large study of 1204 adults showed that smokers had a relatively lower presence of the Phylum Proteobacteria (4.6%) compared with non- smokers (11.7%). Bacterial count depleted with smoking and was related to altered carbohydrate and energy metabolism. This leads to shifts in functional pathways and could have some serious implications (49). Though they could not establish a clear relationship between dysbiosis due to smoking and oral cancer, this is an exciting area for future research.
It has been reported in the literature that nicotine and high sugar in smokeless tobacco affects the growth of Streptococcus mutans and Lactobacillus (50–52). Smokeless tobacco users are at a higher risk of periodontal diseases due to increased abundance of Aggregatibacter actinomycetemcomitans, Tannerella forsythia, Porphyromonas gingivalis and Treponema denticola, especially in periodontal pockets (53). Although further research is needed in regard but smokeless tobacco is a known factor for oral dysbiosis
| Cancer Vaccines and the Microbiome|| |
It took close to 200 years for humanity to eradicate smallpox after the discovery of its vaccine (54). Interests in cancer vaccine developed towards the end of the 20th century. Identification of specific vaccine targets was essential to create a cancer vaccine. Today vaccine primarily target tumour antigens, hence are used in a therapeutic setting after surgical treatment. Prophylactic cancer vaccines which are administered to a healthy individual to prevent cancer are minimal as of now but have proved successful for the primary prevention of hepatocellular carcinoma secondary to hepatitis B virus and cervical carcinoma secondary to human papillomavirus infection. The development of a prophylactic cancer vaccine can target individual microbiomes appears futuristic. The association of poor oral hygiene, dental caries and periodontal disease are well established in oral cancer. And the oral microbiome plays a pivotal role in the development of caries and periodontitis (55–57).
Vaccination against Streptococcus mutans has been developed, which is the primary causative organism for dental caries. Childers et al. immunised adults by orally administering enteric-coated capsules filled with crude Streptococcus mutans Gs-5 GTF antigen preparation leading to the production of salivary IgA antibodies .
So far, the development of preventive vaccinations is limited to targeting individual risk factors like caries, identification of specific dysbiosis leading to oral cancer. Narrowing down to specific organisms is hence essential for targeted vaccine development. Identification of specific microbes and microbiome research holds the potential to develop a prophylactic cancer vaccine.
| Conclusion|| |
Currently, the interlink between the deranged oral microbiome and its causal relationship in non-habit related oral cancer is not very clear. Experience with microbes such as H. pylori and its link to gastric cancer invites for further research studying subtle microflora changes and the risk of malignant transformation.
The microbial profile in tobacco/alcohol vs non-tobacco/alcohol needs to be explored to trace the possible aetiology of oral cancers in young cohorts. The growth and culture of certain specific virulent microbial species in the salivary samples and its possible link to oral cancer may seem to be promising. And this shortly would pave way for early non-invasive detection at a molecular level.
The casual link between microbes and cancer also underlines a possibility that changes in the commensal microflora occur in conjunction with cancer development, which is a potential diagnostic indicator. The microflora associated with oral malignancy and how micro-organisms interact with the oral mucosa on a cellular level are concepts that warrant further investigation. Oncobiome and oral cancer may share a collinear relationship which is more robust than what was thought to exist.
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