Introduction. Squamous cell carcinoma is a highly aggressive type of oral cancer (OC). It is the most common cancer among men, and accounts for almost 90 % of all oral cancers in India. Consumption of tobacco is a leading factor contributing to maximum oral cancer incidences as per the WHO. Hypothesis/Gap statement. Researchers reported a direct association of microorganisms with dysbiosis in various oral lesions including oral cancer. However, there is a dearth of information related to compositional changes in the oral microbiome in long-term tobacco chewers and the Indian oral cancer population. Aim. The aim of this study was to identify and correlate the bacterial diversity in the oral cavity of tobacco chewers, patients with oral cancer and healthy subjects in the Indian population. Methods. Oral rinse samples were collected for ten subjects in each group followed by DNA extraction. The variable regions of the bacterial 16S rRNA gene (V6-V8) were amplified, sequenced, processed, and analysed using QIIME2 platform to assess alpha and beta diversity between the study groups. Results. This pilot study showed genus Streptococcus dominated the control group (18.54 %), and the abundance decreased in tobacco and OC group (9.63 and 5.45% respectively); whereas genus Prevotella dominated the tobacco and OC group (21.01 and 26.03% respectively). A shift in abundance of microbiome was observed from control population to oral cancer via the tobacco chewing population. Maximum alpha diversity of oral microbiome was found in Indian tobacco chewers. Beta diversity of tobacco chewers was similar to both the healthy population as well as oral cancer patients suggesting transitioning of the oral microbiome from healthy to oral cancer microbiome via the tobacco chewers microbiome. Conclusion. The data provides evidence of oral bacterial dysbiosis due to tobacco chewing habits that can further lead to progression towards cancer.

Dysbiosis of the oral microbiome has been found to play a key role in the genesis and progression of oral cancer (OC). Tobacco chewing, a risk factor for oral cancer, is also associated with oral dysbiosis. Since tobacco chewing is a lifestyle habit in the South Asian subcontinent, including India, and contributes to one-third of the global oral cancer burden; we aimed to identify the oral bacterial diversity of Indian oral cancer patients and tobacco chewers. We used 16S rRNA amplicon sequencing to study the composition of oral microbiota in OC patients and tobacco chewers in India and compared it with healthy controls. The abundance of predominant phyla, Firmicutes, and Bacteroidetes varied between the study groups. Our study identified Leptotrichia, Treponema, Lautropia, and Cardiobacterium as significantly enriched in tobacco chewers, whereas genera Pseudomonas, Capnocytophaga, and Mycoplasma were enriched in oral cancer, which could be potential biomarkers for the Indian population. Furthermore, the functional prediction revealed that genes involved in lipid biosynthesis and fatty acid elongation were upregulated in the oral cancer group, whereas those for the reductive TCA cycle were upregulated in the tobacco group. As the role of bacteria in oral cancer is becoming more evident, identification of bacterial diversity and biomarkers for tobacco chewers and OC patients can aid in the early diagnosis of OC in high-risk individuals.

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