Oral Microbiota Alterations and Potential Salivary Biomarkers in Colorectal Cancer: A Next-Generation Sequencing Study

Colorectal cancer (CRC) has a high mortality rate worldwide. Oral and intestinal microbiota members may have an effect on gastrointestinal tumors' pathogenesis, particularly in CRC. Designed as a pilot study, this study's aim was to investigate the relationship between CRC and oral microbiota and to identify potential biomarkers for CRC diagnosis. Saliva samples were collected from recently diagnosed CRC patients (<i>n</i> = 14) and healthy controls (<i>n</i> = 14) between March 2023 and December 2023. Microbiota (16S rRNA) analyses were conducted on these saliva samples using a next-generation sequencing method. Phylogenetic analyses, including alpha diversity, principal component analysis (PCA), principal coordinate analysis (PCoA), beta diversity, biomarker, and phenotype analyses, were conducted using the Qiime2 (Quantitative Insights Into Microbial Ecology) platform. Alpha diversity indices (Shannon: <i>p</i> = 0.78, Cho1: <i>p</i> = 0.28, Simpson: <i>p</i> = 0.81) showed no significant difference between CRC and control groups. Beta diversity analysis using Bray-Curtis PCoA indicated significant differences in the microbial community between the two groups (<i>p</i> = 0.003). Examination of OTU distributions revealed that the <i>Mycoplasmatota</i> phylum was undetectable in the oral microbiota of healthy controls but was significantly elevated in CRC patients (CRC: 0.13 ± 0.30, Control: 0.00 ± 0.00, <i>p</i> < 0.05). Additionally, <i>Metamycoplasma salivarium</i>, <i>Bacteroides intestinalis</i>, and <i>Pseudoprevotella muciniphila</i> were undetectable in healthy controls but significantly more prevalent in CRC patients (<i>p</i> < 0.05 for all three species). LEfSe analysis identified eight species with an LDA score > 2, <i>Granulicatella adiacens</i>, <i>Streptococcus thermophilus</i>, <i>Streptococcus gwangjuense</i>, <i>Capnocytophaga</i> sp. FDAARGOS_737, <i>Capnocytophaga gingivalis</i>, <i>Granulicatella elegans</i>, <i>Bacteroides intestinalis</i>, and <i>Pseudoprevotella muciniphila</i>, as potential biomarkers. The results of this study contribute critical evidence of the role of oral microbiota in the pathogenesis of colorectal cancer. Alterations in the microbiota suggest potential biomarkers in understanding the biological mechanisms underlying CRC and developing diagnostic and therapeutic strategies.