Colorectal cancer (CRC) is a malignant disease associated with substantial morbidity and mortality rates, and the implementation of early screening has been shown to greatly enhance survival outcomes. Currently, early screening methods for CRC rely on stool-based tests and colonoscopy; however, the limited adherence of patients to these screening protocols hinders their widespread adoption. The utilization of innovative microbiological sequencing technique known as 2bRAD-M holds promise for the detection of low biomass samples. In this study, the 2bRAD-M technique was employed to ascertain a diverse microbiota consisting of different microorganisms in the serum of patients diagnosed with CRC, as well as in the serum of healthy control individuals. This study included 3 patients with nonmetastatic CRC and 3 healthy individuals. Additionally, the microbiota present in CRC tumor tissues and paraneoplastic tissues were also examined. Furthermore, the metabolic pathways of these microorganisms were predicted. The findings indicated that the microbiota community structures in serum and tissues were distinct, while the microbiota composition in tumor tissues and adjacent tissues was largely similar. Microbiota in serum such as Enterobacteriaceae and tissue-associated RC9ARalstonia may serve as novel biomarkers for CRC screening. Our results suggest that both serum microbiota and cancer tissue microbiota can serve as a valuable basis for conducting early in vitro screening for CRC.  Kulcsszavak: colorectal cancer, microbiota, 2bRAD-M, serum, tumor tissue, sequencing