Table 3.

Quality assessment of the selected studies according to the star score of the Newcastle–Ottawa Scale, NOS, based on which * are assigned to three criteria, i.e., selection (with a maximum of 4 stars [****]), comparability (with a maximum of 2 stars [**]), and outcome (with a maximum of 3 stars [***]) for a maximum of 9 stars. Higher scores indicate lower risk of bias.

	Selection	Comparability	Outcome	Total Score
Farrell, J. et al. Gut 2012 [30]	***	*	**	6
Chen, X. et al. PloS ONE 2015 [53]	**	**	**	6
Hu, J. et al. Biomed. Res. Int. 2015 [48]	**	**	**	6
May, X. et al. J. Periodontol. 2015 [35]	**	**	***	7
Torres, P. et al. Peer J. 2015 [43]	****	*	**	7
Han, S. et al. Int. J. Oncol. 2016 [39]	****	*	**	7
Kato, I. et al. J. Epidemiol. Res. 2016 [16]	**	*	**	5
Lu, H. et al. Sci. Rep. 2016 [57]	**	**	**	6
Peters, B.A. et al. Cancer Res. 2017 [6]	**	**	**	6
Olson, S. et al. Cancer Causes Control. 2017 [44]	****	*	**	7
Fan, X. et al. Gut 2018 [45]	***	**	**	7
Flemer, B. et al. Gut 2018 [3]	**	*	***	6
Russo, E. et al. Front. Microbiol. 2018 [36]	**	*	***	6
Sun, J. et al. Oncol. Rep. 2018 [50]	****	*	**	7
Wu, J. et al. J. Cancer 2018 [51]	***	*	**	6
Xu, J. et al. Microb. Pathog. 2019 [52]	**	**	**	6
Lu, H. et al. J. Oral Microbiol. 2019 [46]	***	*	**	6
Schmidt, T. et al. eLife 2019 [38]	**	*	**	5
Yang, Y. et al. Int. J. Cancer 2019 [37]	**	**	**	6
Wang, Q. et al. Sci. Rep. 2019 [54]	**	*	**	5
Guven, D.C. et al. Biomark. Med. 2019 [41]	**	**	**	6
Vogtmann, E. et al. Cancer Med. 2020 [42]	**	**	**	6
Zhao, Q. et al. Front. Cell. Infect. Microbiol. 2020 [55]	**	**	**	6
Zhang, S. et al. Theranostics 2020 [40]	***	**	**	7
Kawasaky, M. et al. Cancer 2020 [56]	***	**	**	7
Li, D. et al. Microb. Phatog. 2020 [58]	**	**	**	6
Wei, A.L. et al. World J. Gastroenterol. 2020 [47]	**	*	**	5
Huang, K. et al. Front. Cell. Infect. Microbiol. 2021 [49]	**	**	**	6