Red Complex Periodontal Pathogens and Their Potential Role in Colorectal Carcinogenesis: A Narrative Review

Periodontal disease (PD), a chronic inflammatory condition driven by oral microbial dysbiosis, is increasingly implicated in systemic diseases, including colorectal cancer (CRC). The "red complex" bacteria-Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola-play a central role in PD progression and exhibit virulence factors that promote inflammation, immune evasion, and epithelial colonization. A literature search in PubMed, Google Scholar, and ScienceDirect (English and Slovenian, up to September 2025) identified 12 eligible studies. Only original clinical, in vivo, or in vitro research directly addressing red complex pathogens and colorectal cancer was included. The search results showed that most of the literature focuses on the association between Porphyromonas gingivalis and CRC, particularly its role in tumor immune evasion, alteration of the gut microbiota, creation of a pro-inflammatory microenvironment, and promotion of carcinoma cell proliferation. Infection with Porphyromonas gingivalis has also been linked to poorer cancer prognosis. The other red complex bacteria are primarily mentioned in the context of generating a pro-inflammatory microenvironment and contributing to chronic inflammation, which supports tumor growth and survival.