Characterization of Clinical Fusobacterium nucleatum Isolates from Oral Squamous Cell Carcinoma Patients

Fusobacterium nucleatum (Fn), a Gram-negative anaerobe primarily residing in the oral cavity, has garnered increasing attention for its role in a broad spectrum of human diseases. While typically absent or rarely detected outside the oral cavity in healthy individuals, Fn is frequently found at extra-oral sites under disease conditions and has been implicated in cancer progression and prognosis1. In oral squamous cell carcinoma (OSCC), the abundance of Fn significantly increases as the disease progresses, promoting cell invasion and metastasis. Furthermore, substantial evidence links Fn to accelerated tumor growth and metastatic progression in colorectal cancer (CRC), where its presence is also associated with chemotherapy resistance and poor prognosis. Here, to further elucidate the pathogenic mechanisms of Fn in cancer progression, this study characterized the physiological traits, virulence factor expression, and impacts on cancer cells of 10 Fn strains, including two well-characterized ATCC strains and eight clinical isolates. The clinical isolates consisted of three strains from saliva samples of 117 OSCC patients and five strains from 160 non-cancer individuals. Results indicated that oral isolates, regardless of disease origin, all belong to Fn subspecies polymorphum. The ATCC strains (23726 and 25586) exhibited shorter cell lengths and faster growth rates compared to the clinical isolates. Both ATCC strains formed stable biofilms and expressed key virulence genes, including aim1, fadA, fomA, and radD. All isolates tested showed sensitivity to a panel of eight different antibiotics. Interestingly, only one clinical isolate displayed similar stimulation of CRC cell migration as the two ATCC strains, while the other seven displayed no such ability. Collectively, these findings suggest that the most virulent strains, in terms of biofilm formation and virulence gene expression, are not necessarily the most pathogenic in the context of cancer cell interactions. Future pan-genomic analyses, incorporating whole-genome sequencing of the clinical isolates, will aim to delineate the genetic determinants contributing to the carcinogenic potential of Fn.