Research trends and hotspots of intratumor microbiome: A comprehensive visualization and bibliometric study from 2012 to 2024

To the Editor: As early as in the mid-19th century, microbiologists first recognized multiple microorganisms within human tumors. Nevertheless, the perceptions of intratumor microbiome have made little progression during the first 50 years of the 20th century owing to contamination interference, experimental irreproducibility, and technical limitations. Strikingly, in the past decade, the advances in detection techniques and analytical tools have been opening unprecedented opportunities to widely investigate the manifold characteristics of low-biomass intratumor microbiome and host–microbe interplays. In the early second decade of the 21st century, researchers focusing on the intratumor microbiome gradually emerged. Initially, this adventurous perspective failed to draw much attention because tumors were still considered sterile in the mainstream belief at that time. As researchers realized the significant role of microbiome in tumor biology, immunity, and metabolism as a non-negligent new component of tumor microenvironments (TME), intratumor microbiome soon became hot. Gradually, intratumor microbiomes have emerged as tumor diagnostic and prognostic biomarkers, and even strenuous endeavors have been devoted to tapping their potential as therapeutic targets.[1–5] Yet, it remains much left to translate microbes-based tumor-specific therapies into clinical practice. Therefore, there is great significance in delineating the research status and evolution trend of intratumor microbiome. We conducted a bibliometric analysis of 487 related publications extracted from the Web of Science Core Collection from inception to April 11, 2024 [Supplementary Methods and Supplementary Figures 1 and 2, https://links.lww.com/CM9/C186], mapping the current landscape in the hope of proffering fresh clues for future research. There is a significant blowout trend in the annual publications, and the predictive annual/cumulative publications are 221 and 614, respectively, in 2024 [Supplementary Figure 3, https://links.lww.com/CM9/C186]. Fifty-six countries published documents, wherein less than 20% of them contributed more than 10 publications [Supplementary Figure 4A and Supplementary Table 1, https://links.lww.com/CM9/C186]. Most cooperation is confined to Europe, North America, and a handful of Asian countries/institutions [Supplementary Figure 4B–E, https://links.lww.com/CM9/C186]. China has emerged as the leading producer of publications, and 7 out of the top 10 productive institutions are also held by China. But the United States has continued to dominate the field as a global cooperation center with overwhelming citations. This is partially because China has recently cut a striking figure [Supplementary Figure 4C, E, https://links.lww.com/CM9/C186]. A total of 3505 authors provided publications, wherein 84.9% of them contributed a single publication [Supplementary Figure 5A, https://links.lww.com/CM9/C186]. Only 489 authors have co-authorship relationships [Supplementary Figure 5B, C, https://links.lww.com/CM9/C186]. Just Ravid Straussman and Jennifer A Wargo are concurrently among the top 10 prolific and cited authors [Supplementary Table 2, https://links.lww.com/CM9/C186]. Deborah Nejman is endowed with higher citations and H-index but does not possess more publications, reflecting his stringent quality control over publications. Deborah Nejman is also the most co-cited author [Supplementary Figure 5D, https://links.lww.com/CM9/C186]. The top prolific researchers have come to prominence in the last couple of years except that the publications of Jennifer A Wargo have been distributed discontinuously from 2018 to 2023 [Supplementary Figure 5E, https://links.lww.com/CM9/C186]. Included publications are sourced from 232 journals, wherein 14 journals are judged as core journals [Supplementary Figure 6A, https://links.lww.com/CM9/C186]. Science has the highest citations albeit with only two publications and Cell possesses the highest total link strength despite five publications, whereas Cancers has the highest publications [Supplementary Figure 6B and Supplementary Table 3, https://links.lww.com/CM9/C186]. The top 10 co-cited journals are classified into 4 clusters [Supplementary Figure 6C, https://links.lww.com/CM9/C186]. Citing journals are chiefly from Molecular/Biology/Immunology and Medicine/Medical/Clinical, called research frontiers; whereas the cited journals are chiefly from Molecular/Biology/Genetics, called the knowledge base [Supplementary Figure 6D, https://links.lww.com/CM9/C186]. Thirteen documents are concurrently ranked in the top 20 documents with the highest local citation score (LCS) and global citation score (GCS) [Supplementary Table 4, https://links.lww.com/CM9/C186]. These incipient documents with high LCS proffer a theoretical basis for subsequent exhaustive study on biological characteristics and functions. Supplementary Figure 7A, https://links.lww.com/CM9/C186 portrays the citation relationships, while Supplementary Figure 7B, https://links.lww.com/CM9/C186 demonstrates the document’s impact in the field. Supplementary Figure 7C, https://links.lww.com/CM9/C186 further shows the relationship of the top 20 publications by LCS. Supplementary Figure 8A, https://links.lww.com/CM9/C186 depicts the reference co-citation network, forming 20 clusters for thorough examinations [Supplementary Figure 8B, https://links.lww.com/CM9/C186]. Most of the highly cited references have emerged since 2015, and many of them continue to be widely cited [Supplementary Figure 8C and Supplementary Table 5, https://links.lww.com/CM9/C186]. The hotspots have already shifted from initial topics (e.g., #12, #17, #21, and #23), whereas #0 pancreatic cancer, #1 electroporation, #2 engineered bacteria, and #5 microbiome-based therapy, etc., represent current hotspots. Especially, microbiome-based therapy including engineered bacteria has generated multiple highly cited references over the past one or two years. The reference citation burst can be traced back to Grivennikov SI, 2012, Nature, whereas the latest took place in 2021, attributed to Li S, 2017, Microbiome [Supplementary Figure 8D, https://links.lww.com/CM9/C186]. Notably, Geller LT, 2017, Science experienced the strongest burst. Keyword frequency and evolution are examined with word clouds and heatmap [Supplementary Figure 9A–C and Supplementary Table 6, https://links.lww.com/CM9/C186], generally evincing the research emphasis. The hotspots were relatively homogeneous before 2021, principally focusing on prostate cancer and gender; nevertheless, hotspots have suddenly surged since 2021 [Supplementary Figure 9D, https://links.lww.com/CM9/C186]. Specifically, the emergence of some new keywords in the top-left corner (e.g., diagnosis, prognosis, biomarker, and TME) gained prominence over the last 2 years, reflecting a discernible shift in the orientation of future endeavors, wherein diagnosis and prognosis continue to be of intense research interests. Similarly, immunity, prognosis, metastasis, metabolism, cancer therapy, tumor immune microenvironment, etc., have evolved into salient emergence since 2021 [Supplementary Figure 9E, https://links.lww.com/CM9/C186]. Notably, gut microbiome, etc., have been enduring in recent years. Colorectal cancer, dysbiosis, and 16s rRNA belong to Motor Themes, denoting they are the core themes with high maturity [Supplementary Figure 9F, https://links.lww.com/CM9/C186]. Whereas metastasis belongs to Niche Themes, indicating it is well-developed but banal. Conversely, gut microbiome, immunity, metabolism, cancer therapy, biomarker, tumor immune microenvironment, etc., belong to Basic Themes, demonstrating they are significant but the current research is not enough, therefore they may become hotspots or future trends. The above analysis prefigures future trajectories, encompassing but not confined to the gut microbiome, immunity, metabolism, diagnosis and prognosis, and cancer therapy. Below, we will elucidate their far-reaching meaning for this realm and implications for future directions. Correlation between intratumor and gut microbiome: Gut microbiome could be the main origin of intratumor microbiome. Gut microbiome could directly invade gastrointestinal tumor tissues via impaired mucosa, but also could translocate into tumor in non-mucosal organs, such as pancreas through pancreatic ducts. Furthermore, given intratumor microbiome is chiefly in cancer cells and immunocytes, gut microbiome may be transported to the tumors in the form of fractions or intact cells through cell migration. The conjectures must be validated through holistic metagenomic sequencing and genetic identification. Apart from the direct association by microbial translocation, remote communication mediates the latent connection. Gut microbiome may indirectly affect TME through metabolites or immune system, culminating in altering intratumor microbiome. Conversely, can intratumor microbiome modulate gut microbiome? Further research is warranted to elucidate their interplay and its effects on TME. Tumor immunomodulation: Intratumor microbiome reshapes tumor immune microenvironment (TIME) by modulating pro-tumoral and antitumor immunity. Intratumor microecology facilitates the recruitment and activation of pro-tumoral immunocytes, but also counteracts pro-tumoral immunity. Meanwhile, intratumor microbiome augments antitumor immunity as a focus of local immune activation, but also exerts inhibitory effects on antitumor immunocytes. Although exact mechanisms of multitudinous immunomodulation are yet to be elucidated, the underlying mechanisms may include (1) the valid presentation of microbial antigens, especially microbial antigens mimicry shared with tumor antigens; (2) microbe-induced immunogenic cell death; (3) microbe-derived metabolites, and (4) pattern recognition receptor-mediated microbial adjuvanticity. Collectively, intratumor microbiome is also a potential target for supporting immunotherapy, and their effects are context-dependent and require further clarification prior to clinical translation. It is a fresh topic facing grand challenges but deserves further exploration. Metabolic regulation: Intratumor microbiome reprograms convoluted metabolism within the TME [Supplementary Table 7, https://links.lww.com/CM9/C186], but their metabolites also play a double-edged sword in TIME as non-negligible mediators of host–microbe interactions. The complicated impacts of metabolites on tumors depend on several factors, encompassing tumor type, tumor stage, host genotype, and specific metabolite, which necessitates further study. Targeting microbial metabolic regulation should be a monumental subject for future research, which could proffer a comprehensive perception of tumor microbe microenvironment and enlighten fresh antitumor strategies. Diagnosis and prognosis: Intratumor microbiome characteristics are associated with the clinicopathological features (e.g., age, gender, cancer subtypes, cancer stage, and histological grade), therefore it may be a valuable indicator for tumor diagnosis, subtype stratification, and staging classification. Meanwhile, the abundance of intratumor microbiome is related to therapy responses and clinical outcomes, therefore it may proffer a promising tool for cancer prognosis. Given the invasiveness and difficulty of obtaining tissue samples, accumulating studies typically utilize alternative samples, such as sputum and bronchoalveolar lavage fluid. Notably, tumor-associated circulating microbial DNA, a potential biomarker in tumor liquid biopsy, will be more valuable owing to the potential for microbiome to be transported into tumor through tumor cells or immunocytes. Additionally, combining multiple-omics data with other emerging sampling techniques, such as ingestible capsules for microbiome sampling, will favor further precise diagnosis and prognostic strategies. Cancer therapy: Given the multidimensional immunomodulation and metabolic regulation of intratumor microbiome, it plays significant roles in determining response to antineoplastic therapies. Consequently, targeting intratumor microbiome could proffer fresh adjuvant strategies for antineoplastic therapy, which can be classified into following points: (1) eliminating detrimental and carcinogenic microbial species; (2) fostering beneficial microbiome; (3) microbial metabolite-based intervention to reprogram TIME; (4) modulating microbiome to ameliorate immunotherapy, chemotherapy, and radiotherapy; (5) microbe-inspired drug delivery. In particular, technological development has reformed microbial design that presents preferential targeting of tumors, aggressive tissue penetration, easy recognition, and controllable cytotoxicity. Specifically, genetically engineered bacteria can not only release products or trigger specific reactions handicapping tumors, but also function as hitchhiking for the targeted delivery of immunostimulators, toxins, or other drugs. Nevertheless, it remains somewhat challenging to determine the optimal initial dose and dosing schedule for live bacterial substances, and clinical administration of live bacterial substances has the risk of life-threatening infections and sepsis. Further research is warranted to combine engineered bacteria with conventional therapies, which may proffer fresh ideas and upgrade the antitumor efficacy. Notably, traditional Chinese medicine is successfully applied in the study of gut microbiome, but there is few report about intratumor microbiome. The research approach of traditional Chinese medicine in gut microbiome provides valuable experience for its study in intratumor microbiome. Furthermore, we can speculate that there is a triangular interaction among traditional Chinese medicine, gut microbiota, and intratumor microbiome. There are inevitable limitations to this study. Other database-derived or non-English publications may be omitted. The latest publications are not presented owing to temporal limitation. The bibliometrics are constricted to metadata rather than full-text analysis. But these limitations would not alter the study findings. Overall, a comprehensive visualization and bibliometric study is conducted to explore frontiers and hotspots of intratumor microbiome. According to the systematical analysis, intratumor microbiome endows TME with additional multivariate characteristics divergent from the conventional one and has emerged as an attractive biomarker and target. Nevertheless, it remains much left to depict the intratumor microbiome-mediated molecular networks within the tumor microbe environment. Perhaps the eventual destination is to translate microbes-based tumor-specific therapies into clinical practice. Funding This work was supported by the National Natural Science Foundation of China (No. 82150006), Chinese Society of Clinical Oncology Research Foundation (Nos. Y-HR2017-117, Y-HH202102-0060), Beijing Medical and Health Foundation (No. YWJKJJHKYJJ-F3046D), Qujiang District Quzhou City Life Oasis Public Service Center (No. BJHA-CRP-040), and Wu Jieping Medical Foundation (No. 320.6750.2023-05-10) to Yawen Gao. Conflicts of interest None.