Pancreatic cancer (PC) is reported to be the third most common cause of cancer-related deaths in both sexes in the USA, with a very low 5-year survival rate of 12%, because most patients are identified in the late stages of disease [1]. It is estimated that there will be 64,050 new cases and 50,550 deaths due to PC in 2023 [1]. The high mortality associated with PC increases the importance of identifying new diagnostic markers and therapies to enable the initiation of early intervention [2]. The most prevalent form of PC is pancreatic ductal adenocarcinoma (PDAC) [3]. Tumor growth is triggered by mutations and subsequent inactivation of tumor suppressor genes that cooperate with KRAS oncogene mutations [3]. PDAC is a highly aggressive cancer, and currently, only approximately 15–20% of patients have tumors that are suitable for surgical resection, offering a chance of potential cure. However, surgical intervention is a highly invasive procedure, and even after successful resection, the 5-year survival rate remains limited, reaching only 20% [4].

In addition to genetic alterations, the tumor microenvironment is likely to play a pivotal role in the pathogenesis of PC [5]. Non-neoplastic cells, including endothelial cells, immune cells, and fibroblasts, interact with PDAC cells and may determine tumor growth and the effectiveness of therapy [6]. The microbiota can stimulate persistent inflammation, causing alterations in the antitumor immune system, leading to changes in cellular metabolism in the tumor microenvironment [7, 8]. As a result, the microbiota is able to greatly influence the prognosis of malignancies and affect therapeutic efficacy in some patients [9]. There is still no single screening procedure recommended for PC diagnosis that is applicable to the entire population [10,11,12,13]. Based on international guidelines, MRI/MRCP and endoscopic ultrasound plus fasting glucose or HbA1C, monitoring new-onset diabetes, should be performed on patients with an elevated risk of familial pancreatic cancer (FPC). These guidelines aim to identify Stage I pancreatic cancer and high-grade dysplastic precursor lesions, namely pancreatic intraepithelial neoplasms (PanINs) and intraductal papillary mucinous neoplasms (IPMNs) [14]. Currently, screening tests are only suggested for family members of PC patients with a higher chance of being affected by FPC [15]. A window of opportunity exists before the manifestation of clinical symptoms, during which the detection of precursor lesions offers a chance for preventive measures to hinder invasiveness. To enhance opportunities for potential treatment, early detection is essential [16]. Lowering the detection limit can be achieved through the integration of novel diagnostic tests and methods. Given the widespread presence and significance of the GI microbiota, monitoring this aspect within the GI holds promise for enhancing outcomes in future patients.

Several novel biomarkers and techniques have been introduced to diagnose cancers including PC at an earlier stage of the disease, including serum biomarkers, imaging techniques, genetic testing, and identification of high-risk premalignant lesions [17, 18]. The gastrointestinal (GI) tract and pancreas have a continuous ductal structure [19]. The composition of the microbiota in each of these segments of the GI tract has the potential to influence the other and give rise to diseases resulting from abnormally abundant pathogens. 16S rRNA sequencing is the most common approach to identify the diversity and distribution of microbial communities in various reservoirs of the human body [20, 21]. The functional properties of microbiota samples from patients are studied using metagenomic, metaproteomic, and metabolomic methods [22].

Dysbiosis refers to changes in diversity of the microbial population, that is accompanied by alterations in the taxonomic microbial profiles [23, 24]. There is good evidence for the possible role of dysbiosis in the pathogenesis of various GI tract pathologies, including PC [25,26,27,28]. Various sequencing techniques have been used to investigate the importance of different microbial species in the formation and progression of GI tract pathologies and neoplasms [29]. Research on the effects of microbial diversity on PC is still in early stages, but emerging evidence provides a link between the microbiome and PC [30, 31]. Various bacterial, fungal, and viral species exist in the GI tract and may be involved in the development of PC. The microbiota partially exerts its effects by modulation of tumor microenvironment, which may also lead to alter treatment efficacy [32,33,34].

In the present review, we summarize the role that different bacterial, viral, and fungal species may play in the pathogenesis and development of PC. The effects of microbial diversity and dysbiosis on the pathogenesis and effects on therapy and management of the disease are also summarized. Additionally, we present the possible clinical applications of these microbial species as prognostic and diagnostic biomarkers. We also discuss some of the therapeutic methods that may be used to treat PC by influencing GI tract microbial diversity, including bacteriophage therapy, probiotics, antibiotics, and fecal microbial transplantation.

Identification and characterization of biomarkers play a crucial role in improving the early detection, diagnosis, and management of PC [35]. Several techniques and approaches have been employed to identify and validate biomarkers associated with PC.

Studies have utilized techniques such as 16S rRNA gene sequencing and metagenomic shotgun sequencing to characterize the composition and diversity of the gut microbiota in PC patients [36]. 16S rRNA gene sequencing is used to analyze the microbial diversity present in the pancreatic tumor and surrounding tissues [37]. 16S rRNA genes have been found to be highly conserved and are used for taxonomic classification, serving as a basis for accurate characterization techniques such as gene sequencing and amplification [38]. The most recent development in characterizing the gut microbiota is metagenomics which is the study of the genetic material acquired directly from clinical or environmental samples [39]. Metagenomics aid in investigation of the collective genomes of the environment, and crosstalk between microbial components and disease formation to determine causative mechanisms [40]. Metagenomic sequencing involves direct DNA sequencing of the microbial communities present in the pancreatic tumor. It identifies the functional characteristics of the microbiome that are associated with PC [41]. Currently, samples for next-generation sequencing are collected from feces, mucosal biopsy, and intestinal aspiration. However, these methods are not completely accurate reflections of intestinal microbiota composition [42]. Metabolomics is the study of small molecules (metabolites) produced by the microbiome. It can help in identifying specific metabolites as biomarkers for PC diagnosis and prognosis [43]. These approaches have revealed potential associations between specific bacterial taxa, dysbiosis, and PC development [44]. Furthermore, metagenomic and metatranscriptomic analyses have provided insights into the functional properties of the microbiome and its potential impact on PC pathogenesis [10]. Computational methods are used to analyze the microbiome data obtained from sequencing [45, 46]. These methods include machine learning algorithms, statistical models, and network analysis methods that help in identifying microbial biomarkers for cancer [45]. The identification and characterization of biomarkers are critical for improving the early detection, diagnosis, and management of PC. Employing techniques such as 16S rRNA gene sequencing, metagenomics, and metabolomics has provided valuable insights into the composition, diversity, and functional characteristics of the gut microbiota in PC patients. Metagenomics, particularly, has facilitated the investigation of the collective genomes of the environment and the interplay between microbial components and disease formation.

The intricate interplay between PC and the GI tract microbiome has been extensively investigated through numerous in vivo, in vitro, and in silico studies. Studies have been conducted on rodents in order to better understand the role of the microbiome in cancer formation (Fig. 1). In a study on mice, P. gingivalis was orally administered, inducing a significant change in gut microbial composition, before altering systemic inflammatory resistance [47]. Although the underlying mechanism of tumorigenesis remains uncertain, in a study by Tan et al., the presence of P. gingivalis was detected in both oral cavity and tumor tissues in PC patients. To prove that P. gingivalis can migrate from the oral cavity to pancreas, murine PC cell lines Pan02 were implanted into the pancreas of mice while gavaging P. gingivalis. Subsequent analysis revealed that P. gingivalis-gavaged mice exhibited significantly higher tumor burden and increased cell proliferation compared to vehicle-gavaged mice. Further analysis showed that P. gingivalis promotes PC progression by elevating the neutrophilic chemokine and neutrophil elastase secretion [48]. In recent years, there has been growing interest in understanding the role of the gut microbiome and its metabolites, particularly short-chain fatty acids (SCFAs), in PC development, progression, and clinical outcomes [49]. SCFAs, including acetate, propionate, and butyrate, are produced by the gut microbiota through fermentation of dietary fiber and other substrates [50]. SCFAs exert diverse effects on host physiology and have been implicated in various cancer types [51, 52]. Emerging evidence suggests that SCFAs can modulate several processes involved in cancer development, such as inflammation, cell proliferation, and immune responses [53]. SCFAs and the gut microbiome can influence PDAC prognosis by modulating the tumor microenvironment and host immune responses [35]. Certain SCFAs, such as butyrate, have been associated with improved prognosis and enhanced response to chemotherapy in PDAC in vitro and in vivo models [54]. Conversely, dysbiosis and alterations in SCFA production may contribute to an immunosuppressive microenvironment and poorer outcomes [35]. Interventions targeting the gut microbiota and SCFA production, such as probiotics, prebiotics, and dietary fiber supplementation, have shown potential in modulating the tumor microenvironment and enhancing immunotherapy efficacy [55, 56].

Schematic representation summarizing the role of microbiome in the development of pancreatic cancer

Animal models and in vitro studies have played major role in shaping our understanding of cancer formation and treatment, but clinical studies conducted on patients provide more reliable results. Fan et al. conducted a cohort study involving 361 patients with pancreatic adenocarcinoma and 371 controls matched for age, sex, race, and year of sample collection. After characterizing the composition of oral microbiota in oral wash samples using bacterial 16S rRNA sequencing, they found that Porphyromonas gingivalis was associated with a higher risk of PC, and the presence of Aggregatibacter actinomycetemcomitans was associated with a twofold increase in PDAC risk. Phylum Fusobacteria and the genus Leptotrichia were associated with a lower risk of developing PC. The possibility of reverse causation was reduced by excluding cases with neoplastic development 2 years prior to sampling. Nevertheless, the risks related to these phylotypes remained significant [57]. In order to investigate the role of Fusobacteria in PC development, Udayasuryan et al. found that Fusobacterium nucleatum infection in both normal pancreatic epithelial cells and PDAC cells caused an increase in cytokine secretion, including GM-CSF, CXCL1, IL-8, and MIP-3α, promoting phenotypes in PDAC cells associated with tumor progression, including proliferation, migration, and invasive cell motility. This phenomenon occurred in response to Fusobacterium infection regardless of the strain and in the absence of immune and other stromal cells. Blocking GM-CSF signaling markedly limited proliferative gains after infection [58].

In a prospective study, Wei et al. compared 41 patients diagnosed with pancreatic adenocarcinoma and 69 healthy controls [59]. 16S rRNA sequencing was used to identify bacterial taxa. Z-scores were calculated based on operational taxonomic unit values and logistic regressions were performed to calculate the risk prediction for oral bacteria. Compared to healthy controls, The study found that carriers of Streptococcus and Leptotrichina had an increased risk of PDAC development compared to healthy controls. Additionally, Veillonella and Neisseria were associated with a decreased risk of PDAC and promoted protective characteristics. Patients who reported bloating were found to be more likely to carry higher amounts of Porphyromonas, Fusobacterium, and Alloprevotella. Patients with jaundice had a greater abundance of Prevotella. Patients with dark brown urine were more abundant in Veillonella, and patients reporting diarrhea were found to have lower amounts of Neisseria and Campylobacter. Patients with vomiting had decreased values of Alloprevotella. The existence of symptoms such as bloating, jaundice, and dark brown urine might urge patients to seek medical care thus led to an earlier diagnosis and a better prognosis.

Chung et al. conducted a study in which microbiota was isolated from tongue, buccal, supragingival, and saliva samples from 52 subjects. High throughput DNA sequencing was used to characterize 16S rRNA genes. After analysis, significant difference in bacterial taxa between oral cavity and intestinal and pancreatic tissue samples were found. After adjusting for disease status and within-subject correlation, specific co-abundance patterns in the presence and absence of oral and intestinal or pancreatic samples of Fusobacterium nucleatum subsp. vincentii and Gemella morbillorum were observed between PC patients and healthy controls. These findings indicate that concurrent presence or absence of specific microbial clusters across different sites is associated with the progression of PC or other gastrointestinal disorders [60].

These results suggest the potential role of oral dysbiosis in the development of PDAC. Further research is required to establish any causal relationship between the oral microbiome and development of PC, as well as the possible underlying mechanism of pathogenesis. It is unknown if it is the oral microbiota affecting the composition of the intestinal or pancreatic microbiota, or the other way around. By analyzing oral samples collected from subgingival plaque, tongue coating samples, and fecal samples, Iwauchi et al. found a higher prevalence of transitions in the oral microbiota in elderly compared to other adults, suggesting the influence of oral healthcare on gut microbial composition [61]. Kohi et al. compared bacterial and fungal profiles of subjects, including 134 healthy controls, 98 pancreatic cyst patients, and 74 PDAC patients in a case-control study. PDAC patients exhibited reduced duodenal microbiota diversity compared to healthy controls and patients with pancreatic cyst, while no difference was found between the latter two groups. Significantly increased levels of Bifidobacterium were observed in duodenal fluid in PDAC patients compared to the healthy control group. Also, high levels of Fusobacteria and Rothia bacteria were found to be related to short-term survival (STS) of patients with PDAC. From this study, it may be concluded that the retrograde migration of pathogenic microbiota from the upper GI tract can lead to identification of diagnostic microbiome profiles of patients affected by or at risk of PDAC [62]. In another study conducted on duodenal bacteria flora of 62 patients with duodeno-pancreato-biliary cancers, 16S rRNA analyses were performed to determine bacterial composition. Of the patients, 17 were positive for Enterococcus spp., with E. faecalis showing higher survival rates in pancreatic juice compared to the other bacterial species. Thus, alkalinity may be a selective survival factor of E. faecalis, which is able to colonize the pancreatic duct and cause chronic changes in an altered pH condition [63]. Ren et al. divided 87 PC patients into two groups: those with pancreatic head cancer (PCH) (n = 54) and those with pancreatic body and tail cancer (PCB) (n = 31), along with 57 matched healthy controls. PCH group were further divided into two groups of obstructed bile duct (PCH-O) and unobstructed bile duct (PCH-unO). The microbial characteristics of fecal samples were analyzed using MiSeq 16S rRNA sequencing technique. The Lactobacillus, Haemophilus, and Streptococcus genera were found to be more abundant in stage II PC patients compared to stage I patients. Enriched Streptococcus was also more notable in PCH compared to PCB, and Streptococcus showed a significant elevation in PCH-O versus PCH-unO, marking a close association with the bile in PC. The results indicated a significant decrease in intestinal microbial diversity in PC patients, and an increase in LPS-producing pathogens were observed. Their analyses achieved high classification power for PC which can highlight the significance of gut microbiome as a potential non-invasive cancer diagnosis marker [64]. Okuda et al. detected bacteria in all isolated samples from tumor-associated tissues, gastric fluids, pancreatic juice, and bile of 11 biliary tract cancer patients and 4 PC patients who underwent curative resection. Using the detection of 16S rRNA sequences in tumor-associated tissues and pancreatic fluids, they found Akkermansia to have the highest abundance, but it was only detected in patients’ bile samples. They also found that patients who were positive for bile-specific Akkermansia were more likely to have external biliary drainage [65]. Using 16S rRNA sequencing, Riquelme et al. analyzed tumor composition in PDAC patients. They identified Pseudoxanthomonas, Streptomyces, Saccharopolyspora, and Bacillus clausii as an intra-tumoral microbiome signature in long-term survival (LTS) patients. The pancreatic microbiome, coordinated with the gut microbiome, was demonstrated to influence the host immune response and have an impact on the course of disease [44]. In a separate study by Halimi et al., pancreatic cyst fluid samples were acquired in intraductal papillary mucinous neoplastic lesions. MALDI-TOF MS profiling analysis was performed on these samples and showed Gammaproteobacteria and Bacilli dominated in isolated microbiota. Among these, Klebsiella pneumoniae, Granulicatella adiacens, and Enterococcus faecalis demonstrated pathogenic properties in an ex vivo culture environment. They concluded that pathogenic properties included intracellular, cell death induction, and DNA double-strand breaks, suggesting an explanation for pancreatic cystic lesions’ progression to neoplasms [66]. In a study performed by Chakladar et al., intra-pancreatic microbiome was found to be correlated with immune suppression and metastasis, in addition to a poorer prognosis of PDAC in males and smokers. They found 13 microbes in association with advanced tumor progression, while 9 were positively correlated with reduced ability of tumor suppressive pathways. Of these, A. baumannii and M. hyopneumoniae were associated with smoking, which causes genomic changes leading to PDAC. A. ebreus, A. baumannii, G. kaustophilus, and E. coli abundance were found to increase cancer activation and immune-suppression pathways in males compared to females. Citrobacter freundii, Pseudomonadales bacterium, and A. ebreus were positively correlated with proinflammatory immune pathway activation. C. freundii and M. hyopneumoniae were associated with immunosuppression and activation of oncogenic pathways [67]. It is worth noting that in another study conducted on the African American population, no difference in microbiota diversity between PDAC cases and healthy controls were found after accounting for multiple comparisons [68].

There is evidence to suggest that fungal and viral infections may play a role in the development of PC. The study by Aykut et al. showed that the presence of fungi, particularly Malassezia spp., in the pancreas is linked with the development and progression of PDAC. The composition of the mycobiome in the tumor tissue was different from that of the gut or normal pancreas. They showed that the ablation of the mycobiome could be protective against tumor growth in slowly progressive and invasive murine models of PDAC. Moreover, repopulation with Malassezia species accelerated oncogenesis. Further evaluation of the underlying mechanisms revealed that the ligation of mannose-binding lectin (MBL), which binds to glycans of the fungal wall to activate the complement cascade, was required for oncogenic progression [69]. The development of PC has been found to be associated with Candida infection in the oral cavity as per a prospective cohort study carried out in Sweden [70]. In the case of mechanism, the presence of oral Candida induces inflammation and promotes the growth of suppressor cells that are derived from the myeloid lineage [71]. According to certain evidence, it is possible that hepatitis viruses are connected to PC. Several studies have shown a correlation between chronic pancreatitis and hepatitis B virus [72, 73]. The risk of PC is found to be higher in individuals with hepatitis C virus infection, as proven by a meta-analysis carried out by Arafa and colleagues [74]. These investigations connect chronic hepatitis, chronic pancreatitis, and PC, highlighting the fact that the potential involvement of viruses in PC must not be disregarded.

Further study of microbial roles and underlying mechanisms and pathways is needed to gain an imperative understanding of tumor genesis associated with the microenvironment and immune pathways to encourage the development of innovative treatment and diagnostic methods. While the microbial composition of PC patients has been explored in various tissues and fluids, including pancreatic juice, bile, and tumor-associated tissues, questions about the causal relationship between the oral and intestinal microbiota and the mechanisms of pathogenesis remain. Research is required to elucidate the microbial roles, underlying mechanisms, and pathways influencing tumor genesis, microenvironment interactions, and immune responses associated with PC. Table 1 provides a summary of the microbial species discussed, delineating their respective roles in either promoting or inhibiting the progression of PC.

PC can influence the metabolic function of the surrounding tissue environment and potentially alter the composition of the GI microbiota, enabling researchers to identify biomarkers [75]. Current clinical practice guidelines advise primarily screening in high-risk individuals, including those with at least two first-degree relatives with PC. As a result, genetic testing must be considered for eligible relatives who are at risk for FPC [76]. Therefore, patients outside of these criteria might miss the opportunity for early intervention and treatment, which prompts the need for easily performed, non-invasive, and accurate biomarkers to broaden the screening criteria.

Oral samples are a fast way of obtaining insights into patients’ microbial composition. Although the mechanism is not yet completely clear, the literature has proven the association between PC and oral microbiota [77]. Tooth loss, cavities, and periodontal diseases have been found to be independent predictors of PC [78,79,