Background Colonization of the human gut and tumor tissue by non-pathogenic fungi
has emerged as a potential risk factor associated with cancer epidemics. Therefore,
we aimed to conduct a systematic review to assess the role of fungal colonization
in gastrointestinal (GI) tumors in increasing diagnostic efficiency. Methods A PubMed
citation search was conducted for publications up to and including March 2023, followed
by full-text screening. Results were reported according to Preferred Reporting Items
for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. According to the
Patient, Intervention, Comparison, Outcome (PICO) framework, patients diagnosed with
early-and advanced-stage GI cancers, GI adenoma patients, and healthy subjects were
included with metagenomic (MG) or internal transcribed spacer (ITS) sequencing on
tumor tissue, adjacent normal tissue, stool, and blood samples. Results Fourteen studies
were eligible based on the inclusion criteria and methodological quality. Studies
were conducted in stool (n = 8) or tissue (n = 7) as the most common specimens to
be used for molecular analysis. In the collected data, ITS was used in n = 10 cases
and metagenomic analyses in n = 3 cases. Observing the interindividual variability,
we found that the Ascomycota/Basidiomycota (A/B) ratio from healthy to cancer state
decreased in n = 2, increased in n = 1 cases, and did not change significantly in
n = 2 studies. An increase in the relative abundance of Malassezia was identified
in n = 4, of Candida in n = 5, of Saccharomyces in n = 2, and of Aspergillus in n
= 2 cases. Intraindividual differences in the A/B ratio were identified in cancer
and adjacent tissue (n = 4) and cancer vs. stool (n = 1) studies. Intraindividual
variability of the A/B ratio showed an increase in n = 2 and no change in n = 3 studies
for cancer tissue. Conclusion In conclusion, the advent of highly sensitive sequencing
methods may aid in the identification and the differentiation of cancerous from healthy
human fungal colonizations with potential future diagnostic applications. Further
studies are needed to establish reliable biomarkers for GI cancer screening.
