Gastric cancer causes one of the major types of digestive tract tumor worldwide . After the continuous development of high-throughput sequencing technology, research on the correlation between gastric flora (other than Helicobacter pylori) and GC has gradually emerged. In this study, we tried to describe the fungal spectrum associated with GC, which has not been explained to date; the focus was on gastric fungal dysbiosis caused by GC. Compared with fecal samples, the colonization performance of tissue samples can better demonstrate the dynamic changes in the surrounding environment for gastric carcinogenesis. Therefore, we analyzed the ITS metagenome sequences of cancer lesions and adjacent noncancerous tissues to investigate the composition and ecological alterations of fungi associated with GC and identify fungal indicators. To ensure that the most effective data were clustered into OTUs, we filtered low-quality reads, and assembled and refiltered the data. After obtaining the OTUs, under the condition that the GC and control groups were effectively grouped, we carried out species identifications and alpha and beta diversity analysis, and compared differences between groups. Candida albicans was identified for the first time as a key fungus that can be used to distinguish between GC and control groups. We also combined FUNGuild functional annotation to study fungal functions from other ecological perspectives. For the first time, we showed the characteristics of the fungal flora in the stomach tissues of GC patients, demonstrating fungal malnutrition in the GC ecosystem and proving that C. albicans can be used as a biomarker with a certain degree of accuracy.
We clarified specific fungal composition changes in GC. Overall, the GC group showed a lower OTU abundance. At the phylum level, Ascomycota was the most enriched in the GC group compared with the control group, while Basidiomycota was less enriched. We further analyzed the differences at lower taxonomic levels and finally, at the species level, confirmed that C. albicans, Fusicolla acetilerea, Arcopilus aureus and Fusicolla aquaeductuum were excessively colonized in the GC tissue. At present, C. albicans is the most researched of these organisms with regard to its role in various diseases. This species normally exists in the body and does not cause damage. However, when the host's defense capacity is weakened, C. albicans will cause disease. Therefore, C. albicans is recognized as an opportunistic pathogen. Since immunosuppression caused by cancer chemotherapy promotes C. albicans infection, the relationship between C. albicans and cancer development or progression has been widely reported. For example, for hematological malignancies or solid tumors, up to 35% of patients with underlying disease have candidiasis, and the most common underlying disease among patients with candidiasis is also solid tumor . Candida albicans can produce carcinogenic nitrosamines, which can cause abnormal proliferative changes in oral epithelial cancer . The risk of malignant transformation of oral leukoplakia is higher than that of oral lichenoid lesions, and C. albicans strains isolated from patients can produce more carcinogenic acetaldehyde in ethanol . The role of C. albicans in tumor adhesion and metastasis has been associated with TNF-α and IL-18 [37–39]. Recently, Bertolini et al. confirmed that C. albicans induced mucosal bacterial malnutrition and promoted invasive infection .
Notably, we first confirmed the indicative role of C. albicans in GC. In our study, compared with the control, the species richness of C. albicans occupied 22% in the GC group. Both the Welch's t test and Wilcoxon rank sum test confirmed that C. albicans was significantly more abundant in the GC group than the control group. In addition, the ROC curve showed that the AUC value of C. albicans was 0.743. Combined with the results of the Gini index and the mean decrease in accuracy, all results indicated that C. albicans could be used as a biomarker with a certain degree of accuracy. By diversity analysis, compared with the control group, the GC group had a decrease in species richness, diversity and uniformity. The structure of the species flora between the groups also showed a significant change, suggesting that C. albicans has an adverse effect on the diversity and richness of the stomach microbiome. Aykut et al. stated that identifying the species most associated with cancer may guide future attempts to use targeted antifungal drugs to slow tumor growth and avoid side effects and reported Malassezia as a pathogenic fungus associated with pancreatic cancer that promotes pancreatic oncogenesis via activation of MBL. Our discovery that C. albicans may have contributed to the pathogenesis of GC not only lays a scientific foundation for the exploration of innovative therapies for GC but also provides a new idea for treating specific patients by adjusting their intestinal microbial flora as an adjuvant therapy or developing immunotherapies for targeted control of fungal infections, which is worthy of further study.
Due to the current lack of fungal genomic data, we integrated published article data and used FUNGuild to predict fungal functions from other ecological perspectives based on OTU abundance. The guild classification revealed that the most diverse guilds were undefined saprotrophs. Simultaneously, the trophic mode implied that the most diverse fungal type was the saprotrophs. Our analysis clarified the importance of fungal homeostasis in the stomach and showed that fungal dysbiosis eventually promotes the occurrence of GC.