Traditional treatments for colorectal cancer (CRC), such as surgery, radiotherapy and chemotherapy, are unsatisfactory due to the high risk of recurrence, metastasis and drug resistance. These challenges have been linked to the incomplete eradication of cancer stem cells, which are tiny populations of cancer cells with the ability to differentiate and proliferate quickly (Parizadeh et al., 2019). The surface molecular marker leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5) is utilized to precisely identify and extract stem cells, also referred to as crypt base columnar cells (Li et al., 2018). Abnormally high LGR5 expression caused canonical Wnt/β-catenin signaling to be potentiated, which in turn encourages cancer stem cell proliferation and self-renewal (Xu et al., 2019). In CRC patients, elevated LGR5 expression is linked to worse overall survival as well as disease-free survival (Morgan et al., 2018). Salinomycin was initially identified as particularly toxic to breast cancer stem cells after evaluating more than 2000 chemicals. Compared the therapy with salinomycin to the commonly utilized chemotherapeutic medication paclitaxel, the proportion of cancer stem cells decreased by more than 100-fold. Additionally, following three weeks of growth in vivo, mouse mammary tumor cell 4T1 pre-treated with salinomycin demonstrated a 4-fold reduction in metastatic burden compared with vehicle-pretreated cells. Furthermore, tumor cell 4T1 that was resistant to paclitaxel exhibited a 2-fold rise in the sensitivity of salinomycin (Gupta et al., 2009). Studies have also demonstrated that salinomycin inhibits other types of cancer stem cells including those found in uveal melanoma, liver cancer, and CRC (Liu et al., 2021; Wang et al., 2019; Zhou et al., 2019).

The intestinal microbiota normally acts as a homeostatic organ in a physiologically healthy state, taking part in the synthesis of vitamins and short-chain fatty acid, energy release, and the fermentation of complex undigested polysaccharide polymers (Khan et al., 2019). It also contributes to the production of mucus, the expansion of the mucosal surface, the thickening of the villus, the maintenance of epithelial connections, and, lastly, the intestinal development (Bäumler and Sperandio, 2016). In addition, the gut microbiota is involved in the development of the local or systemic immune system. Healthy gut flora promotes cancer suppression and elimination (Yu et al., 2023). However, the intestinal microbiota, which has a symbiotic relationship with the human host, may be disrupted by the antibiotic salinomycin. Reversing the salinomycin-induced intestinal microbial imbalance is therefore advantageous for the restoration of multiple physiological processes, especially for boosting immunity against cancer.

Fecal microbiota transplantation (FMT) has been reported as an effective method to reverse the disorder of the intestinal microbiota by transplanting healthy fecal microbiota from donors into patients. Significant therapeutic efficacy has been observed in patients treated for recurrent Clostridium difficile infection (Cammarota et al., 2017), inflammatory bowel disease (Fang et al., 2018), and irritable bowel syndrome (El-Salhy et al., 2020).

This study investigated the synergistic effect of salinomycin and FMT on intervening in CRC progression. Salinomycin effectively inhibited the proliferation of CRC stem cells. The supplementation of FMT significantly reversed the microbial dysbiosis in CRC mice while boosting the anti-cancer immune response. The combination of the two treatments had a more potent anti-cancer effect.