Introduction
Colorectal cancer (CRC) is the third most common malignant tumor worldwide [
1]. With the development of early diagnosis methods and molecular targeted therapy, the progression free survival (PFS) and the overall survival (OS) of patients with CRC have increased [
2]. However, metastatic colorectal cancer (mCRC) is associated with a poor prognosis [
3,
4]. Chemotherapy is a strongly recommended therapeutic regimen to prevent and control mCRC, and it has been associated with a prolonged life [
5,
6]. Adjuvant chemotherapy may confer a survival advantage in resected CRC patients with stage III or high-risk stage II disease [
7,
8]. Cytotoxic chemotherapeutic agents inhibit the proliferation of cancer cells, but, at the same time, they produce toxicity and side effects on other tissues and organs. How to effectively and efficiently avoid or to alleviate the toxicity and side effects is a critical clinical problem that should be solved.
CID is a common side effect of the digestive system in the antitumor treatment process of cytotoxic drugs. The typical clinical characteristics of CID are as follows: symptoms range from loose stool without pain to severe, watery diarrhea with abdominal pain; symptoms typically last 5 to 7 days and may occur during or after chemotherapy; and patients with CID have a poor treatment response to gentamicin, berberine, and furoxone [
9]. CID can lead to weakness, electrolyte disorders, renal failure, blood volume reductions, shock, and even death [
10]. Its occurrence can result in delaying chemotherapy, increasing hospitalization time and costs, aggravating the psychological stress of patients, reducing the treatment compliance of patients and even altering the entire chemotherapy plan [
11].
A unique microecosystem and characteristic microbial community lives in the colorectal regions owing to their function of storing feces [
12]. Accumulating evidence points to the gut microbiome being involved in CRC [
13]. For example, some researchers have reported that
Streptococcus bovis [
14] and
Streptococcus gallolyticus [
15] are specific bacteria involved in colorectal cancer; Castellarin et al. [
16] found that
Fusobacterium nucleatum infection is prevalent in CRC tissue specimens and Kostic et al. [
17], found that
Fusobacterium nucleatum can generate a proinflammatory microenvironment that is conducive to the progression of CRC through recruitment of tumor-infiltrating immune cells. Moreover, many enzymes, peptides and small molecules secreted by the intestinal gut microbiome are involved in activating and regulating important signaling molecules and signaling pathways involved in the progression of CRC [
18,
19].
Diarrhea and gut microbiome disorders interact as both cause and effect. Diarrhea can disrupt the balance of the gut microenvironment. Meanwhile, the invasion of exogenous pathogenic microorganisms and the imbalance of intestinal microbes can lead to diarrhea [
20]. For example, the invasion of some pathogenic bacteria including
Shigella,
Salmonella and
Klebsiella [
21,
22], and the abundance changes of intestinal parasitic microbes include
Candida albicans,
Escherichia coli, and
Aeromonas can induce diarrhea [
23‐
25]. Chemotherapy has been widely used in the clinic as an effective therapy for colorectal cancer, but diarrhea caused by chemotherapeutic drugs often affects the implementation of chemotherapy regimens [
26]. Many probiotics and antibiotics play a positive role in the treatment of infectious diarrhea by regulating the gut microbiome [
27,
28]. The study of relationships between gut microbiome and CID may provide a new direction for solving this clinical problem.
In the present study, we aimed to explore the association between the gut microorganisms and CID of the CapeOX regimen in resected stage III CRC and to provide some research methods and research ideas for further exploring the relationship between intestinal microbes and CID. The results may provide a fresh approach to the prevention and treatment of CID from a microbiological perspective.
Discussion
The CapeOX regimen is recommended as the postoperative adjuvant chemotherapy regimen in resected CRC patients with stage III or high-risk stage II [
35]. The high-risk factors in stage II may affect the accuracy of experimental results, Thus, CRC patients with stage III were recruited in the present study. According to the NCCN guidelines, the CapeOX regimen for resected stage III CRC has been defined as 8 cycles of capecitabine (1000 mg/m
2 twice daily) combined with oxaliplatin (130 mg/m
2 every 3 weeks) [
36]. CID is one of the most common side effects of the CapeOX regimen [
37]. The degree of diarrhea caused by different chemotherapeutic agents varies. Among the chemotherapeutic drugs for colorectal cancer, the most common drugs that cause diarrhea are fluorouracil, irinotecan and platinum, and the incidence of diarrhea associated with these drugs can be as high as 50–80% [
38]. Our previous clinical investigation found that the probability of CID increased with an increase of chemotherapeutic frequency and chemotherapeutic dosage and most cases of CID occur within 2 weeks after chemotherapy. Therefore, in the present study, stool samples from the patients were collected in 2 weeks after the 8 cycles of chemotherapy of the CapeOX regimen were completed.
Studies of Carroll et al. [
39] suggested that the richness of 16S rRNA sequences was significantly decreased in patients with diarrhea-predominant irritable bowel syndrome. Lee [
40] has reported the community diversity of gut microbiome is lower in patients with kidney posttransplant diarrhea. Similar results have been reported in dogs with acute diarrhea [
41]. The research on these diarrhea diseases is consistent with our findings. We found that the gut microbial community richness and community diversity was lower in CRC patients with CID in the present study. The decrease of microbial diversity may be related to the imbalance of gut microbiome. The dominant pathogenic bacteria leads to the reduction of the resident normal microbiota through the plunder of nutrients or the killing effect of bacterial toxic metabolites.
Our results from the microbial community structure analysis showed
Klebsiella pneumoniae occupied the highest proportion (31.22%) of gut microbiome in CRC patients with CID. Lu et al. [
42] reported that the rate of
Klebsiella pneumoniae was approximately 0.5% in Beijing by culturing the stool pathogens from outpatients with diarrhea syndromes using the Vitek2 Compact instrument. Zhang et al. [
22] only isolated 43
Klebsiella pneumoniae strains from 551 stool specimens from diarrhea patients. Although the study may have be interfered by other factors such as geographical location, nosocomial infection, or pollution contamination in the fecal collection process. The increased proportion of
Klebsiella pneumoniae in patients with CID remains a phenomenon of concern.
Microorganisms can adjust and modify their surroundings due to their metabolic products. A series of enzymes and genes participate in the process of metabolism [
12]. Functional analysis of these enzymes and genes would be conducive to understand the whole microecological environment. In the present study, we screened 75 differentiated microorganisms at the species level by using LDA effect size analysis as well as 23 pathways associated with differential microorganisms by using KEGG databases. These differential pathways are related to microbial metabolites, cell proliferation and death, immune system, and many other aspects. These differentiated microorganisms, their metabolic products and the relevant pathways make up the intestinal microecosystem that causes CID. These results may provide characteristic microorganisms or potential molecular targets for the treatment and prevention of CID.
It must be emphasized that small sample size and potential sample contamination may affect the accuracy of the present study. Strict inclusion and exclusion criteria was performed in this study. Subjects in the experimental group were only included the resected stage III colorectal cancer (CRC) patients who voluntarily accepted and completed 8 cycles of the CapeOX regimen and accompanied with grade 2 of CID. Thus, the small sample size limits the applicability of the findings. A multi-center, large study will provide more powerful data to support the clarification of the microbial differences in CRC patients with CID.
Authors’ contributions
All authors participated in the conception and design of the study; conceived and drafted the manuscript: HS; performed the experiments: ZF, YL, YX and WW; collected the basic patient information, clinical indicators, and imaging data: YX, DM and WW; analyzed the data: ZJ and DM; wrote the paper: HS and ZF. All authors read and approved the final manuscript.