Role of the intestinal microbiome in colorectal cancer surgery outcomes

In recent years, several studies have shown the impact of the intestinal microbiome on host health and disease development. The human intestinal microbiome is a complex community of bacteria, archaea, viruses, and eukaryotes that is subject-specific and stable in healthy individuals [1]. Disturbances in the balance of the composition and function of the microbiome are associated with the onset of various pathologies, including obesity, Crohn’s disease, and gastrointestinal malignancies [2]. Indeed, a growing amount of evidence supports the role of the microbiome as a risk factor for the carcinogenesis of several malignancies, including colorectal cancers [3]. Conversely, the impact of the microbiome on the occurrence of postoperative complications and on the development of local recurrence after colorectal cancer surgery is not clearly defined.

Assessing whether the microbiome is a potential risk factor for postoperative complications of colorectal surgeries could lead to modifying perioperative care, as multiple perioperative interventions, such as mechanical bowel preparation (MBP) and antibacterial therapy (ABT), drastically influence microbiome composition, especially bacterial diversity [4, 5]. MBP, with or without ABT, is widely but empirically implemented before colorectal surgery to carry out a “clean” intervention that minimizes the risk of fecal contamination of the operative field, particularly during the anastomosis preparation. However, it has been shown that the loss of bacterial diversity could be a risk factor for postoperative complications, questioning the role of preoperative MBP and ABT [4].

Studies of animal models have shown that a eubiotic state is important for normal wound healing, including anastomosis repair after colorectal surgery [6]. Bacterial competition and cooperation can either promote or hamper wound healing during the inflammatory phase, influencing cellular activation and fibrosis in the wound repair process [5]. In the eubiotic state, bacteria remain harmless and do not cause infections, whereas following changes in the local environment and the induction of dysbiosis, by surgical injury for example, bacterial invasion and tissue inflammation take place [7].

Due to the major role of the microbiome in inflammation and wound healing [8], it is plausible that dysbiosis may be related to the development of colorectal surgery complications, such as anastomotic leakage (AL), surgical site infections (SSI), and prolonged postoperative ileus (PPI) [9, 10].

The composition of the intestinal microbiome may also impact the long-term outcomes of surgical treatments of colorectal cancer. Local recurrences of colorectal cancer are reported in 1–23% of cases after curative surgery [11]; this rate varies depending on tumor stage, localization, neoadjuvant and adjuvant therapies, and surgical technique [12]. Moreover, the occurrence of major postoperative complications has been related to worse oncological outcomes and overall and disease-free survival [13]. Of note, regarding the occurrence of postoperative complications, the administration of adjuvant chemotherapy may be delayed or contraindicated due to an insufficient patient performance status, which impacts prognosis [14].

In this context, it would be important to characterize the intestinal microbiome not only as a potential contributor to early postoperative complications (e.g., AL) but also as a potential marker of cellular and molecular mechanisms linked to local recurrence.

The present systematic review aims to analyze the available literature about the intestinal microbiome and its association with postoperative complications and long-term oncological outcomes after colorectal cancer surgery. To our knowledge, there is no existing review on the topic that has been performed with a systematic approach.

The present systematic review is the first, to our knowledge, to analyze the existing literature about the intestinal microbiome and its association with short- and long-term outcomes after colorectal cancer surgery in a systematic manner.

Literature about the impact of microbiome diversity on the occurrence of postoperative complications after colorectal cancer surgery is very limited. Only one study focused on the relationship between the composition of the microbiome and AL [29] and showed that a low microbial diversity and high abundances of Lachnospiraceae and Bacteroidaceae were significantly related to AL. These findings were used for a predictive analysis, which revealed that samples with a total sum of Lachnospiraceae and Bacteroidaceae higher than 60% and a Simpson diversity score on the family level < 0.75 are prone to developing AL, with an odds ratio of 28 (95% confidence interval not reported). Interestingly, both bacterial families are normal habitants of the intestinal microbiome and are important for the healthy functioning of the colonic epithelium, with Faecalibacterium prausnitzii and Eubacterium rectale (Lachnospiraceae family) being the two most common species in the human gut [31]. The Lachnospiraceae family is rich in butyrate-producing bacteria that provide a crucial energy source for colon epithelial cells that is essential for the maintenance of gut barrier functions and immunomodulatory and anti-inflammatory properties. To explain the findings with possible pathophysiological effects, it would be crucial to compare the composition of the microbiome before and after surgery.

Half of the included studies focused on F. nucleatum and its association with the long-term oncological outcomes of colorectal cancer patients. All studies showed that high concentrations of F. nucleatum had a negative impact on survival outcomes and were associated with decreased OS [26], decreased median survival [22], decreased CSS [27], decreased DFS [26, 27], decreased RFS [28], and increased cancer-specific mortality [25]. However, we must be careful in interpreting these results because the available data rarely or only partially considered the many potential confounding factors that can influence survival outcomes.

In recent years, multiple reviews have analyzed the relationship between Fusobacteria and colorectal cancer. Several studies that detected a higher abundance of F. nucleatum in colorectal adenomas compared with healthy colorectal tissues [32] suggested the possible involvement of this bacterium in the pathogenesis of colorectal cancer. A study by Flanagan et al. [22] supports this hypothesis, indicating that not only there are higher F. nucleatum levels in individuals with adenomas than in controls but also that F. nucleatum levels increase through adenomatous stage progression from a tubular adenoma to a high-grade dysplasia and colorectal cancer. Indeed, F. nucleatum is able to invade human epithelial cells, activate the β-catenin pathway, and trigger oncogenic gene expression using the FadA adhesion virulence factor; consequently, it can stimulate colorectal cancer cell growth [33]. Moreover, a high abundance of F. nucleatum is associated with cancer progression and lymph node metastases, as shown in the study by Li et al. [34]. The majority of the included studies [25‐28] also found a significant association of F. nucleatum with one or more unfavorable cancer characteristics—advanced stage, tumor invasion, lymph node metastasis, and tumor size. Further important evidence was obtained from the study by Yu et al. [28], which highlighted the negative impact of F. nucleatum on oncological outcomes. This study compared microbiome composition between recurrent and non-recurrent colorectal cancers and found that F. nucleatum is the most enriched bacterium in recurrent cases. Moreover, the study suggests that F. nucleatum may promote resistance to oxaliplatin and 5-FU chemotherapy regimens via the colorectal cancer cell autophagy pathway, therefore leading to decreased recurrence-free survival.

Another bacterium that was found to be associated with unfavorable oncological outcomes was B. fragilis, as shown in the study by Wei et al. [26]. Like F. nucleatum, B. fragilis was also associated with decreased 3-year OS and DFS. However, unlike F. nucleatum, which is an indigenous species in the oral cavity and is associated with only pathological changes in the colon and rectum, B. fragilis is present in the intestinal microbiome of all healthy individuals. In particular, enterotoxigenic B. fragilis (ETBF) has been linked to the development of colorectal cancer. Purcell et al. [35] showed an increased abundance of ETBF in early stage carcinogenic lesions—low-grade dysplasia and tubular adenomas. Viljoen et al. [36] described significantly higher ETBF and F. nucleatum levels in individuals with advanced stage III and IV colorectal cancer. Therefore, it remains to be determined whether the worse oncological outcomes described by Wei et al. [26] are due to specific pathways in carcinogenesis or the B. fragilis association with advanced cancer stages.

Kosumi et al. [24] analyzed the relationship between the Bifidobacterium genus and colorectal cancer characteristics and outcomes. Bifidobacterium has been of interest in many studies, mainly concerning probiotic administration and their effect on gastrointestinal and extra-digestive pathologies. Bifidobacteria are normal habitants of the intestinal tract and are present from an early age. The imbalance of Bifidobacteria has been associated with several non-oncological diseases [37], but few studies have suggested its association with colorectal cancer. Indeed, it has been shown that there are decreased levels of Bifidobacteria in patients with colorectal cancer compared with the levels in controls or patients with diverticular disease [38]. Conversely, Kosumi et al. [24] did not find any significant association between the amount of Bifidobacteria and cancer-specific and overall mortality or any relationship with the clinical, pathological, or molecular characteristics of colorectal cancer. Despite the paucity of literature, it is likely that Bifidobacteria exert a beneficial effect during the early stages of carcinogenesis, acting as a prebiotic agent in improving epithelial defenses against infections [39]. The study by Flemer et al. [23] is the only one that focused on the analysis and comparison of tissue-associated groups of various bacteria rather than individual bacteria. In their previous work [40], the authors defined 6 mucosa-associated bacterial co-abundance groups (CAGs) and showed an increased abundance of the Firmicutes CAG 2, the Prevotella CAG, the pathogen CAG, and the Bacteroidetes CAG 2 in tissues of colorectal cancer compared with their levels in tissues of control subjects. Moreover, they showed that the pathogen CAG and the Prevotella CAG were correlated and that their gene expression levels were previously associated with decreased survival in patients with colorectal cancer. In the most recent study, the results support the idea that the two latter CAGs are in fact associated with longer survival [23]. The authors stressed the importance of further studies to explain this outcome.

The present systematic review is based on 8 original studies with variable quality and risk of bias characterized by a high degree of heterogeneity in terms of study design, sample size, patient population, clinical characteristics, bacterial sampling and detection methods, target microbiome, and target outcome. Although all studies that focused on oncological outcomes detected MAM from tumor samples and in 5 of these studies [22, 25‐28], they analyzed a DNA load of F. nucleatum, it must be stressed that each study used different types of samples. Only Flanagan et al. and Wei et al. used fresh frozen tissue samples as opposed to formalin-fixed paraffin-embedded (FFPE) samples used in the rest of the studies. It has been shown previously that FFPE specimens have lower expression of the analyzed targets than fresh frozen samples, which is of great importance in the quantitative analysis of bacteria [41]. Three studies [22, 23, 27] analyzed the microbiome in healthy tissues from the same patient as a reference. Although this approach is a common practice in studies focusing on microbiome detection in colorectal cancer and is used to exclude interindividual microbiome differences, there is contradictory evidence whether the healthy and cancerous tissues from the same subject are representative of healthy and cancer-related microbiome composition. Finally, it must be highlighted that the included studies were carried out in different geographical areas (e.g., Europe, Asia, the USA), a feature that may influence the diversity of the intestinal microbiome [42]. The mode of subsistence and diet seem to be the most important natural factors impacting the intestinal microbiome, and diet varies between ethnic groups, nationalities, and people who live in rural or urban areas, with the latter being especially enriched in Bacteroides, Bifidobacterium, and Firmicutes [42].

The limited number of studies and the methodological heterogeneity hampered any meta-analytic approach of the present data and it should be carefully considered when interpreting the available body of evidence.

The present systematic review demonstrates that the current literature about the role of intestinal microbiome on colorectal cancer surgery outcomes is limited. However, there is consistency in the available data supporting a plausible linked between the intestinal microbiome composition and the occurrence of postoperative complications in colorectal cancer surgery patients. Further investigations are awaited to assess this association and elucidate whether intestinal microbiome characterization could be used as a prognostic marker in colorectal cancer patients.