Introduction

Diverticular disease (DD) is one of the most common disorders in western countries, with its prevalence increasing throughout the world, likely because of occidental habits and lifestyles [1, 2]. People affected by colonic diverticulosis remain mostly asymptomatic [3]. However, in about 10% to 25% of patients the obstruction of the diverticulum sac by fecalith leads to colon inflammation and congestion, evolving into overt diverticulitis [4]. Presentation of diverticulitis can be acute or chronic, ranging from mild gastrointestinal symptoms—similar to those of irritable bowel syndrome (i.e., abdominal pain and discomfort, bloating, constipation and diarrhea)—to severe manifestations resembling appendicitis (i.e., fever, acute abdominal pain and leukocytosis), recurrent severe attacks of abdominal pain, fever or acute abdomen [5]. Through the years, among different score systems proposed to stratify treatments for this pathology, the Hinchey classification has been the most used [6]. In July 2016, the Word Society of Emergency Surgery (WSES) [6] proposed the latest classification, distinguishing DD in uncomplicated (thickening of the colonic wall or increased density of the pericolic fat) and complicated (including different stages in relation to abscess size or state of colic perforation, leading to medical treatment or surgery).

Although DD pathogenesis remains poorly elucidated, several factors including genetics, age, colonic motility, vitamin D levels, obesity, diet and fiber intake may influence the disease progression [4]. In recent years different studies have been performed investigating the impact of gut microbiota on intestinal and extraintestinal disease development, including cancer [7] and neurological disorders [8]. In healthy individuals, gut microbiota is a balanced community of bacteria, viruses, archaea and eukaryotes involved in host homeostasis. Alterations of microbiota equilibrium have already been related with DD pathogenesis and progression in acute diverticulitis (AD), even if supported by non-homogeneous results and small samples. Hence, the aim of this study is to analyze the available literature on intestinal microbiota's association with DD and AD in different stages. To our knowledge, no existing review on the topic has been performed with a systematic approach.

Methods

Data Sources and Search Strategy

A systematic review was performed following the Cochrane Collaboration-specific protocol and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [9].

Studies investigating the association between intestinal microbiota and DD/AD were sorted in the following databases without any date restriction: Medline (through PubMed), Scopus, Embase, Cochrane Oral Health Group Specialized Register, ProQuest Dissertations and Theses Database, and Google Scholar. A specific research equation was used for each database, using the following keywords and MeSH terms: diverticulosis, diverticulitis, diverticular disease, microbiome and microbiota. According to the PICOS schema, the following criteria were used for literature search and selection:

P, population: adult patients with DD/AD. Any stage of the disease (according to the Hinchey classification) was considered.

I, intervention: analysis of the luminal or mucosa-associated microbiota from fecal samples or colorectal biopsies. Both culture-dependent and genome sequencing methods were considered.

C, comparisons: patients with and without DD/AD, patients with different bacterial DNA loads in their microbiota samples.

O, outcome(s): type of microbiota, disease treatment (medical therapy or surgery).

S, study design: randomized and non-randomized clinical trials, including cohort and case-control studies.

Studies exclusively investigating colorectal diseases—differing from DD/AD or evaluating the impact of probiotic treatments—were not eligible for inclusion. Our search was limited to human studies published in English. Then, the literature review was completed by using the “related articles” function in PubMed to ensure an extensive approach. Moreover, the reference lists of eligible records and pertinent review articles not included in this study were double-checked to identify potential additional articles.

Literature search and selection were performed by two independent reviewers (ER and EF). Records were removed from the selection if both reviewers excluded the articles at the title/abstract screening levels. Disagreement was resolved via a discussion with a third reviewer (SG). Overall concordance rate between the two reviewers was 99%. Figure 1 reports the PRISMA flowchart of study selection.

Fig. 1
figure 1

PRISMA 2009 flowchart

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Data Extraction

Both reviewers performed an independent full-text analysis and data extraction by filling in an electronic database. Extracted data included first author’s name, year of publication, number of patients, type of microbiota, microbial analysis performed, stage of disease and type of treatment (medical or surgical).

Stage of disease was reported according to the WSES classification [6].

Study Quality Assessment and Risk of Bias

Two reviewers (EF and ER) carried out the study quality assessment and risk of bias evaluation of the selected articles. According to the study design, the New Castle-Ottawa scale (NOS) was used [10].

Results

The initial search yielded 79 results; once duplicates were removed, 78 articles were screened for eligibility according to title and abstract, while 9 articles were retrieved for a full-text evaluation. A total of nine studies [11,12,13,14,15,16,17,18,19] fulfilled inclusion criteria and were finally included in the review.

Study Characteristics

The selected studies were published between 2007 and 2018, counting three cohort studies and six case-control studies. The studies were carried out in Europe (n = 7) [11,12,13,14,15,16, 19] and the USA (n = 2) [17, 18]. Study period was not specified in three studies [11, 15, 19]. The general characteristics of the studies examined are summarized in Table 1.

Table 1 General characteristic of the included studies on diverticulosis/diverticulitis. SUDD, Symptomatic uncomplicated diverticular disease
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In between the nine selected articles, only one was focused on patients with AD, reporting the disease stage according to the current classification [12]. Diseases differing from DD/AD were considered in two studies [11, 16]. Four articles considered symptomatic uncomplicated DD without specifying the correct stage according to current classifications [11, 13,14,15]. Only one study described the stage of the disease [12], while all other articles only considered patients with DD without acute inflammation signs [16,17,18,19].

Daniels et al. [12] compared patients with AD and controls, while Tursi and Mastromarino [13] and Barbara et al. [14] compared AD with patients with DD and controls. Two studies [11, 15] were only focused on AD. Among studies exclusively on DD, three studies [16, 18, 19] compared patients with DD and controls, and only one study was only focused on DD [17].

Four studies collected fecal samples through rectal swabs [12] or stool samples [13, 15, 16]. Five studies collected mucosa samples during colonoscopy [14, 18, 19] or surgery [11, 17].

Disease was defined through CT scan in one study [17], through endoscopy in another one [19], and two studies [15, 16] did not report the grade of the disease. AD was defined by endoscopy in one study [11] and by CT scan in another one. Kvasnovsky et al. [15] reported the use of both CT scan and endoscopy in AD definition, while two studies [13, 14] did not report the diagnostic imaging performed to achieve diagnosis.

All of the included studies used microbial DNA analysis as a method for microbiota detection and characterization. Barbara et al. [14] also performed metagenomics profiling of mucosa-associated microbiota, while Schieffer et al. [17] evaluated the fungal ITS sequencing as well.

Most of the studies focused on the general composition of the microbiota, while Linninge et al. [19] and Tursi and Mastromarino [13] focused on target microbiota profiling (Enterobacteriaceae and Akkermansia, respectively). Culture-independent profiling of bacterial communities in tissue samples was performed using amplification of bacterial 16S ribosomal RNS gene (16S rRNS) in five studies [11, 14,15,16,17], while in four studies DNA detection and bacteria quantification were performed by real-time quantitative PCR (qPCR) [12, 13, 18, 19]. Only one study [15] reported the use of antibiotics and probiotics in patients with AD, not reporting the type of antibiotics used and not adjusting the statistics for their confounding effect. Type of medical therapy in AD stages was not reported in all the other studies [11,12,13,14], but Daniels et al. [12] reported performing the swab before medical therapy administration.

Study Quality Assessment

Based on the Newcastle-Ottawa Quality assessment scale (NOS) [10], three studies received 7/9 stars, three studies received 6/9, and three studies received 5/9. It must be stressed that a high heterogeneity in studies design, population, microbiota analyzing methods and outcomes was observed. All included studies were cohort or case-control studies.

Microbiota Association

Table 2 summarizes the main findings of the selected studies. Kvasnovsky et al. [15] discovered that patients with different episodes of AD showed an overgrowth of Pseudobutyrivibrio, Bifidobacterium and Christensenellaceae families, with an overall microbiota biodiversity positively correlated with fecal calprotectin levels. Bloating severity score was correlated with an abundance of Ruminococcus and decreasing of Roseburia, while the intensity of pain was significantly related to the relative abundance of Cyanobacterium. According to these results, Gueimonde et al. [11] found an overrepresentation of Bifidobacterium, associated with colonic inflammation. Barbara et al. [14] focused on different disease stages (DD and AD), including a control group, showing a significantly lower abundance of Clostridium cluster IV in patients with diverticula. Enterobacteriaceae were positively correlated with AD [12] and DD [19] in two studies, although both were limited by the small sample sizes. However, other studies suggested limited or absent association between AD [13] and DD [16, 18] and microbiota composition. As showed in Table 1, the included studies showed large heterogeneity in population characteristics.

Table 2 Samples and microbiome characterization
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Discussion

According to the current evidence, several factors including age, lifestyle, obesity, fiber intake and medications, together with altered colonic motility, have been proven to impact DD pathogenesis and progression [20,21,22,23,24], with a mutual influence on microbial dysbiosis. Different studies showed a possible overlap between AD and inflammatory bowel disease (IBD) such as Crohn’s disease (CD) and ulcerative colitis (UC) [25, 26]. In these conditions, the association with an alteration of gut microbiota is proven by different studies [27, 28]. Given the link between AD and IBD, similar alteration patterns could also be found in patients with DD.

However, the role of gut microbiota in DD/AD is not defined because of numerous limitations such as small casuistic, heterogeneous samples type (tissue, mucosa or fecal samples) and sampling methods, laboratory techniques (PCR, metagenomics), insufficient clinical characterization of symptoms and disease burden. A systematic review should therefore become useful in the quest for a rationalization of available scientific sources.

To the point, asymptomatic DD does not seem to be linked to significant intestinal microbiota alterations. As shown in Table 1, most of the studies [16,17,18] focusing on DD could not find a concrete association between disease and dysbiosis, except in one case [19] in which a connection between Enterobacteriaceae and DD was demonstrated. Contrarily, studies focused on AD showed that DD progression implies an inflammation component with a consequent change in microbiota composition [29]. Specifically, Barbara et al. [14] pointed out a reduction of taxa with anti-inflammatory activity (Clostridium cluster IV, Lactobacilli and Bacteroides) in patients with AD, while other studies suggested an overgrowth of Bifidobacteria [11, 15], Enterobacteriaceae [12, 17] and Akkermansia [13] in acute stages. Nevertheless, in other experiences these taxa revealed both anti-inflammatory (Bifidobacteria, Akkermansia) and pro-inflammatory (Enterobacteriaceae) activity [30]; therefore, their definitive role should be clarified in future studies.

According to the modulation of intestinal microbiota based on the inflammatory stage [30], patients with more advanced AD could provide more consistent results. Even if Enterobacteriaceae represents one of the most distinctive features in severe dysbiosis [30, 31], all the cited studies were characterized by small sample size, and the largest study to date [18], including 226 DD patients and 309 healthy controls, showed no differences among the groups.

Concerning sample collection and analysis, we found great heterogeneity: as shown in Table 2, microbiota analyses were performed on stool and mucosa samples indiscriminately, even though studies comparing fecal- and mucosa-associated microbiota in the same subjects indicated only partial overlap between results [32, 33]. Fecal microbiota composition depends on stool consistency and bowel movement, being thus influenced by diarrhea or constipation occurrence [34, 35], but there is more! Taking part in the digestion process, the intestinal microbiota synthetizes essential amino acids and vitamins. A symbiotic microbiota is normally characterized by a high diversity, and it can resist changes occurring during physiological stress, being able to stimulate the immune response [36]. Quality and amount of the intestinal microbiota could be modified by different factors such as diet, alcohol and smoking, impairing its ability to interface with the immune system and leading to an uncontrolled degree of inflammation [37, 38].

Hence, further studies should be oriented to determining the reliability of different sampling methods among the different stages of DD.

Considering disease manifestation and related therapies, most of the studies included patients with early stage disease, not requiring emergency surgery (Table 1). Only one article focused on patients submitted to emergency surgery, but did not specify the disease severity according to the current guidelines [11], and this is peculiar, given the fact that AD is one of the most common conditions encountered by surgeons in the acute setting [6], often burdened by postoperative complications such as intra-abdominal abscess, anastomotic leak, bleeding, wound infection and wound dehiscence with a considerable social impact [39]. Current guidelines [6] suggest that patient-related factors (i.e., immunodepression) should be considered in planning elective surgery for DD, but again the authors did not provide information about an elective surgery rationale. The possible role of gut microbiota on postoperative course and complications is suggested by several studies [7] concerning different diseases; therefore, future studies should include patients undergoing surgery for elective DD and emergency AD to clarify the role of intestinal microbiota in the postoperative course, ideally stratifying the results according to the current disease classification.

Study Limitations

The present systematic review aims to show the current literature on the association between DD and the intestinal microbiota. Given the high heterogeneity in study design, variables reported and outcome measures, it was not possible to perform a meta-analysis and quantify the association, thus representing a potential limit of the study. Moreover, included studies often performed different types of comparison as some articles [12, 16, 18, 19] compared disease and controls, others [13, 14] compared AD with DD and controls, while other studies did not make a comparison [11, 15, 17]. Although this could be seen as a limitation, the heterogeneity of the selected studies underlined the need of further studies on the topic more closely aligned with the current guidelines on DD management.

Clinical Impact and Future Perspectives

Evidence deduced from preliminary experiences showed a relationship between gut microbiota composition and DD development and progression in AD. Considering the high prevalence of DD in the general population, this connection represents a promising field of research, with potentially high future clinical relevance. In fact, specific microbiota signatures with a negative impact on colonic microflora can be used as a top biomarker for DD diagnosis, screening and progression, reducing the occurrence of disease development, acute manifestations and emergency surgery requirements and promoting the introduction of targeted probiotic therapies as well, effective for both prevention and cure. In conclusion, actual literature concerning the role of intestinal microbiota in DD progression and AD is still limited, showing non-homogeneous results as a consequence of the evident casuistic heterogeneity. Ongoing studies could possibly translate microbiological concepts into clinical practice, suggesting microbiota-targeted therapies for both surgical and non-surgical stages and reducing the disease recurrence rate, emergency surgery requirement and potential postoperative complications.