Inflammatory bowel diseases, Crohn’s disease and ulcerative colitis, are life-long disorders characterized by the chronic relapsing inflammation of the gastrointestinal tract with the intermittent need for escalation treatment and, eventually, even surgery. The total proctocolectomy with ileal pouch–anal anastomosis (IPAA) is the surgical intervention of choice in subjects affected by ulcerative colitis (UC). Although IPAA provides satisfactory functional outcomes, it can be susceptible to some complications, including pouchitis as the most common.
1. Introduction
In humans, a wide number of different microbial species are located in the bowel, which hosts several trillion microbial cells
[1]. This ensemble of microbial species is generally called gut microbiota
[2]. There is a profound interplay between the gut microbiota and the human biology
[3]. Indeed, the gut microbiota is important for various physiological functions such as eliciting immune maturation
[4], defending against pathogens colonization and overgrowth
[5], influencing epithelial proliferation
[6] and intestinal vascular density
[7], modifying bile acids in the large bowel
[8], promoting metabolic homeostasis
[9] and hormone modulation
[10], synthesizing vitamins
[11] and neurotransmitters
[12], supplying energy
[1], and regulating bone metabolism
[13]. The intestinal microbiota of a healthy subject is composed predominantly by
Bacteroidetes and
Firmicutes, with also other smaller sections comprised by
Actinobacteria,
Proteobacteria,
Verrucomicrobia, methanogenic archaea,
Eucarya, and various phages
[1]. Modifications in the constitution and function of gut microbes lead to dysbiosis
[1], and several diseases are associated with gut dysbiosis
[1,14][1][14]. In particular, intestinal dysbiosis is an important feature of inflammatory bowel diseases (IBD)
[14[14][15],
15], playing a crucial role in the onset of the disease in predisposed subjects
[16]. Indeed, some important alterations of intestinal microbiota have been identified in IBD, including an underrepresentation of
Firmicutes (in particular
Faecalibacterium prausnitzii)
[17],
Bacteroidetes, and
Lactobacillus [16] with increased levels of
Proteobacteria [18]. IBD are life-long disorders characterized by the chronic relapsing inflammation of the gastrointestinal tract
[19,20][19][20] with the intermittent need for escalation treatment, eventually requiring surgical intervention
[21]. In particular, although decreasing over time, subjects affected by UC still have a 5- and 10-year risk of colectomy of 7.0% and 9.6%, respectively
[22]. Indications for colectomy comprise refractory acute severe UC, medically refractory disease, and colorectal cancer
[19]. For these cases, restorative total proctocolectomy with ileal pouch–anal anastomosis (IPAA) is the surgical intervention of choice
[19,23,24][19][23][24]. Although IPAA provides a good quality of life and satisfactory functional outcomes
[25[25][26],
26], it can be subject to some complications, including pouchitis as the most common
[27]. Pouchitis is an active, non-specific, idiopathic inflammation of the IPAA mucosa
[28]. Approximately 25% of subjects develop pouchitis a year after IPAA with an increasing trend that reaches up to 45% at 5 years
[29]. Approximately 10–20% of the pouchitis may also progress to chronic pouchitis, leading to antibiotic dependency or refractoriness requiring immunosuppressive therapy
[30]. Furthermore, pouchitis is a risk factor for hospitalization
[31] and pouch failure
[32], which can occur in 5–10% of cases
[33,34,35][33][34][35]. The etiology of pouchitis is mostly unclear. However, the efficacy of antibiotics in pouchitis suggests that the IPAA-related dysbiosis of the microbiota could play an important role in its pathogenesis
[36,37,38][36][37][38].
2. Ileal PAA ouch–Anal Anastomosis (IPAA) Microbiota Evolution over Time
Although derived from the small intestinal tissue, the microbiota of the IPAA changes over time into a microbiota with a colonic profile
[39,40,41,42][39][40][41][42]. These modifications can arise as early as two months after surgery and achieve a more stable composition as the years go by after the creation of the IPAA
[40,42][40][42].
Clostridium coccoides,
Clostridium leptum,
Bacteroides fragilis,
Atopobium,
E. coli,
Klebsiella,
Veillonella,
Staphylococcus (coag-), and
Enterobacter are the more counted bacterial species in functional IPAA
[40,41,43][40][41][43]. In particular, it seems that the microbiota of healthy IPAAs try to recover to a composition comparable to that observed prior to surgery
[43], and it has been hypothesized that the presence of
Veillonella,
Lachnospiraceae, Ruminococcus gnavus, and clostridial cluster IV (i.e.,
Faecalibacterium prausnitzii) might be a marker of regularity of the IPAA flora
[37,43,44][37][43][44]. Furthermore, a comparison between the microbiota of IPAA in subjects with UC and subjects with familial adenomatous polyposis (FAP), which exhibit a low incidence of pouch inflammation, might help to understand the microbial families potentially implicated in the pathogenesis of pouchitis
[37]. Indeed, a higher presence of sulfate-reducing bacteria (SRB) in UC-IPAA has been observed compared to FAP-IPAA
[45,46][45][46]. SRB produce hydrogen-sulfide, which inhibits butyrate oxidation and prevents its utilization by the intestinal epithelial cells, potentially resulting in the damage of the mucosa of IPAA
[37,46][37][46]. Other findings confirm the presence of differences between UC-IPAA and FAP-IPAA observing less bacterial diversity, an increased proportion of
Proteobacteria, and decreased levels of
Bacteroidetes and
Faecalibacterium prausnitzii in the UC-IPAA group
[47,48][47][48].
Table 1 shows the most common microorganisms and the main differences between healthy adults, IBD patients, UC-IPAA, and FAP-IPAA patients.
Table 1.
Most common microorganisms and main differences between healthy adults, IBD patients, UC-IPAA, and FAP-IPAA patients.
Healthy Adults |
IBD |
UC-IPAA |
FAP-IPAA |
Bacteroidetes * |
↓ Bacteroidetes |
↓ Bacteroidetes |
↑ Bacteroidetes |
Firmicutes * |
↓ Firmicutes |
↓ Firmicutes |
↑ Firmicutes |
Actinobacteria * |
↓ Lactobacillus |
↑ Proteobacteria |
↓ Proteobacteria |
Proteobacteria * |
↑ Proteobacteria |
Presence of SRB |
Absence of SRB |
Verrucomicrobia |
↑ Enterobacteriaceae |
|
|
Methanogenic archaea |
|
|
|
Eucaria (i.e., yeasts) |
|
|
|