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]. In particular, intestinal dysbiosis is an important feature of inflammatory bowel diseases (IBD)
[14][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] 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]. Although IPAA provides a good quality of life and satisfactory functional outcomes
[25][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]. 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].
2. Ileal Pouch–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]. 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].
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]. 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]. 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]. 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]. 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].
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) |
|
|
|