Role of Intestinal Microbiota in Ulcerative Colitis

indicates that a change in the microbiota from ‘healthy’ to ‘unhealthy’ occurs during the remission
period.
Background
Ulcerative colitis (UC) is an idiopathic inflammatory bowel disease (IBD) which is characterized by
chronic inflammation of the colonic mucosa. UC is usually associated with recurrent attacks and
complete remission of symptoms in the interim. The classical symptoms of patients in relapse are
diarrhea with passage of blood or mucus, or both, occasional abdominal cramping and pain as well
as systemic symptoms such as fever and weight loss in severe cases [2,13].
The aetiology of inflammatory bowel disease remains an enigma, and no known infectious agent
has yet been demonstrated [30,44]. However, the intestinal microbiota seems to be involved in
triggering inflammation, as observed in several animal models, where colitis could not be induced
in the absence of a microbiota [45]. Hence, the intestinal microbes may influence some of the key
mechanisms that are involved in the inflammatory processes of the gut mucosa [20,44].
Lipopolysaccharides (LPS) released by Gram‐ negative bacteria are able to trigger an inflammatory
response through the nuclear factor κB (NF‐κB) pathway leading to over‐expression of pro‐
inflammatory cytokines [42]. However, certain Gram‐positive bacteria, particularly bifidobacteria
and lactobacilli, have been shown to have a beneficial therapeutic effect in inflammatory bowel
disease [23,54]. Additionally, in vitro studies show that strains of lactobacilli and bifidobacteria do
not possess any intrinsic pro‐inflammatory characteristics but exert anti‐inflammatory actions
such as inhibition of NF‐κB activation and IL‐8 secretion [16,42]. Both Gram‐negative and Gram‐
positive bacteria may therefore be involved in the pathogenesis of inflammatory bowel disease.
A recent study, including a large metagenomic sequencing approach, has shown that the fecal
microbiota differs between subjects with UC and healthy controls [41]. However, only very few
investigations [51,52] have so far examined if there are differences between the fecal microbiota
from UC patients in relapse and in remission by quantitative methods.
The aim of our study was therefore to compare fecal microbiota derived from healthy controls,
patients with UC in remission, and patients with active disease. Additionally, we wanted to
investigate if the composition of Gram‐negative bacteria and Gram‐positive bacteria differed
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