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FEMS Microbiology Letters 244 (2005) 1–7
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Sporadic colorectal cancer – role of the commensal microbiota
Mairi E. Hope a, Georgina L. Hold a, Renate Kain b, Emad M. El-Omar
a
a,*
GI Research Group, Department of Medicine and Therapeutics, Institute of Medical Sciences, University of Aberdeen, Foresterhill,
Aberdeen AB25 2ZD, UK
b
Department of Pathology, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK
Received 22 November 2004; received in revised form 14 January 2005; accepted 17 January 2005
First published online 25 January 2005
Edited by R.I. Aminov
Abstract
There are vast numbers of bacteria present within the human colon that are essential for the hostÕs well being in terms of nutrition
and mucosal immunity. While certain members of the colonic microbiota have been shown to promote the hostÕs health there are
also numerous studies that have implicated other members of the colonic microbiota in the development of colorectal cancer, a
prominent malignancy within the western world. In this review we consider the evidence for the role of bacteria in colorectal cancer
from molecular and animal model studies. We focus on some of the mechanisms by which the colonic microbiota drives the
progression towards colorectal malignancy including generation of reactive metabolites and carcinogens, alterations in host carbohydrate expression and induction of chronic mucosal inflammation.
Ó 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.
Keywords: Colonic microbiota; Colorectal cancer; Inflammation; Carbohydrate expression; Reactive metabolites
1. Introduction
Colorectal cancer (CRC) is principally a disease of the
western world and in the UK alone accounts for approximately 17,000 deaths annually, with 30,000 new cases
diagnosed each year [1]. The average age of incidence is
in the seventh decade and it is the most common cause
of cancer related death after lung cancer in men and
breast cancer in women (http://www.cancerresearchuk.org/aboutcancer/statistics/incidence). Over 90% of
CRC cases are sporadic in nature and the most common
type of CRC, accounting for 95% of cases, is adenocarcinoma, which develops from glandular cells lining the
wall of the bowel (Fig. 1). A tumour of the colon is usu*
Corresponding author. Tel.: +44 1224 553021; fax: +44 1224
555766.
E-mail address: [email protected] (E.M. El-Omar).
ally first detected as a polyp (a mass of cells projecting
from the colon wall) although it is now possible to detect
smaller lesions affecting crypts, termed aberrant crypt
foci (ACF). Tumours can appear anywhere within the
colon although the majority of sporadic CRC occur in
the left side of the colon distal to the splenic flexure
(including the rectum and sigmoid) [2].
Sporadic CRC results from a series of somatic genetic
mutations and there are several genetic events commonly occurring in CRC that have been described at a
molecular level. These include inactivation of the tumour suppressor genes such as APC, DCC, DPC4 and
p53, along with activation of the oncogenes, of which
the ras family are the best described [3] (see Fig. 1).
Not all CRC progress down this pathway as not all tumours need to acquire every mutation and there are
other genetic events that may have to occur in order
for a tumour to progress. The fact that some polyps
0378-1097/$22.00 Ó 2005 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.femsle.2005.01.029
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M.E. Hope et al. / FEMS Microbiology Letters 244 (2005) 1–7
Fig. 1. Endoscopic views of the different stages of colorectal neoplasia starting with normal mucosa, small (early) adenoma, large (late) adenoma,
and cancer. These macroscopic stages are associated with distinct genetic alterations that are thought to mediate the neoplastic transformation. The
genetic alterations and their sequence are based on the work by Fearon and Vogelstein [4]. This genetic pathway does not necessarily apply to the
majority of sporadic colorectal cancers but is a useful model for the pathogenesis of the disease.
progress to cancer whilst others do not indicates that
there are other factors that can influence malignant
transformation. It has been shown that migrating human populations can adopt the cancer risk of the area
to which they relocate proving that the environment
plays a crucial role in the development of CRC [5].
There is also increasing evidence supporting the role of
inflammation in the pathogenesis of CRC, with evidence
indicating that commensal colonic bacteria are important in influencing this process [6,7]. Nevertheless, presence of the colonic microbiota is critical for the normal
structure and function of the colon.
2. Colonic microbiota and normal gut function
The human colon is known to contain several hundred different bacterial species with obligate anaerobes
contributing over 99% of the total diversity [8]. This
complex ecosystem develops as a result of interaction
between the hostÕs physiology and the bacteria that are
introduced from the environment soon after birth [9].
Studies suggest that each individual harbours a distinct
microbial cohort that remains relatively constant once
adulthood is reached, however, the composition of the
resident biota may alter as a result of environmental factors such as diet and antibiotic usage. There is also evidence for changes in the colonic microbiota as the body
ages, which has been attributed in part to a decrease in
gastric acid barrier function [10].
One of the primary functions of the intestinal microbiota is to salvage energy from dietary elements that would
otherwise be lost through excretion [11,12]. Bacterial
degradation of complex carbohydrates and other nutrients is an important process that occurs in the colon.
Polysaccharides that remain undigested when they enter
the colon are metabolised to short chain fatty acids
(SCFA) such as butyrate, propionate and acetate by
the residing microbes and then absorbed by passive diffusion [13]. Production of SCFA is dependent on the
substrates available, starch for example is strongly butyrogenic whilst non-starch polysaccharide fermentation results in more acetate and propionate production [13].
SCFA concentrations are higher in the right side of the
colon compared to the left and this is most likely due
to greater carbohydrate availability [14]. SCFA have also
been shown to play a part in maintaining integrity of the
epithelial layer with colonic epithelial cells thought to acquire up to 70% of such energy from butyrate [11]. Butyrate also acts as a trophic factor for cells in intact tissues
[15]. However, it has been shown that butyrate induces
different levels of apoptosis, cell cycle arrest and differentiation in animal models of colon cancer, as well as in
CRC cell lines compared to normal cells. These alterations have been reported to occur via the Wnt signaling
pathway, which has been shown to be deregulated in
most CRC cases [16]. It is interesting to note that the
majority of CRC occur in the distal (left) side where
the SCFA concentration is at its lowest and contact with
luminal contents (including bacteria) is increased due to
the more solid nature of luminal contents and also due to
the slower transit through this segment of the bowel.
Another important role of the residing microbiota is
their ability to resist the colonisation of new strains of
bacteria (pathogenic or non-pathogenic) entering the colon, a mechanism termed colonisation resistance. The
M.E. Hope et al. / FEMS Microbiology Letters 244 (2005) 1–7
microbiota is thought to achieve this by a variety of
means including (A) competition for substrate and/or
mucin adhesion sites, (B) alteration of physiological
conditions including redox potential, pH and (C) production of substances including bacteriocins that create
an environment physiologically inhibiting to other bacteria [17].
3. Deactivation of reactive metabolites and carcinogens by
the commensal microbiota
Intestinal bacteria maintain human health beyond
basic nutrition, a fact first discovered by Elie Metchnikoff at the beginning of the 20th century. He observed
that Bulgarians who ate large amounts of fermentedmilk products had a relatively long life expectancy.
Organisms such as lactobacilli, bifidobacteria and streptococci, belonging to the group of lactic acid-producing
bacteria (LAB), are thought to confer benefits such as
stimulation of the immune system by non-pathogenic
mechanisms and inhibition of colonisation by harmful
microbial species [18,19]. It has also been suggested that
LAB may have the potential to protect against colonic
tumours by influencing metabolic, immunologic and
protective functions in the colon [20]. In animal models,
LAB ingestion has been shown to prevent carcinogeninduced pre-neoplastic lesions or tumours (reviewed by
[21]). It has also been demonstrated that LAB are involved in the detoxification of various carcinogens such
as polycyclic aromatic hydrocarbons (PAH) and heterocyclic aromatic amines [22]. The mechanisms by which
intestinal bacteria play a role in the inactivation of carcinogens remain unclear. It is possible that LAB (A)
bind directly to the carcinogens and exert degrading effects, (B) catalyse detoxification reactions and (C) produce metabolites that directly lead to carcinogen
detoxification [21,22]. However, the protective effects
conferred by LAB are only demonstrated when high
numbers of the organisms are present. Therefore, regular ingestion of the bacteria is required to maintain a
sufficient beneficial population [23].
4. Inflammation, the microbiota and CRC
The link between inflammation and cancer was first
suggested by Rudolf Virchow in 1863 when he noted
the presence of leukocytes in neoplastic tissues. He
hypothesised that this infiltrate mirrored the origin of
cancer at sites with chronic inflammation [24]. There is
increasing evidence of the microbiotaÕs involvement in
the induction of chronic colonic inflammation. The microbiota provide a major stimulus in the activation and
development of the normal intestinal immune system.
This is effectively demonstrated in adult germ-free ani-
3
mals that have little or no mucosal lymphoid tissue or
secretory IgA. The bacteria induce continual T and Blymphocyte activation, though not all members of the
microbiota are equally capable of doing so (reviewed
in [6]). This immune response is generally thought to
be limiting and for this reason the colon is said to be
in a perpetual state of controlled or physiological
inflammation [6]. Chronic inflammation is characterised
by infiltration of damaged tissue by mononuclear cells
such as macrophages, lymphocytes and plasma cells, together with tissue destruction and attempts at repair
[25]. The macrophage is a major player in the chronic
inflammatory response due to the great number of bioactive products it releases such as complement components, cytokines, chemokines and nitric oxide [26].
These mediators form part of the bodyÕs powerful defense against invasion and injury. However, persistent
or pathological macrophage activation can result in continued tissue damage. It is well established that individuals with inflammatory bowel disease (IBD) are at an
increased risk of developing IBD-associated cancer, with
the risk relating to the duration and severity of mucosal
inflammation. It has been suggested that there are some
similarities in the mechanisms by which both IBD-associated cancer and sporadic CRC arise [27], for example,
the multiple mutations that are a requisite for cancer
progression, along with similar patterns of allele loss
and the occurrence of chromosomal instability. It is
therefore reasonable to hypothesise that sporadic CRC
may also arise as a result of chronic inflammation. There
is increasing evidence to support this view. One particularly elegant study was performed by Okada and workers. They showed the conversion of human colonic
adenoma cells to adenocarcinoma cells by foreign
body-induced chronic inflammation in nude mice, thus
indicating the contribution of chronic inflammation directly to colorectal tumour progression. The study also
showed that once the adenoma cells acquired the tumourigenic phenotype, transfer into nude mice without a
foreign body also resulted in the promotion of tumourigenicity [28].
Evidence from animal models has shown that a dysregulation in the T cell response can lead to the development of chronic intestinal inflammation, tissue damage
and, in humans, an increased risk of IBD. This process
is mediated in part by the resident microbiota as inflammation does not occur when the same animal models are
maintained in a germ-free environment [29]. Rhodes and
workers recently showed that Escherichia coli species
isolated from cancerous colonic mucosa were capable
of inducing interleukin-8 (IL-8), a pro-inflammatory
cytokine, in colonic epithelial cells in vitro [30]. This
induction was not dependent upon adherence or invasion of the bacteria. If similar processes occur in vivo
with other bacterial species then one can see how
alterations in the gut microbiota towards less beneficial
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M.E. Hope et al. / FEMS Microbiology Letters 244 (2005) 1–7
populations may have a contributing effect on mucosal
inflammation. In contrast, Pitari et al. showed that a
heat-stable enterotoxin produced by enterotoxigenic E.
coli (ETEC) was capable of suppressing colon cancer
cell proliferation by a guanylyl cyclase C-mediated signalling cascade. This effect was forwarded as a possible
reason for the low incidence of CRC in ETEC-endemic
regions in developing countries [31].
5. Promotion of carcinogenesis
It is now apparent that the metabolic activities of the
colonic microbiota may be responsible for a wide range
of products that are detrimental to the host by various
mechanisms. One such category of products includes
reactive oxygen intermediates (ROI). These are molecules that result in oxidative DNA damage, with
numbers increasing in the presence of chronic inflammation. There is increasing evidence to support the role of
ROI in the development of CRC with ROI already
incriminated in a wide range of cancers [32]. ROI are
derivatives of molecular oxygen and commonly include
superoxide, hydrogen peroxide, hypochlorous acid, singlet oxygen, and the hydroxyl radical. They are produced in all cells through the process of normal cell
metabolism and may react with lipids or proteins to generate intermediates that react with DNA [33,34]. In
addition, they can also cause alterations in DNA such
as base modifications, deoxyribose damage and breakages of the DNA strand [33]. This effect, coupled with
a relatively slow and often incomplete repair process,
can lead to chromosomal instability in the form of
mutations, deletions, sister chromatid exchanges and
chromosomal translocations [34]. The generation of profuse ROI has been demonstrated in the faecal matrix,
with the lack of ROI present in autoclaved faeces leading to the suggestion that ROI are a bacterial metabolite
[35]. In contrast, others have reported that the generation of ROI were instead due to a soluble component
within the matrix [32]. However, it is interesting to note
that the study reporting this was performed using aerobic and anaerobic culturing techniques after a period of
sample storage at 80 °C. This, coupled with the fact
that the majority of the colonic microbiota have yet to
be cultured, would have major implications for the accuracy of this study. A study by Huycke and workers demonstrated that the commensal Enterococcus faecalis was
capable of producing extracellular superoxide and
hydrogen peroxide that damaged colonic epithelial cell
DNA both in vitro and in vivo. By using both a wildtype strain and a strain with attenuated extracellular
superoxide production they showed significantly more
DNA damage in the colon of rats colonised by the
wild-type E. faecalis [36]. Studies by Balish and Warner
[37] have also showed that this bacterium is capable of
inducing IBD, dysplasia and adenocarcinoma in IL-10
knockout mice, with no pathology observed in germ-free
counterparts.
6. Animal models implicating the colonic microbiota in
CRC
Animal models have become a powerful tool in
studying the role of bacteria in the development of
CRC. Genetic knockout mice and germ-free mice are
two animal model systems that have been used extensively with the majority of these studies implicating the
colonic microbiota as an important factor in the development of CRC. There are a number of genetically engineered models of intestinal neoplasia including the
T-cell receptor beta-chain and p53 double-knockout,
IL-10 knockout, Smad 4 with APC, TGFb-1 and
Rag2, many of which are discussed in a recent review
by Boivin et al. [38]. Many of these models have shown
that, under germ-free conditions, colitis and subsequent
tumour formation are suppressed compared to
either mono-associated or conventionalised animals
[29,37,39–42]. It must be noted however, that results obtained from mono-association studies may not take into
account interactions between members of the commensal microbiota and between the microbiota and the host.
For example, Scharek and workers inoculated germ-free
rats with either Bifidobacterium adolescentis or Bacteroides thetaiotaomicron, showing only B. thetaiotaomicron
capable of inducing a systemic immune response. However, when rats were di-associated with the two species,
the specific immune response towards B. thetaiotaomicron was reduced, showing that B. adolescentis was
capable of down-regulating humoral immunity to B.
thetaiotaomicron [43].
Animal studies have also shown the ability of members of the commensal microbiota to produce metabolites that are potent direct-acting mutagens [44]. Onoue
and workers [45] demonstrated that intestinal bacteria
could function as promoters of carcinogenesis by
increasing both the number and rate of progression of
chemically induced aberrant crypt foci (ACF). These
studies provide compelling evidence that the commensal
microbiota is important in the development of CRC,
although not all bacteria are equally capable of causing
(or protecting against) pathology. However, there are
only limited studies on the ability of individual colonic
bacteria to promote colonic tumourigenesis. A study
by Ellmerich and workers showed that supplements containing live Streptococcus bovis were capable of inducing
ACF formation and increased expression of proliferation markers in rats treated with carcinogens [46]. A
more detailed study conducted by Horie and colleagues
used 1,2-dimethylhydrazine-treated germ-free mice,
which were subsequently mono-associated with a variety
M.E. Hope et al. / FEMS Microbiology Letters 244 (2005) 1–7
of bacterial species. They showed that the incidence of
colonic adenomas ranged from 30% to 70% depending
on the colonising species [47].
7. Evidence from clinical studies
Evidence from clinical studies has also demonstrated
the role that various members of the colonic microbiota
may play in CRC development. Swidsinski et al. [48]
showed the presence of intracellular bacteria in the
majority of colon biopsies taken from adenoma and carcinoma patients whilst showing that the colonic mucosa
of asymptomatic controls were not colonised. The predominant bacterial species involved was shown through
sequence analysis to be E. coli or E. coli-like species.
However, it is yet to be discovered whether the presence
of such bacteria is due to the disease or is itself the cause
of the pathology. More recently, Martin et al. [49] obtained mucosal biopsies from colon cancer patients
and showed the presence of both mucosa-associated
and intramucosally associated bacteria in a high proportion of colon cancer biopsies whereas biopsies from patients with negative findings were relatively free from
aerobic bacteria.
There have also been a few studies that have implicated sulphate-reducing bacteria (SRB) in the development of CRC. SRB are considered relatively normal
inhabitants of the human intestine and their major
metabolite is hydrogen sulphide, which can damage
the colonic epithelial barrier leading to impaired butyrate oxidation [50]. Faecal samples obtained from IBD
patients have demonstrated increased H2S concentrations compared to controls [51], and similar findings
were reported studying the H2S levels in the colons of
healthy males compared with patients with re-occurring
CRC [52].
8. Effect of the commensal microbiota on host
carbohydrate expression
There is increasing evidence that the colonic microbiota are involved in alterations in host glycosylation.
Studies comparing germ-free and conventionalised rats
show that the intestinal microbiota have a strong influence on mucin content, thickness, composition and
structure of the pre-epithelial mucus layer within the
colon [53]. Further studies have shown that commensal
bacteria can also induce more complex glycosylation
patterns in the colon by changing the cellular and subcellular distribution of glycans [54]. Alterations in host
cell surface glycosylation in cancer are a universal feature of the neoplastic process, and changes occurring in
adenomatous and metaplastic polyps, CRC and other
precancerous disorders such as IBD are well docu-
5
mented [27]. The importance of these alterations are
demonstrated in mice that are genetically deficient in
Muc2, the most abundant secreted gastrointestinal mucin. These animals show aberrant intestinal crypt morphology and frequently develop small intestinal and
colonic tumours [55]. One of the most common
changes in glycosylation is increased mucosal expression of a galactose disaccharide known as the Thomsen
Friedenreich (TF) blood group antigen, which allows
the binding of galactose-binding lectins via unsubstituted terminal galactose residues [56]. Other alterations
that have been demonstrated include reduced mucosal
sulphation and increased expression of fucosylated
and/or sialylated structures such as the Lewis and sialyl
Lewis antigens and ABO blood groups, which are now
considered tumour–associated antigens [27]. These increases are mirrored by increased expression levels of
several fucosyl- and sialyltransferases, demonstrated
in microarray studies comparing normal and neoplastic
colon tissue [57].
The importance of individual bacteria in altering host
glycosylation was first shown using the human commensal Bacteroides thetaiotaomicron to colonise germ-free
mice. Colonisation resulted in the modification of mucosal surface glycosylation patterns by the production of
fucosylated glycans [54,58]. These studies showed that
by altering intestinal glycosylation B. thetaiotaomicron
was able to gain a foothold and create a favourable
niche for itself. Another study showed that B. thetaiotaomicron was also capable of inducing other more subtle
glycosylation changes that affect the expression levels
and subcellular structure of the glycoconjugates. A recent study conducted by Freitas [59] showed that it
was also possible to modulate the glycosylation patterns
progressively by adding individual bacteria to germ-free
mice. It is interesting to note that colonisation with B.
thetaiotaomicron is shown to affect numerous other cellular functions which are potentially relevant to CRC
progression including angiogenesis, mucosal barrier
function and xenobiotic metabolism [60].
It is now apparent that alterations in host epithelial
carbohydrate expression play a pivotal role in determining the proliferative, metastatic and invasive potential of
tumour cells [27]. The mechanisms by which the glycosylation patterns are altered in the development of
CRC remain undefined. It is not clear whether changes
in glycosylation in CRC are a result of a change in the
composition of the colonic bacteria, with subsequent
changes in host–microbial interactions, or that alterations in glycosylation as a result of the neoplastic process are responsible for the recruitment of different
microbial populations. It is interesting to note, however,
that changes in glycosylation can predate malignant
cytological changes, demonstrated in unaffected normal
mucosa of cancer patients showing alterations at the
molecular level [27,57]. Rhodes and workers [61]
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M.E. Hope et al. / FEMS Microbiology Letters 244 (2005) 1–7
proposed that glycosylation alterations result in functional changes as a result of recruitment of a different
range of lectins (carbohydrate-binding proteins) of
microbial or dietary origin that pass through the colon.
They demonstrated that, in certain individuals who expressed increased TF antigen in their rectal mucosa, peanut lectin was capable of increasing the rectal mitotic
index in healthy human colonic mucosa by 41% after a
period of five days of peanut ingestion [62]. Although a
similar effect has yet to be established for lectins of microbial origin, this is a direct demonstration of how alterations in the host carbohydrate repertoire are capable of
resulting in changes that may be detrimental to the host.
Alterations in host carbohydrate expression may also
result in unfavourable consequences at later stages of
CRC development. As the neoplastic process progresses
it has been shown that cancer cells undergo metabolic
shifts in order to cope with a hypoxic environment.
These changes are partly mediated by transcription factors known as hypoxia-inducible factors (HIF). A recent
study by Koike and colleagues demonstrated that one of
these factors, HIF-1a, was capable of significantly
enhancing cancer cell adhesion to endothelial cells by
increasing cell surface carbohydrate expression of sialyl
Lewis antigens. This is a process that facilitates haematogenous metastasis and tumour angiogenesis [63]. It has
also been demonstrated that bacterial lipopolysaccharide (LPS), a component of Gram-negative bacterial cell
walls, is a potent inducer of HIF-1a in macrophages in
non-hypoxic conditions [64].
9. Conclusions
Over the years it has become apparent that the colonic microbiota is capable of influencing a wide range of
host processes and functions that may lead to beneficial
or detrimental effects within the colon. Whilst the exact
nature of some of these host–bacterial interactions is
becoming more apparent, there are many others that remain to be elucidated. The role of the colonic microbiota in the development of CRC is undoubtedly a
multifactorial one that can affect the various stages of
the neoplastic process. It may be that induction of
mucosal inflammation, production of mutagens and
reactive metabolites and alterations in carbohydrate
expression are all processes that act in concert to set
the colonic mucosa on the first step of the adenomacarcinoma sequence.
Acknowledgments
Editorial limitations mean that we are unable to cite
all relevant literature and we must apologise to authors
whose papers in this field are not cited here. The GI re-
search group receives funding from Cancer Research
UK and Tenovus Scotland. RK is funded by a senior research fellowship (SF 3/2000) through the National Kidney Research Fund. The endoscopic pictures are
courtesy of Jannsen-Cilag Ltd and Eisai Ltd.
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