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Introduction

 

One of the most
diverse and abundant microbial niches is found in the human body. The
Gram-negative anaerobe Akkemansia
muciniphila, which belongs to the Planctomycetes-Verrucomicrobia-Chlamydiae
superphylum, is present in the alimentary canal of more than 90% of the
analyzed cases. A. muciniphila is
especially adapted to the human gut environment and uses glycosolated proteins
of the epithelial mucus layer as its carbon and nitrogen source. Earlier
studies indicate that A. muciniphila
is related to gut health; i.e. a low density of A. muciniphila is related to diabetes
type 1, Crohn’s disease (CD) and in Ulcerative colitis (UC), both during
remission and in clinically active disease, than in vigorous individuals. Furthermore,
A. muciniphila is related to restoration of the thickness of the mucos layer in
the intestines and reducing endotoxemia.

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State of the art

 

The evidence of
the positive effect of A.
muciniphila on gut health increases but less is know about the interactions
between the host and the bacterium and how it copes with different
circumstances. The interaction of the bacteria and its hosts starts with colonizing in which they can adhere by binding to the mucus layer of the intestines
epithelium or via the cells underneath, the enterocytes. To which surface A. muciniphila adheres hasn’t been studied
yet even as the ability of coping with an oxygen rich environment. This study
will answer which mechanisms A. muciniphila uses to adhere to the mucus
layer or the epithelium cells of the gastrointestinal tract.

 

Recent findings

 

Although the human
colon consists out of an anaerobic microbe community, it turned out that A. muciniphila is capable of surviving
in both oxic as well as anoxic conditions. As A. muciniphila is aerotolerant, contrasting incubation conditions were compared
in an adhesion experiment. The binding efficiency with epithelial cells  HT29 and Caco-2 do not differ between aerobic
and anaerobic atmosphere. Thus, A.
muciniphila does not have to be treated as a true anaerobe, but is able to
cope with oxygen.

 

Also, it turned
out that the only significant binding of A.
muciniphila, compared to BSA, occurred with laminin. The bindingprocess of A. muciniphila with other extracellular matrix (ECM) proteins, was not significant. As
the adhesion between A. muciniphila and
the intestinal mucus is less than 1%, it can be stated that there is no
adhesion at all. As other bacteria, for example L. rhamnosus and B. bifidum,
show strong connection to human colonic mucus, it was unexpected that A. muciniphila did not. An explanation
for this is that these species do not utilize and degrade the mucus, like A. muciniphila does. Although the
adhesion of A. muciniphila and L. rhamnosus on colonic mucus was not
compareble, A. muciniphila adhered to
both enterocrytes equally well as L.
rhamnosus. This might indicate that the enterocytes are true binding sites
for A. muciniphila.

 

A. muciniphila and B. fragilis were both cocultivated for 24 hours and indicated an expansion
in transepithelial electrical
resistance (TER). Compared to the Caco-2 cultures, without bacteria the TER of
Caco-2 cocultures of Escherichia coli declined
significantly. This shows that at this timepoint E. coli cells increased and that there is no cell suspension for
the OD600 values of A.
muciniphila and B. fragilis,
which indicates a stagnation of growth. The positive impact of cell monolayer
integrity for the first 24 hours was the most succesfull with B. fragilis, followed by A. muciniphila.
After 48 hours the transepithelial
electrical resistance of
Caco-2 cocultures of A. muciniphila became
equal to the cocultures of B.
fragilis. During the 48 hour incubation the cell density of B. fragilis and
A. muciniphila did not diversify, neither seems that the bacteria are
severly compromised. Under the same circumstances, E. coli affected TER
development negatively and the coculture increased during the second 24 hours,
which will probably result in a further decline of the transepithelial electrical resistance in E. coli cocultures.

 

Earlier studies
have associated obesity and diabetes to decreased gut health and inflammation,
which result in lipopolysaccharide (LPS) induced endotoxemia. When LPS is
released, enterocytes start producing the chemokine interleukin-8 (IL-8) which
leads to inflammation. Needless inflammation can cause disorder in the
intestinal epithelium and can disturb the homeastasis of the colonal mucus.
Compared to the IL-8 production by E.
coli, A. muciniphila produced less IL-8 in HT-29 cells. Thus, there will
be no strong inflammation when A. muciniphila is present in the gastrointestinal tract. Since there almost
was no inflammatory response in the presence of A.
muciniphila, it was checked
wether it does or does not produce LPS and whether it is different compared to E.
coli. The results show that A. muciniphila does produce LPS, however
it does not activate HT-29 cells to produce a lot of interleukin-8. Therefor it is
likely that the produced LPS by A. muciniphila is different compared to that of E. coli.

 

Discussion

 

The results of
this study show that A. muciniphila does not bind to the intestinal mucus but prefers
to bind to the epithelial cells Caco-2 and HT-29 and the ECM laminin. It remains unknown how this
organism is able to live in this constantly changing environment and should be
studies to answere this question. A possible justification could be that A. muciniphila releases a certain enzyme
which decreases the colonic mucus, making it hard for the bacteria to
effectively bind to the mucus.

As A. muciniphila was able to bind to the extracellular
matrix laminin it might suggest that pathogens are competing with A. muciniphila for bindingsites at
locations where the epithelial cell layer of the colon is damaged.

Furthermore,  it is likely that A. muciniphila is able to strengthen the barier of the intestinal
track. Future research could study the helpful role of A. muciniphila in connection with its host in for example obesity
and diabetes.

 

 

Since it was
noticed that A. muciniphila binds
laminin and Caco-2 cells, it is likely that A.
muciniphila might participate in the exclusion of pathobionts from the
sites of injury and has an significantly increased the TER in coculture with
Caco-2 cells. The strengthening of the epithelial barrier could explain the
observations linking A. muciniphila to
systemic health and not only gut health.

In the observation that A. muciniphila fortifies epithelial barrier function could provide
a working hypothesis for the findings
connecting decreased fecal A. muciniphila
levels with diabetes and obesity and could be the mechanism behind the
protective effect of the bacterium against high-fat-diet-induced LPS
endotoxemia. 

Although A. muciniphila
induced IL-8 in enterocytes at cell concentrations100-fold higher than those
for E. coli, indicating that A. muciniphila does not provoke a
strong inflammatory cascade in the epithelium. 

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