Introduction

 

The human body is
one of the most rich microbial niches in the world. The Gram-negative anaerobe Akkemansia muciniphila, which belongs to
the Planctomycetes-Verrucomicrobia-Chlamydiae superphylum, is found in the alimentary
canal of more than 90% of the evaluated cases. A. muciniphila is well adapted to the human gut environment and
uses glycosolated proteins of the epithelial mucus layer as its C and N source.
Earlier studies show that there is a relation between A. muciniphila not being abundant in the intestinal track and gut
health. For example, when the density of A.
muciniphila is low this is associated to diabetes type 1, Crohn’s disease
(CD) and in Ulcerative colitis (UC). Furthermore, the recovery of the mucus
layer in the intestines and decreasing endotoxemia are accociated with A. muciniphila.

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State of the art

 

There is not much
known about the interactions between A.
muciniphila and the host, nor how it handles the different environmental
circumstances. Previous studies do suggest that 
A. muciniphila has a positive
consequence on gut health so further investigation is needed for future application.
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 affected.
Under the same circumstances, E. coli affected TER development
negatively and the coculture increased during the second 24 hours, which will most
likely 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 continually adjusting habitat 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 connect 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 barrier 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.