Introduction The human body isone of the most rich microbial niches in the world. The Gram-negative anaerobe Akkemansia muciniphila, which belongs tothe Planctomycetes-Verrucomicrobia-Chlamydiae superphylum, is found in the alimentarycanal of more than 90% of the evaluated cases. A. muciniphila is well adapted to the human gut environment anduses 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 guthealth. 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 mucuslayer in the intestines and decreasing endotoxemia are accociated with A.
muciniphila. State of the art There is not muchknown about the interactions between A.muciniphila and the host, nor how it handles the different environmentalcircumstances. Previous studies do suggest that A. muciniphila has a positiveconsequence 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 intestinesepithelium or via the cells underneath, the enterocytes. To which surface A. muciniphila adheres hasn’t been studiedyet even as the ability of coping with an oxygen rich environment. This studywill answer which mechanisms A. muciniphila uses to adhere to the mucuslayer or the epithelium cells of the gastrointestinal tract. Recent findings Although the humancolon consists out of an anaerobic microbe community, it turned out that A.
muciniphila is capable of survivingin both oxic as well as anoxic conditions. As A. muciniphila is aerotolerant, contrasting incubation conditions were comparedin an adhesion experiment. The binding efficiency with epithelial cells HT29 and Caco-2 do not differ between aerobicand anaerobic atmosphere. Thus, A.muciniphila does not have to be treated as a true anaerobe, but is able tocope with oxygen.
Also, it turnedout 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. Asthe adhesion between A. muciniphila andthe intestinal mucus is less than 1%, it can be stated that there is noadhesion 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 explanationfor this is that these species do not utilize and degrade the mucus, like A. muciniphila does. Although theadhesion of A. muciniphila and L.
rhamnosus on colonic mucus was notcompareble, A. muciniphila adhered toboth enterocrytes equally well as L.rhamnosus. This might indicate that the enterocytes are true binding sitesfor A. muciniphila. A. muciniphila and B.
fragilis were both cocultivated for 24 hours and indicated an expansionin transepithelial electricalresistance (TER). Compared to the Caco-2 cultures, without bacteria the TER ofCaco-2 cocultures of Escherichia coli declinedsignificantly. This shows that at this timepoint E.
coli cells increased and that there is no cell suspension forthe OD600 values of A.muciniphila and B. fragilis,which indicates a stagnation of growth. The positive impact of cell monolayerintegrity for the first 24 hours was the most succesfull with B. fragilis, followed by A. muciniphila.
After 48 hours the transepithelial electricalresistance of Caco-2 coculturesof A. muciniphila became equal to the cocultures of B. fragilis. Duringthe 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 developmentnegatively and the coculture increased during the second 24 hours, which will mostlikely result in a further decline of the transepithelial electrical resistance in E.
coli cocultures. Earlier studieshave associated obesity and diabetes to decreased gut health and inflammation,which result in lipopolysaccharide (LPS) induced endotoxemia. When LPS isreleased, enterocytes start producing the chemokine interleukin-8 (IL-8) whichleads to inflammation. Needless inflammation can cause disorder in theintestinal 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 willbe no strong inflammation when A.
muciniphila is present in the gastrointestinal tract. Since there almostwas no inflammatory response in the presence of A.muciniphila, it was checkedwether 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, howeverit does not activate HT-29 cells to produce a lot of interleukin-8. Therefor it islikely that the produced LPS by A. muciniphila is different compared to that of E. coli. Discussion The results ofthis study show that A. muciniphila does not bind to the intestinal mucus but prefersto bind to the epithelial cells Caco-2 and HT-29 and the ECM laminin.
It remains unknown how thisorganism is able to live in this continually adjusting habitat and should bestudies to answere this question. A possible justification could be that A. muciniphila releases a certain enzymewhich decreases the colonic mucus, making it hard for the bacteria toeffectively bind to the mucus.As A. muciniphila was able to connect tothe extracellular matrix laminin it might suggest that pathogens are competingwith A. muciniphila for bindingsitesat 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 intestinaltrack. Future research could study the helpful role of A. muciniphila in connection with its host in for example obesityand diabetes.