Introduction We live in a microbial world. Everywhere we go, we are surrounded by microbes. Even the human bodyitself is home to a large amount of these microscopic organisms. The area with the greatest diversity andabundance of microbes in the human body, is the intestine.
It houses more than a thousand differentspecies. One of these species that is commonly present is the Gram-negative anaerobe bacteriaAkkermansia muciniphila. This species is especially adapted to the gut environment and specializes in theutilization of intestinal mucus as a source of carbon and nitrogen.
Unsurprisingly, it has been detected inhigh numbers in the mucus of the human colon, an area completely covered by a thick mucus layer whenhealthy. State of the art Previous studies have associated the presence of A. muciniphila with intestinal health and improvedmetabolic status. For example, a low cell count of A.
muciniphila has been correlated to type 1 diabetes,obesity and Crohn’s disease. Furthermore, A. muciniphila plays a role in restoring the mucus layer thicknessin the intestines and reducing endotoxemia. Even though evidence of the positive effects of A.
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muciniphila grows, still little is known about the basicways it interacts with the host and how it copes with different environmental circumstances. In order for abacteria to colonize and interact with the host, it has to bind to the intestine epithelium. This can beachieved by either binding to the protective mucus layer or to the cells underneath this layer, theenterocytes. Which surface A. muciniphila adheres to has not been studied thus far.
Also, the copingcapacities of this species in aerobic environments are unknown. Gaining knowledge on the mechanismsthat A. muciniphila uses can be of large value for possible future implementations of this promising gutbacteria. Recent findings In the human colon all bacteria are anaerobe. However, not all species cope with oxygen the same way.
The survival of 80% of A. muciniphila cells exposed to atmospheric oxygen in a plate count, indicate thatthis bacterium is an aerotolerant anaerobic species. Also, in an aerobic atmosphere the levels of bindingefficiency with the human colonic epithelial cells, Caco-2 and HT-29, did not differ from an anoxicatmosphere.
Thus, when working with this bacteria it does not have to be treated as a highly oxygensensitive anaerobe. Also, the following possible binding sites in the human colon for A. muciniphila were studied: humanenterocytes (Caco-2 and HT-29 cell lines), colonic mucus and extracellular matrix (ECM) proteins.
First, thebinding levels of A. muciniphila with colonic mucus were less than 1%, thus negligible. This was unexpected,because of the close interaction between the mucus and the bacterium. Other intestinal bacteria such as L.
rhamnosus and B. bifidum do show strong binding to human colonic mucus. However, these species do notdegrade and utilize the mucus, which A.
muciniphila does. Thus, this result may reflect the mucinolyticnature of A. muciniphila.
Secondly, A. muciniphila did show strong binding to the human enterocyte lines,Caco-2 and HT-29. This may indicate that the enterocytes are the true docking sites for A. muciniphila cells.In the colon these enterocytes are normally covered in a thick mucus layer.
However, in the small intestine this layer is more permeable to bacteria. Thus, there A. muciniphila may bind directly to the enterocytes.
The binding levels of A. muciniphila to enterocytes were not much affected in vitro by variation indevelopmental state of the enterocytes. This indicates that surface molecules on the host cell surface, usedby A.
muciniphila for adhesion, are expressed irrespective of the cell’s developmental state. Thus, A.muciniphila may also be able to bind to enterocytes of different states in vivo. Third, binding of A.muciniphila to ECM proteins was at background levels. Furthermore, the results show that the trans-epithelial electrical resistance (TER) of a Caco-2 monolayersignificantly increased after 24h and 48h of cocultivation with A.
muciniphila. Whereas, cocultivation withE. coli significantly decreased the TER. This indicates that the presence of A. muciniphila positively affectedthe integrity of the Caco-2 monolayer. It is also speculated that A.
muciniphila may competitively excludepathobionts in areas where the epithelium is recently damaged. When the epithelium is damaged, ECMproteins will be exposed. These proteins can serve as binding sites for pathobionts. However, because A.
muciniphila is able to bind to undifferentiated Caco-2 cells it may outcompete other bacteria in the earlystages of epithelial recovery. The role of A. muciniphila in the strengthening of the epithelial barrier couldexplain previous observations linked to gut health and systemic health. Finally, previous research has linked diabetes and obesity to decreased gut epithelium integrity and low-grade inflammation, which can lead to LPS-induced endotoxemia. The release of LPS induces the productionof Interleukin-8 by enterocytes.
This production leads to inflammation, which is a defence againstpathogens. Unnecessary inflammation in a healthy intestinal epithelium can lead to a disturbance of themucosal homeostasis. The results in this study show that A. muciniphila only provokes a minor IL-8production in HT-29 cells, compared to E.
coli. Thus, the presence of A. muciniphila does not lead to a stronginflammation of the epithelium. Because of the low inflammatory response it was also studied whether A.muciniphila produces LPS and if it is different from that of E. coli. Genome analysis and immunoelectronmicroscopic analysis shows that A. muciniphila can and most likely does produce LPS.
However, it does notinduce a strong IL-8 release by HT-29 cells. Thus, the LPS produced by A. muciniphila probably differsstructurally from that of E. coli. Discussion and future developments The results showed that A. muciniphila does strongly bind to enterocytes, but not to colon mucus. However,this study cannot conclude with certainty that this is due to the binding capacities of A.
muciniphila or dueto the aerobic experimental conditions used in the mucus binding assay. It is also still unknown how A.muciniphila manages to colonize the thick mucus layer of the colon. Therefore, future research is needed. Also, future research on the properties of LPS produced by A. muciniphila is needed.
This can give moreinsight into the precise immuno-signalling properties of the bacterium and the host receptors involved. Furthermore, the in vitro results suggest a fortifying effect of A. muciniphila on the epithelial barrier. Thiscan be used to hypothesise in future in vivo research about the beneficial role of host interaction with A.
muciniphila in for example diabetes, obesity and the reduction of high-fat-diet-induced LPS endotoxemia inobese mice.
