The human gut houses a large
number of microbial species, who’s total genome is many times bigger than that
of humans. This microbiota performs a lot of functions essential to human life.
It regulates metabolism, it develops defence against pathogens, and it plays a
key role in the immune system. The microbiotic composition and functioning is
therefore directly linked with health. Even though their importance, many
microbes have not yet been cultured nor studied. Improvements in culturability
of the human gut microbiota led to the successful isolation of Akkermansia muciniphila. This
Gram-negative bacterium is the only microbial isolate of the phylum
Verrucomicrobia, and has been linked with intestinal health and improvement of metabolic
status. However, adhesive interactions of A.
muciniphila with the host have been poorly studied.
State of the art
Adhesion of colonizing
bacteria in general can be either adhering to the enterocytes or to the
protective mucus gel layer covering the epithelial cells. A thick mucus gel
layer would fully cover the epithelial cell layer in a healthy colon. A healthy
small intestine, on the other hand, is characterised by the epithelial cell
layer being not fully covered with a thinner mucus layer. Thus, the small
intestine is suitable for direct contact with host enterocytes, whereas the
colon provides more nutrients in the form of a thick mucus layer. A. muciniphila uses intestinal mucins as
its sole source of carbon and nitrogen. It is present in over 90% of adult
gastrointestinal (GI) tracts all over the animal kingdom. A. muciniphila has been described as a strict anaerobic bacterium,
complicating successful culturing. Since its discovery associative in vivo studies have been carried out
linking A. muciniphila to systematic
health. For instance, in animal studies low A.
muciniphila abundance has been linked with metabolic impairments, such as
obesity, inflammatory bowel disease (IBD), and type-2 diabetes (T2D). In an
unhealthy state intestinal permeability increases, giving lipopolysaccharides
(LPS) the ability to disperse through the body, which can lead to metabolic
endotoxemia. However, opposing trends have been found where A. muciniphila was associated with
increased inflammation. The exact proinflammatory characteristic is not yet
clear. Moreover, the results of these associative in vivo studies call for further research in human situations.
Reunanen, J. et al. (2015)
examined A. muciniphila adhesion to
human colonic mucus, multiple intestinal epithelial cell lines (Caco-2; HT-29),
and extracellular matrix (ECM) proteins. They hypothesised A. muciniphila to adhere to colonic mucus as this is its sole
source of nutrients. In contrast they found A.
muciniphila to have a low level of binding (less than 1%) to human colonic mucins.
However, A. muciniphila did bind to
both enterocyte lines. Interestingly level of adherence did not change with
epithelial cell development. This while surface molecules are known to
differentiate over different developmental stages. The equal adherence over
time suggests that A. muciniphila is
able to bind at various differentiation stages in vivo, despite changing surface structure. It remains unclear
whether this low mucus binding is a result of their mucin-degrading enzymatic
activity or because of the experimental conditions. The experiments were conducted
in aerobic circumstances, as A.
muciniphila did not appear to be harmed by oxygen. So, A. muciniphila may be classified as an aerotolerant anaerobic
bacterium rather than strictly anaerobic. Binding with ECM proteins was studied
to investigate possible adherence after mechanical stress of digestion,
exposing the subepithelium with its ECM networks. Reunanen, J. et al. (2015)
found A. muciniphila to only bind to
laminin at an above background level. In order to study in what way epithelial
integrity and interleukin-8 (IL-8) release would be affected by A. muciniphila, Reunanen, J. et al. (2015)
used in vitro models. To assess the
effect of A. muciniphila on the
monolayer integrity of the epithelial cell line Caco-2, the development of the
transepithelial electrical resistance (TER) was assessed. This way the
epithelial barrier function is studied measuring the ion passage across the
tissue. Escherichia coli was also
studied, as it is known to adversely
affect monolayer integrity. Additionally, Bacteroides fragilis was added to allow for comparison. As
expected E. coli adversely affected the Caco-2 monolayer integrity,
decreasing the TER. However, both A.
muciniphila and B. fragilis significantly
increased the TER, strengthening the epithelial barrier function, which could
mean A. muciniphila can not
only be linked to gut health but also to systematic health. Reunanen, J. et al.
(2015) studied the effect on IL-8 production in HT-29 cells to assay the
proinflammatory capacity of A.
muciniphila. Again using E. coli and B. fragilis for comparison. A. muciniphila only triggered IL-8 production at the highest dose
tested (1:100 dilution; 106 bacteria/mL-1). Compared to E. coli the proinflammatory effects are
minor, as much smaller E. coli concentrations
were found to have similar IL-8 production. This low proinflammatory
stimulation of A. muciniphila is in
line with prior studies linking A.
muciniphila to noninflamed mucosa. The minor proinflammatory activity found
in A. muciniphila is suggestive of a
low LPS production by the bacterium. Reunanen, J. et al. (2015) found the
genetic elements necessary for LPS production. However, A. muciniphila LPS probably differs from E. coli LPS as they did not induce an equally strong IL-8 release
from HT-29 cells. This, as well as other findings, have revealed many subjects
for further exploration.
Discussion and future developments
This recent study shows the
surprising adherence preference of A.
muciniphila. Binding directly to epithelial cells and laminin rather than
colonic mucus. Additionally, Reunanen, J. et al. (2015) demonstrates the
strengthening of the enterocyte monolayer integrity in vitro, which is suggestive of the ability to restore a weakened
gut barrier. This finding, on the supposed aerotolerant anaerobic bacterium,
gives way to new questions on colonization, stable occupation of a constantly
changing niche, and immunosignaling properties of this species. With a total of
eight different Akkermansia species
identified, asks for further exploration of human gut microbiotic diversity, as
there is still a lot yet to be discovered, which can possibly promote the
understanding of metabolic interactions with the host.