Despite the superficial similarities between the small and
the large intestine, there are considerable differences between
the two organs, arising from differences in physiology.
The thickness of the mucus layer tends to increase from the
jejunum (120 mm) to the colon (830 mm), particularly the
loosely adherent mucus layer, while there is little difference
in thickness between the duodenum and the jejunum (36).
This may provide a more extensive mucosal habitat for
microbes in the colon compared with the upper parts of the
small intestine. The main mucus types in the small intestine
and colon are MUC2 and MUC3, although they differ in
the secretion of other mucus types (37). These differences
may influence the composition of the mucosa-associated
microbiota. Probiotics and the normal microbiota, in turn,
influence the composition, degradation and secretion of the
intestinal mucus (38). The differences in flow rates between
the small intestine (on average about 1 m/h) and the colon
(about 5/10 cm/h) (6) render the need for adhesion to the
mucosa, in order to persist, more important in the former
than in the latter. The surface area of the small intestine is
increased through villi, which are absent in the colon. The
176 A.C. Ouwehand and S. Salminen
crypts of Lieberku¨hn in the small intestine contain Paneth
cells, which produce antimicrobial substances like lysozyme
and defensins, which are not present in the crypts in the
colon (33). However, these latter are deeper, making the
mucosa thicker, and contain more mucus-producing goblet
cells, providing the thicker mucus layer (39).
For adhesion studies on probiotics, it is important to
know to what extent adhesion is relevant for the endogenous
microbiota. This is still uncertain, although mucosal
microbial populations in the colon have been found to
contain lower numbers and their composition is different to
that of luminal populations (40/42). Ljungh and co-workers
(43) observed that B/10% of randomly isolated lactobacilli
from the human colonic mucosa were highly
adhesive. This would indicate that adhesion is important
for certain members of the normal microbiota, although it
may be a small fraction. The adherence of bacteria in
different mucosal and intestinal regions remains less clear.
The standard preparation of electron microscopy samples
removes the mucus layer; bacteria present in the mucus
layer may therefore seem to colonize the underlying
epithelium (44). Careful preparation of specimens indicates
that both mucus and the underlying epithelial cells are
colonized by the normal microbiota (44/46).
A good mucosal adhesion model should ideally combine
all three (or four) compartments of the mucosa. However,
this has proved difficult, both for the development of the
model and for interpretation of results. A number of
intestinal mucosal models have therefore been developed,
using different parts of the intestinal mucosa as substratum.