Tissue culture cells and cells from hosts
The most widely applied models in assessing the adhesion
of microorganisms, not just probiotics, are tissue culture
cells. For the study of adhesion to intestinal epithelial cells,
in particular, Caco-2 and HT-29 cells are commonly used
(47) (Table I). A potential disadvantage with the use of
tissue culture cells is that they are cancer cells (48), which
may or may not be different from normal intestinal
epithelial cells. Caco-2 cells are derived from colonic
adenocarcinoma and express protein characteristics for
both colonocytes and enterocytes immediately after confluence.
Thereafter, the content of colonocyte-specific
proteins decreases and those specific for enterocytes increase.
This makes timing in the use of these cells
particularly important (49). HT-29 cells are likewise derived
from colonic carcinoma and differentiate less well than
Caco-2 cells (50). The cell lines currently used for probiotic
adhesion studies differentiate to varying degrees into
enterocytes. Currently, no cell line for colonocytes exists.
Tissue culture cells, with the exception of HT-29 MTX cells,
do not have the mucous layer which normally covers the
enterocytes in the intestine and thus represent in general
only one part of the intestinal mucosa. Because of the
difference in cellular structure, it is not surprising that
bacteria exhibit different adhesion to the three types of
epithelial cells / adhesion appears to be higher to HT-29
MTX cells than to HT-29 or Caco-2 cells (22, 24).
Alternative sources for cells are buccal epithelial cells,
urinary tract epithelial cells and vaginal epithelial cells
(obtained, for example, with swabs), also from biopsies, and
resected tissue epithelial cells can be released and collected
(51). These cells may be covered with mucus and have a
normal microbiota, and are thus more representative for the
in vivo conditions. However, as there appears to be wide
person-to-person variation and even time variation for the
same person, collection should be standardized. Furthermore,
in women the properties of the cells may vary
depending on the hormonal levels during the menstrual
cycle (52). Although the viability of the collected cells can
readily be assessed by determining the exclusion of erythrosin
B, this appears not to be done routinely. Enterocytes
and brush border membranes can also be obtained from
experimental animals (12). The use of these collected cells
and brush border membranes poses a problem in that only
binding on the apical side is relevant. When the cells or
membranes are immobilized, the orientation cannot be
influenced, and when the cells are left in suspension the
microorganisms may also bind to the basolateral side (53).