Numerous criteria have been defined for the selection
of probiotic strains (reviewed in 11). Obviously, the most
important criterion is that selected strains have to be safe
for use in the host and for the environment. One of the
most commonly reported selection criteria is the ability to
survive during passage through the gastrointestinal tract
(GIT) of the host for which the capacity of a strain to
withstand the conditions prevailing in the stomach (high
acid level) and intestinal tract (bile acids, pancreatic and
other digestive enzymes) is crucial. Adhesion to intestinal
epithelial cells is considered important for immune modulation, pathogen exclusion and prolonged residence time
in the GIT. Viability of the probiotic strain is assumed to
be important and metabolic activity may be pivotal for
the expression of anti-pathogenic activity. However, there
is increasing evidence that bacterial compounds such as
DNA (some CpG motifs) or cell-wall fragments and ⁄ or
dead bacteria can elicit certain immune responses