Probiotics are viable bacteria that beneficially influence the
health of the host (1, 2). Probiotic bacteria selected for commercial
use in foods and in therapeutics must retain the characteristics
for which they were originally selected (1–3). These
include characteristics for growth and survival during manufacture
and, after consumption, during transit through the stomach
and small intestine. Importantly, probiotics must retain the
characteristics that give rise to their health effects. Consequently,
it is necessary to test the stability of these characteristics
during manufacture and storage and to ensure that they are
retained in different types of foods (3, 4). The initial screening
and selection of probiotics includes testing of the following
important criteria: phenotype and genotype stability, including
plasmid stability; carbohydrate and protein utilization patterns;
acid and bile tolerance and survival and growth; bile metabolism;
intestinal epithelial adhesion properties; production of
antimicrobial substances; antibiotic resistance patterns; ability
to inhibit known gut pathogens, spoilage organisms, or both;
and immunogenicity.
Examples of how each of these criteria can be unstable are
most abundant in the area of acid stability, which can vary considerably
by strain depending on how the strains are used in different
food products. This illustrates the necessity for ongoing
quality control of probiotic bacteria during manufacture and use
and for continual monitoring of the effectiveness of probiotics in
humans. It also indicates the need for selection of more stable
probiotic strains for commercial use.
THE QUALITY OF METHODOLOGY USED FOR
SELECTION
Many in vitro tests are performed when screening for potential
probiotic strains. The ability to adhere to the intestinal mucosa is
one of the more important selection criteria for probiotics
because adhesion to the intestinal mucosa is considered to be a
prerequisite for colonization (5). As substrata, enterocyte-like
Caco-2 tissue culture cells and intestinal mucus are currently
used. However, these represent only a distinct part of the intestinal
mucosa. In this respect, mucus-secreting HT29-MTX tissue
culture cells would come closer to the true situation in the intestine.
In addition to these models, human ileostomy glycoproteins
have been used to study adhesion to the small-intestinal mucosa
(6). All of these in vitro systems provide valuable information on
the ability of probiotics to adhere and colonize the intestine.
Adhesion to colonic or intestinal biopsy samples, if possible,
should be considered as a final in vitro adhesion test that would
be most like the in vivo situation. Not only would this be a better
approximation of the in vivo situation, it would allow for the
study of adhesion to different parts of the intestine. This is especially
important regarding immune stimulation by oral administration
of probiotics.
Adhesion is also considered important for stimulation of the
immune system. Adhesion to M cells or Peyer’s patches may