Adhesion is also considered important for stimulation of the
immune system. Adhesion to M cells or Peyer’s patches may therefore be an important determinant of possible immunestimulating
properties of probiotic microorganisms.
Fecal samples have been used in most colonization studies with
probiotic bacteria. These, however, reflect only the bacteriologic
situation in the fecal material and do not give an accurate picture
of the different parts of the gastrointestinal tract or the mucosal
layer of the gut. The use of biopsies from the intestinal mucosa is
a more accurate means of determining colonization. Lactobacillus
strains were found to adhere to rectal mucosa obtained from volunteers
who had consumed a fermented oatmeal soup (7).
When tested in vitro, probiotics are usually grown in laboratory
media. With many probiotics, the aim is at least transient
colonization, in which case the probiotics may need to grow or
at least metabolize in the intestine. The adhesive properties,
metabolism, and morphology of probiotics grown in intestinal
contents or intestinal mucus have been shown to be different
from those of probiotics grown in laboratory media. These differences
may affect the health effects of the probiotics. By using
culture media more resembling the nutrients available in the
intestine, one may obtain a more accurate understanding of the
properties of probiotics in vivo.
One of the selection criteria for probiotics is the production of
antimicrobial substances, and many probiotic bacteria have been
shown to produce them (8). Among these substances are not only
growth-inhibiting metabolites, eg, organic acids and hydrogen
peroxide, but also bacteriocins, adhesion inhibitors, and a range
of small antimicrobial substances. These substances have been
shown to be produced in laboratory media but their production
and efficacy in vivo remain uncertain (8). It has not been tested
whether administration of purified bacteriocins alone has effects,
eg, on diarrheal disease. Nor has it been tested whether bacteriocins
are produced in vivo. If bacteriocins are produced and
active in vivo, it may be necessary to assess their effects on the
indigenous microflora. There is the potential risk that beneficial
strains in the indigenous microflora are also affected by the presence
of a bacteriocin and that the bacteriocins may thereby alter
the natural resistance of the indigenous microflora to colonization.
Because the production of antimicrobial substances is
regarded as an important selection criterion for probiotics, it
must be confirmed whether these substances are indeed produced
in vivo and exert a beneficial effect. Intestinal or fecal
microflora studies are needed to confirm these properties.