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.