These in vitro tests for selection of acid- and bile-tolerant
strains can readily be applied to ensuring the quality of probiotic
cultures during manufacture and storage and throughout the shelf
life of the product. Both short-term environmental factors affecting
gene regulation (eg, culture growth phase and stress leading
to the production of shock proteins), and selection of variants
through long-term subculturing may produce changes in culture
performance (3, 9). The former of these is the most likely to have
the greatest influence on probiotic performance (potentially positively)
because appropriate culture maintenance procedures limiting
the number of passages should prevent the selection of
genetic variants in industrial processes. Acid tolerance is likely to
be a relatively intrinsic property of bacteria and acidification of
culture broth during fermentation would also make selection of
less acid-tolerant variants unlikely if serial subculturing was practiced.
However, data showing the long-term stability of acid and
bile tolerance in probiotics during subculturing are lacking.