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.