2. It should be non-pathogenic and non-toxic.
3. It should be present as viable cells, preferably in large
numbers.
4. It should be capable of surviving and metabolising
in the gut environment, e.g., resistance to low pH and
organic acids, and should be able to maintain genetic
stability in oral microflora.
5. It should be stable and capable of remaining viable
for periods under storage and field conditions.