Eli5 how bacteria in probiotic pills can be alive and stable in a solid on the shelf

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Answer 1

They don’t choose bacteria to make pills off at random. They select bacteria that are known to be able to form spores.

Spores are a special form some bacteria can take that has minimal metabolism and intracellular water, so as to survive extremely adverse conditions, either high temperature, low temperature or extreme dehydration.

Some spores are thought to be able to survive for thousands of years.

When presented with favourable conditions, spores return to be active bacteria that can multiply again.

If you want to see a similar process*, though not exactly the same, you can add dry yeast to water and sugar and see the small bubbles of CO2 created when cellular respiration restarts.

*Yeasts are not bacteria.