Probiotics may not colonize the gut permanently and often wash out, though prolonged use might help with some colonization.

“Now, that mechanism isn't well understood but it's fairly well recognized that these organisms that you can buy as supplements or you find in fermented foods don't take up permanent residence in the gut typically but they do something as they're passing through this community. It's known that they can be viable, they're alive, and they can actually have interactions either with the microbiota or the host's immune system. So I think, a nice way to think of it is just using probiotics as place holders while your microbiota is recovering using those organisms that are present in fermented foods for instance can actually help to prevent pathogens, bad bacteria from taking up residence during that time.”