How do probiotics survive stomach acid exposure?

We put this question to naked scientist Chris Smith:

Chris - The evidence is that they do make it and people have done a lot of studies to prove this. The way they've done this is you feed people with having established a baseline of what's already in their gut. Genetically, you feed them some of these probiotics. You then collect samples that are leaving their body and again, you can either try to directly culture or you genetically interrogate by getting DNA out of the poo, what's growing in there and you can prove what the lag time is, when these bugs turn up and how long they dwell in the body after you've eaten them. And you can prove therefore that they must be surviving because they're going in, coming out but then they're also hanging around for a while afterwards. So, that proves it works. How do they survive? Well, some of them are pretty tough and they're actually known as acidophili. They are lactobacilli. They actually eat lactic acid to produce an acid environment and as a result, they are actually quite well-adapted to surviving an acid. If you eat them with food and with glucose, they tend to survive much better than if you just eat them on an empty stomach. So the evidence is, if you eat these things regularly, they will make it through. At least a proportion of them will and these bugs grow fast. As a result, even if a few get through, they're still going to grow and colonise and we think therefore, they've got the potential to change the environment of your intestine in a good way.

Kat - So they're basically just pretty nailed hard.

Chris - Yeah, but in a good way because they will actually help to rebalance your intestine because part of the whole story about your microbiome is that there are more bacteria living in you and on you than there are cells in your entire body. There's maybe 100 trillion cells that are you. There's probably 100 times that many cells that are bacteria living in you. There's probably 2 kilos of bugs living in your gut, just bugs. If we filtered everything that comes out of your gut then actually, you'd find there's probably 2 kilos of bacteria in there.

Kat - Nice.

Chris - Yeah, very nice. Actually, they do a very good job of keeping you healthy. They eat your food for you, they pass on the results of what they digest and they therefore bring to the table a microbiological metabolic knife and fork that you haven't got in your own cells. They can break stuff down and make stuff you haven't otherwise got and they feed it to you. If they go out of kilter, your body is deprived of this source of energy, the source of chemicals, biochemical, and also, some vitamins. And so, rebalancing that keeps out the bad guys, and keeps you ticking along with the supply of the right source of chemicals that your body has adapted to need.