After my column in last Sunday’s Seattle Times on probiotics and prebiotics, a reader asked me how probiotics and fermented foods make it through the stomach digestive acids.

It’s an excellent question. I mean, our stomach and small intestine digest our food, why wouldn’t they digest any passing bacteria? It’s true, our digestive tract is a harsh environment, full of acids and digestive enzymes, but some bacteria are up to the task. Unfortunately, this includes some illness-causing bacteria (hello, food poisoning).

Different bacteria adapt to different environments. Take temperature, for example. Some bacteria can survive freezing temperatures (which is why you should never freeze food that “might” have spoiled in the hopes that freezing will make it OK), while others thrive in hot springs.

Now let’s consider the acidic environment of the stomach. While most bacteria prefer a environment that hovers near neutral (pH of 6-8), some have adapted to survive or thrive in acid. Helicobacter pylori is able to survive stomach acid (pH 1.5-3.5) long enough to burrow through the stomach’s mucosal lining (which protects the stomach from being digested by its own acids) and make a home close to the layer of cells lining the interior of the stomach, which can cause ulcers and other problems. There are other bacterial species that can use sulfuric acid as an energy source! These bacteria have developed mechanisms that allow them to maintain their required internal pH by compensating for an extreme pH (either acidic or alkaline) in their environment.

So back to probiotics. Some of these beneficial bacterial species will make it through your stomach and small intestine unscathed, alive and kicking and ready to do good things in the large intestine. Others will not survive. Partly, it’s a numbers game, and partly it depends on how well adapted the species is to an acidic environment. The most common probiotic species, Lactobacillus and Bifidobacteria (the ones you frequently see in yogurt and other fermented dairy products) have adapted thusly.

The survival odds are enhanced when the probiotics are in a dairy product, or in a supplement that comes packaged in an enteric-coated capsule (the same type used for many over-the-counter pain relievers). Essentially, to be considered health-promoting, the probiotic species has to be able to survive. It’s a requirement! I stumbled across one study that looked at the survival of yogurt bacteria in the human gut. They followed the little guys all the way from input (down the hatch) to, um, output.

As for what happens to fermented foods in general, if you’re talking about fermented foods that still have live, active beneficial bacteria, then the same rules apply regarding whether those species survive. But the fermenting process, which is sort of like pre-digestion, brings other potential benefits, including making some foods more edible, digestible and nutritious.