Pretty much this. I've been wondering if any of the yogourt and other "health" foods containing living probiotic cultures survive digestion to populate our intestines? If so, is there peer-reviewed evidence you could point me to?
Apparently, some do and some don't.
I just tried searching for yogurt lactobacillus survival on Google, and the first hit I got was an article titled "Survival of yogurt-containing organisms and Lactobacillus gasseri (ADH) and their effect on bacterial enzyme activity in the gastrointestinal tract of healthy and hypochlorhydric elderly subjects" by Pedrosa et al. (1995), published in the American Journal of Clinical Nutrition, vol. 61, pp. 353–359. The abstract reads:
"The effect of the live bacterial yogurt cultures, namely Streptococcus thermophilus and Lactobacillus bulgaricus, and a mucosal adhering strain of Lactobacillus gasseri (ADH) on small intestinal and fecal bacterial characteristics was examined in 10 elderly subjects with atrophic gastritis and 23 elderly normal volunteers (11 received yogurt and 12 received ADH). Neither S thermophilus nor L bulgaricus was recovered from the stomach or small intestine of subjects fed yogurt or pasteurized yogurt. ADH was recovered from gastric or small intestinal aspirates in three of four subjects and in the stools of four of five subjects diagnosed with atrophic gastritis. In 11 of 12 normal subjects, ADH was isolated from stools. There was a significant reduction in fecal bacterial enzyme activity in both normal volunteers and subjects with atrophic gastritis after being fed with viable ADH. Adherent strains of bacteria such as ADH are likely to survive passage through the gastrointestinal tract and thus have greater metabolic effects."
Here's another result from the same search, "Survival and therapeutic potential of probiotic organisms with reference to Lactobacillus acidophilus and Bifidobacterium spp." (PDF) by Kailasapathy & Chin, Immunology and Cell Biology (2000) 78, 80–88, which says:
"Lactobacillus delbrueckii ssp. bulgaricus and S. thermophilus (yoghurt starter cultures) are not bile resistant and do not survive the passage through the intestinal tract [Gilliland, 1978]. However, L. acidophilus and B. bifidum incorporated into the yoghurt starter culture have the ability to establish themselves among the gut flora [Tamime & Robinson, 1985]."
but also points out that, for survival in the gastrointestinal tract to matter, the bacteria must first survive long enough to get there:
"Strains of bifidobacteria used in some commercial products neither survive gastric transit nor product acidity during storage [Varnam & Sutherland, 1994]. [...] Eight commercial yoghurt samples claiming to contain viable bifidobacteria, sold in London, were enumerated for the presence of this organism. Only five of the eight yoghurts tested contained viable bifidobacteria at > 106 per mL, while the remaining three did not contain any bifidobacteria [Masuda et al., 1993]. Modler and Villa-Garcia  reported that bifidobacteria do not survive in several yoghurt products in North America, due to highly acidic conditions. [...] It is considered misleading to describe probiotic yoghurt as having health promoting properties unless the minimum level of viable cells is present at the expiry date."
When I took Microbio we had a entire lecture on this. My professor's view was that they do not survive due to the pH of the stomach. He told us that there are microbiologist on both sides as to whether they do make it to the intestine (or enough of them to make much difference) or that they do survive and help. I have added a link below to a article by the American Society of Microbiology. Here it is saying that it may be helpful, but that more studies are being conducted.
Thanks for the searching and quoting Ilmari. According to some theories, and perhaps there is some research on stool to support these, we can aid our intestines for producing beneficial bacteria by eating PREbiotic foods which supposedly help create a good environment in the intestines for producing healthy bacteria on its own. Some foods that are supposed to be prebiotic are onions, garlic, and fermented foods such as sauerkraut, kimchi, cortido, miso, kombacha, and sour cream, and also saturated fats such as butter and animal lard. Non-pasteurized full-fat yogurt is not only supposed to be PRObiotic but also PREbiotic as well. That many priobiotic cultures do not appear to survive digestion perhaps our focus should be more on eating PREbiotic and not so much on consuming PRObiotic cultures.
There are also foods that are could be harmful to the environment in the intestines. These, according to theories, include refined sugar, soy, processed foods, and seed oils (the process of extracting oil from seeds and dry plant components is highly refined). For some people, again according to theories, the harmful foods also include gluten and lactose from Holstein Friesian cattle (the most common dairy cow in the USA and I think Europe).