The answer is… not a lot. Well, actually, a bit. Look, it’s complicated. There’s a new study out that tries to answer this question and I’ve written about it for Nature News. Here’s the summary, but do read the full piece – the really interesting thing here is the potential for future experiments that will actually be able to test some of the grandiose claims that adorn yoghurt packets.
Many yoghurts are loaded with live bacteria, and labelled with claims that consuming these microorganisms can be good for your health. But a study published today shows that such yoghurts have only subtle effects on the bacteria already in the gut and do not replace them.
Nathan McNulty, a microbiologist at Washington University in St Louis, Missouri, recruited seven pairs of identical twins, and asked one in each pair to eat twice-daily servings of a popular yoghurt brand containing five strains of bacteria.
By sequencing bacterial DNA in the twins’ stool samples, the team showed that the yoghurt microbes neither took up residence in the volunteers’ guts, nor affected the make-up of the local bacterial communities.
McNulty also fed the five bacterial strains from the yoghurt to ‘gnotobiotic’ mice — animals raised so that the only microorganisms that their guts contain are 15 species found in humans.
As with the twins, the yoghurt bacteria did not change the composition of the rodents’ resident communities. However, the activity of genes that allow the native bacteria to break down carbohydrates did increase.
Meanwhile, if you’re interesting in the bacteria in our guts and bodies, have a look at this introductory slideshow:
Image by Coda.
October 26th, 2011 at 6:00 pm
I’m wondering about the rather limited grouping of bacteria in commerical yogurt as compared to a yogurt cultured from raw milk? And I wonder if you looked at the effect of the latter?
October 26th, 2011 at 6:03 pm
The slide show was excellent. Gave me a good solid introduction to the topic.
I wonder how certain bacteria take a hold in our bodies (like the seaweed one), and some simply pass through (like the yogurt one). And do our gut bacteria get rid of invasive species simply by out-competing them, or do they use something else?
October 26th, 2011 at 11:13 pm
A container of warm milk, raw or pasteurized, is a living environment completely unlike any portion of the human gut. Yes, both are warm and wet, but in all other ways they are different habitats. Bacteria evolve to thrive in very specific environments, so it would be quite surprising if yogurt bacteria were flexible enough to live happily in both human guts and containers of warm milk.
While yogurt is often soothing to an irritated intestine, the bacteria themselves are not likely to be the soothing agents. Researchers have known for a long time that yogurt bacteria do not take up residence in human intestines, but mythology is often stronger than facts – and that myth seems to be very persistent.
October 27th, 2011 at 1:57 am
Even small changes in gut flora could be significant if this recent research holds up:
Gut Microbiome Metagenomics Analysis Suggests a Functional Model for the Development of Autoimmunity for Type 1 Diabetes. (2011) PLoS ONE 6(10): e25792
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0025792
October 27th, 2011 at 6:34 am
I’ve always wondered why we expect yogurt and probiotic bacteria that aren’t metabolically set up for the human intestine to survive. Bacteria that grow in milk often don’t have a requirement for iron, for instance, as it’s not available to them in milk. Mixing these with competitive iron-hungry bacteria in the more nutrient-rich intestine doesn’t give them a chance. We published a study showing that probiotics can’t respond to iron in the way most gut bacteria can, and I’d like to see whether this could be one of the factors limiting them in colonising humans.
October 27th, 2011 at 8:14 am
I wonder about the effect of eating yoghurt in people whose intestinal flora have been largely removed by antibiotics or physical cleansing. Does it speed the reestablishment of a healthy bacterial flora?
October 27th, 2011 at 7:36 pm
BJM – the answer is no, yogurt bacteria cannot colonize a human intestine, it is a hostile environment for them.
Close association with other humans who have not used antibiotics recently – the closer and more intimate the better – is currently the fastest way to re-colonize your intestinal flora (and your skin flora) after antibiotic use. As those who study the bacteria of urban surfaces say, “poo is everywhere”. It’s mostly a good thing those beneficial bacteria are spread so easily from person to person, since some of them are symbiotes necessary for our health.
Physical cleansing, such as using enemas, does not have an appreciable effect on your intestinal flora. Vast numbers of your normal bacteria remain behind on the intestinal walls after the bulky contents are washed out, and the next food which moves through is properly processed.
October 29th, 2011 at 1:12 pm
Persons taking antibiotics are, sometimes, advised to consume probiotics to prevent yeast infections. Why?