Yogurt bacteria knock back influenza

9 Sep

Homemade yogurt

I recently mastered the art of yogurt-making…  or, I guess I  could say it more precisely: I learned how to manipulate a commercially available consortium of lactic acid bacteria (LAB) to make yogurt for me using common, household materials in my very own kitchen.  As a soil microbiologist, I love running these little experiments at home – the do-it-yourself mad-scientist approach to homemaking.

According to all the “make your own yogurt at home” websites, blogs, and YouTube videos, the number of live organisms in the homemade stuff is substantially higher than in store-bought stuff, which means it confers much greater probiotic benefits; unfortunately, I couldn’t find any original research to back this claim.  In my futile attempts to find more information on the proven health benefits of homemade yogurt (I need something to convince my friends and family to eat the stuff now that I’ve made loads of it), I came across a pretty cool study that I decided to share; it’s not exactly environmental microbiology, but a very cool case of microbial/human ecology (and particularly relevant as flu season approaches).

By way of introduction to the subject…

The most common bacteria in store-bought yogurt are Lactobacillus delbrueckii and Streptococcus salivarius; these little guys and their cohorts earn the label “live active cultures” in the fine-print on the side of the yogurt container.  Some commercial yogurts may also contain certain bifidobacteria, and you can even purchase a “yogurt starter” which is a mixture of two or three lactobacilli, along with the Streptoccocus and a bifiobacterium.  These LAB function to convert the lactose (milk sugar) into lactic acid via fermentation, and voila! Yogurt!

But it just so happens that many of these types of organisms (LAB) naturally live in the human gut (not to mention a variety of other locations on and in our bodies) and help us digest our food.  So, if for any reason a person’s gut microflora get perturbed (let’s say by a dose of antibiotics for that ragweed-induced sinus infection), then we can safely and effectively re-introduce these good bacteria to our digestive tract by eating yogurt and virtually eliminate the upset stomach and diarrhea that might have resulted otherwise.

This all seems pretty straightforward, and science has had a good hold on this aspect of probiotics for quite some time now.  However, a much more interesting aspect of this story has arisen recently, regarding the human immune system.

“Emerging evidence from recent clinical and animal studies supports the notion that probiotic lactobacilli, especially some selected strains, can modify host innate and acquired immune responses by which they can protect against respiratory infections.” – Kawase, et al. (2010)

The flu study

The authors administered live cells of either Lactobacillus rhamnosus or Lactobacillus gasseri (also found in yogurt and closely related to the other LAB mentioned earlier) to a group of 13 mice (one group for each species of bacteria, plus a control group = 39 mice), once each day for 19 days.  On day 14, the mice were also inoculated with the mouse-version of the H1N1 flu virus.  From that point on, the mice were visually monitored for flu symptoms, and the level of virus in the lungs of the mice was measured at the end of the experiment (I’ll spare you the gory details of how they performed that last part).

What they found was that all the mice seemed just exactly the same until 2 days after they had received the H1N1 virus.  The effects of the probiotics were not evident until the control mice, who had not received either of the LAB, began to fall ill.

By 6 days after infection with the virus, the control mice were displaying clinical symptoms of the flu (headache, fever, glued to the couch with a blanket sipping chicken soup, etc), while the mice on the probiotics were looking significantly better and acting healthy (jogging, biking, playing horseshoes, you get the picture).

But seriously, not only did the treated mice look better and seem to human observation to be feeling better, but the level of virus in their lungs at the end of the experiment was significantly lower (less than half) compared to  those mice who had not received either of the lactic acid bacterial treatments.  The researchers also found pathological changes in the lining of the bronchial tubes in control mice that did not exist in the treated mice.  There appeared to be no difference in the protective effects of each species of bacteria; both conferred disease resistance equally.

I have to admit I was skeptical at first read, but with a little more digging, I found these results were supported (albeit indirectly) by in vivo (Harata et al. 2009) as well as in vitro work with immunocompromised model animals (Yasui et al. 2004).  These studies demonstrated altered immune response in terms of cytokine and IgA production and increased survival rate, all in response to LAB probiotics, but the mechanism remains unclear.  How does it work?  We don’t really know yet; it just does.

Of course, these aren’t humans we’re talking about, and quite a few studies have attempted to reproduce a similar effect in children and infants, to decrease allergies and/or asthma and the results are a mixed bag.  There are so many additional factors that could confound work with human subjects, though… at least with the mice, you know exactly what they eat, when, how long they sleep, what they do all day, and it seems to me that you could never know all that with human subjects, no matter what they say on that questionnaire.  Just another reason I prefer to do my science with microbes.

In a nutshell

It seems certain species of lactic acid bacteria like those we find in yogurt, when ingested, have beneficial effects on disease resistance in mice, and potentially in humans as well.  I suspect this work by Kawase et al. could be just the tiniest tip of a very fascinating iceberg.

In the meantime I’ll be watching (and taking notes) on how many of my home-made yogurt connoisseurs come down with the flu this year.  Eat up!

ResearchBlogging.orgKawase, M., He, F., Kubota, A., Harata, G., & Hiramatsu, M. (2010). Oral administration of lactobacilli from human intestinal tract protects mice against influenza virus infection Letters in Applied Microbiology DOI: 10.1111/j.1472-765X.2010.02849.x

Harata, G., He, F., Kawase, M., Hosono, A., Takahashi, K. and Kaminogawa, S. (2009) Differentiated implication of Lactobacillus GG and L. gasseri TMC0356 to immune responses of murine Peyer’s patch. Microbiol Immunol 53, 475–480.

Yasui, H., Kiyoshima, J. and Hori, T. (2004) Reduction of influenza virus titer and protection against influenza virus infection in infant mice fed Lactobacillus casei Shirota. ClinDiagn Lab Immunol 11, 675–679.


10 Responses to “Yogurt bacteria knock back influenza”

  1. Cam September 10, 2010 at 5:35 am #

    It seems to me that the probiotic movement as a whole lacks biological plausibility – sure, good bacteria in the gut can outperform bad most of the time, but what does that have to do with influenza, a respiratory virus? Do lactobacilli from yogurt colonize the gut and help provide extra resistance? What’s the mechanism? I’d be interested to see more information on where it all fits in, in terms of the medical picture.

    • Microbial Modus September 10, 2010 at 9:11 am #

      The mechanism is exactly the question! How on earth could this possibly work? I believe the answer lies in microbial signaling molecules much like the one described by Woodward and colleagues and succintly blogged here on Small Things considered. The Listeria bacteria excrete a molecular “calling card” that activates host immune response. As far as I know, there isn’t any evidence yet to believe that LAB are doing this in the human gut, but it hasn’t been ruled out either.

  2. Bill February 13, 2011 at 9:59 pm #

    What is fascinating is that there is no published list in the entire world of microbe strains used in yogurt, one of the top foods of the world.

    I am trying to put together such a list: http://mryogurt.info/probiotics/

    Please, post links where I might find more strains to add to the list. From what I read, thousands of strains are used for yogurt culture.

    • Microbial Modus February 14, 2011 at 10:39 am #

      That’s a great question Bill! If I find out anything for you, I’ll let you know…

  3. Joanna Elizabeth May 17, 2011 at 7:11 pm #

    Hm… are you able to tell if what you’re identifying is alive?

    • Microbial Modus May 19, 2011 at 9:21 am #

      Nope. The assumption is that dead organisms and their DNA should be rapidly degraded and digested by other living organisms, but to date, no one has really been able to prove that or establish how long free DNA can exist in a given system. There’s also the question of organisms that may be technically “alive” but not active, so not really a part of the ecosystem. It’s a potential problem, for sure, but still the most commonly used method.


  1. Bacterial Bonanza (via NPR) « Microbial Modus - September 15, 2010

    […] science pieces.  The following post (and associated broadcast) is especially relevant to my most recent post on probiotics.  […]

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