Dogs, Dust Microbes, and Allergies

Dust-borne bacteria from houses with dogs can prevent allergies in mice by changing their gut microbes.

By Ed Yong | December 16, 2013

WIKIMEDIA, BEV SYKESA pet dog can bring joy, companionship, and security to a home. It could also change the microbes in your gut.

Susan Lynch from the University of California, San Francisco, has shown that cohabitating with a dog can increase the diversity of bacteria in house dust. When she fed this bacteria-rich dust to mice, it shifted the microbes in their guts towards species that prevented their immune systems from overreacting to airborne allergens. The team also showed that a single bacterial species could duplicate many of these benefits.

The results are published today (December 16) in PNAS. If Lynch’s findings can be confirmed in humans, they could guide the development of probiotics for infants, which could reduce the risk of allergic conditions later in life. 

Lynch was inspired by a decade-old study showing that people who spent their first year of life in a household with dogs or cats were less likely to be sensitive to airborne allergens as adults. In 2010, her team showed that the dust from houses with dogs (and to a lesser extent, cats) harbor a broader range of bacteria than that the dust from houses without pets. And most of these microbes are also found in the human gut.

Since earlier studies had shown that gut microbes can influence the immune system, Lynch reasoned that pets could change their owners’ propensity for allergies by affecting the microbes in their house dust and, therefore, their guts.

To test this hypothesis, Kei Fujimura, a postdoc in Lynch’s lab, fed mice dust samples collected from two homes: one with a dog and one with no pets.

When the mice ingested the dog dust, they were less likely to show allergic reactions to varied substances, including cockroach allergens and a protein found in egg whites. When they came into contact with these chemicals, they produced fewer immune cells, showed less inflammation in their airways, and had less immunoglobulin E (IgE)—an antibody associated with allergic diseases like asthma and atopic dermatitis.

Fujimura showed that the dusty meals from homes with dogs changed the microbe communities in the rodents’ guts, increasing the numbers of over 100 groups. They particularly enriched one genus, Lactobacillus, and one species within it—Lactobacillus johnsonii.

L. johnsnoii is a known human gut colonizer and becomes a dominant member of the maternal vaginal microbiome just before birth,” said Lynch. She suspects that the species helps to structure the initial community of microbes in a baby’s guts.

When Fujimura supplemented her rodents’ diet with L. johnsonii alone, she saw much the same effects as feeding them dog dust: a restructured gut microbiome, less inflammation, and a weaker immune response to allergens. Supplementing the mouse diets with this lone species even prevented the rodents from overreacting to respiratory syncytial virus (RSV)—a common virus that infects the lower airways of many human infants and increases the risk of asthma.

Lynch said that the “magnitude and breadth of the protective effect” exerted by the bacteria was surprising, and suggests that L.johnsonii triggers immune pathways that work against a variety of different airborne threats. Her team is now trying to understand the molecular mechanics at work.

“It’s a neat study, and it’s encouraging to see that they can identify single Lactobacilli that are involved in moderating the immune response,” said Brett Finlay, a microbiologist from the University of British Columbia, in an e-mail to The Scientist.

However, Thomas Abrahamsson, a pediatrician at Linköping University in Sweden noted via e-mail that “they do not actually show that L. johnsonii originated from the dust.” He thinks the team should identify the bacteria in the dust samples themselves, supplement the mice with these species, and see what happens.

Lynch says she’s working on this. Her team is trying to understand whether the dust bacteria colonise the gut themselves, or simply change the colonization patterns of the existing microbes. “There are other components, such as allergens, present in the dust which may impact immune responses, thus altering microbiome composition,” she added.  

The protective effect that corresponds with the presence of furry pets is likely time-sensitive, with the greatest impact experienced by newly born organisms, Lynch said. She added that she hopes to develop probiotics that could duplicate the immune-enhancing effects of dog dust. Indeed, several studies have already examined whether probiotics can reduce the risk of atopic eczema, with more than half showing a benefit.

K. E. Fujimura et al., “House dust exposure mediates gut microbiome enrichment of Lactobacillus johnsonii and immune defense against airway allergens and respiratory virus infection,” PNAS, doi:10.1073/pnas.1310750111, 2013.

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Avatar of: DavidT


Posts: 2

December 17, 2013

I agree with Thomas Abrahamsson's criticism. L. johnsonii is an obligate anaerobe. As such, it's very unlikely to survive in meaningful amounts in the air or in dust. It can form spores, and it's possible that some may survive and transfer from dog to human.... but I think it's much more likely that the increase in  L. johnsonii in the gut is an indirect effect of something else, likely an immune response to a dog-originating antigen. Still interesting work though, and this microbe may have pretty strong potential in anti-allergy developments. 

Avatar of: jeenious


Posts: 45

December 19, 2013

If any microbes get from the gut of a house-kept dog, surely a primary source would be from dog saliva.

I spent the first year of my life being licked frequently by a dog-wolf hybrid,

My physician says I am the healthiest 79-year old he ever has known, which is anecdotal, of course.  But I've never been completely without a household pet dog.  And I've never contracted any kind of pathology I know of, as a result.

I do, however, have what may be a gut-related intolerance for certain categories of fats.  I could drink fresh vegetable oil without regurgitating it, but once it ages (even in the refrigerator) it goes in only one direction -- upwardly regurgitated.  If I consume any amount of chicken fat, the result is total regurgitation of it. 

In discussing this with my private physician, we both tend to agree that, despite this intolerance, I get enough fat, evidently, to be healthy, and not enough to result in any bad blood test results.

Again, this is anecdotal.

Whether dog dander, or dried dog saliva in dust, would have killed me, I'd never have lived through my first year.

One of the least understood areas in science, I think, is immunology and parasitology.  And, if current trends in research are any indication, then that field is now waxing into a triplex one:  immunology, parasitology and oral intake of any or every substance that makes its way into the alimentary system by any means, and from any source.

There is a tendency, I perceive, even among those trained in the sciences, to reduce, or over-simplify, some things that are so complex as to require the most cutting edge data processing of today, to control any research project in this triplex dynamic set, for all the hundreds, if not thousands, of variables required to be controlled for, if ever we are to arrive at truly effective analyses of individual (not to mention various COMBINATIONS) of etiological actors entailed in gut flora and fauna, and in what results various kinds of population mixes may or may not lead to in millions of individual human subjects.

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