Observational studies in humans have shown that early life exposure to microbes in a variety of situations is associated with a decreased risk of asthma.
– Exposure of a pregnant mother to microbes in order to protect her baby
– Exposure of children to livestock or dogs
– Growing up with numerous siblings
Meanwhile, disruption of normal colonization processes (such as antibiotic use and C-section delivery) is associated with an increased risk of allergy.
Some gut microbiota changes also track with asthma risk. For example, one study showed a greater risk of developing childhood allergic diseases when infant feces contained higher levels of Escherichia coli or Clostridium difficile.
Whereas observational studies don’t tell us about causality, mouse models can be useful first steps for identifying causal links.
A recent mouse study delved more deeply into how microbial exposures were associated with allergic immune responses. Researchers at University of California San Francisco investigated how the air we breathe may impact GI microbiome composition and airway disease outcomes .
They collected dust from two homes: one without a dog, the other with an indoor/outdoor dog. Using a sterile process, they vacuumed a small area in the house for three minutes. Four times as much dust was collected from the home with the dog.
They then exposed mice to the collected house dust, and found that immune response to these airway allergens was down-regulated in mice treated with the dust from the home with a dog. Observed changes included reduction in the total number of airway T cells, down-regulation of Th2-related airway responses, reduced mucin secretion and an altered microbiome of the cecum. Specifically, Lactobacillus johnsonii levels were higher in mice exposed to dust from the house with a dog.
Researchers took this information and found that giving mice an oral dose of live L. johnsonii (3.9×107 CFU) by itself was protective, down-regulating antibody responses and airway inflammation.
This study is unique in that it evaluates the impact of inhaled exposures, rather than dietary ones. The second part reinforces that protection from allergic responses can also be achieved through exposure to certain dietary probiotics. The authors conclude that this study suggests that “GI microbiome manipulation represents a promising and efficacious therapeutic strategy to protect individuals against both pulmonary infection and allergic airway disease.”
The next step: to confirm these findings in humans.
 Fujimura KE, Demoor T, Rauch M, Faruqi AA, Jang S, Johnson CC, Boushey HA, Zoratti E, Ownby D, Lukacs NW, Lynch SV. (2014) House dust exposure mediates gut microbiome Lactobacillus enrichment and airway immune defense against allergens and virus infection. Proc Natl Acad Sci U S A.
2014 Jan 14;111(2):805-10.