A nice summary in The Hindu by D. BALASUBRAMANIAN Coalition dharma and asthma of Bacteria and Asthma: Untangling the Links by Jennifer Couzin-Frankel (the full article needs subscription). Abstract:
"The number of asthma cases is soaring, but the causes remain elusive. Researchers have some striking clues: For example, children on farms are much less likely to get the lung disease. There's mounting evidence that bacteria matter. Babies born via cesarean section, who experience a more sterile entry into the world than those born vaginally, are more likely to get asthma. So are young children treated with many courses of antibiotics. Along with animal studies, these observations suggest that the balance of bacteria and other microbes help guide immune development—and that when the balance is disrupted, disease may follow. The picture can be dishearteningly complicated. Thousands of species of bacteria have constructed virtual cities inside us, along with fungi and viruses—a world called the microbiome. And it's not so much the presence or absence of bacteria, or even certain species, that matter, but rather the shape of the whole community. All of us play host to bacterial residents. But children who develop asthma, researchers are learning, are home to different bacteria—and sometimes a less diverse mix—than those who stay healthy."
The article also says "Although researchers assume that a child's microbiome is affected by the environment, they don't know this for sure. And proving definitively that bacteria help cause asthma is remarkably difficult. “The only proof lies in a randomized controlled trial, where you somehow manipulate exposure” and see who gets sick, says Bisgaard.
Researchers are experimenting with this approach in the gut. Probiotics, microorganisms like Lactobacillus found in yogurt, could in theory be helpful, but small trials testing whether they prevent allergic disease haven't been definitive. In 2006, the University of California, San Francisco, began recruiting about 200 babies who have at least one parent with asthma. Half receive a probiotic and half get a placebo, and the researchers are focusing on early markers linked to asthma, like eczema and wheezing. In January, they reported that 6 months of probiotics in infancy did alter the balance of microbes in the babies' guts, but final results are several years off."
In other words, only hints so far, and defibitive solutions may be years away.
Along the way, the article also points out the changes in the accepted wisdom "he work also upended how researchers think about lung biology. “If you read a medical textbook even now, it will say the lungs and the airways are sterile; there aren't any bacteria down there,” says Cookson. He became certain that the conventional view was incorrect when he and an Imperial College colleague, geneticist Miriam Moffatt, conducted their own variation of Bisgaard's study in babies. They had at their disposal advanced gene-sequencing techniques that allow for a much more comprehensive census of bacteria flourishing in the lungs. In January, the two and their colleagues wrote in PLoS ONE that they'd sequenced more than 5000 different species in 43 people, including some with asthma and others who were healthy."
Similar changes and progress are decribed by Ed Yong in The dark side of oxytocin, much more than just a “love hormone”.