Infants are more likely to be at risk of asthma if four specific types of bacteria are missing from their guts, a study has found.
Animal tests also showed that the bugs appear to protect against airway inflammation and asthma in offspring.
The research provides strong support for the “hygiene hypothesis” that suggests over-clean conditions may be making babies more vulnerable to asthma and other allergy disorders.
Scientists identified four bugs that were completely absent or present at very low levels in stool samples from 22 three-month old infants who had an increased chance of being diagnosed with asthma by the age of three.
Another 297 children from the study group had significantly higher numbers of the microbes in their guts, and were not considered to be at risk according to a range of test results.
The high-risk infants were also more likely to have been treated with antibiotics before the age of one.
Newborn asthma-prone mice harbouring the bugs turned out to be much less likely to suffer air way inflammation and symptoms than those which did not, indicating a protective effect from the microbes.
Lead scientist Professor Brett Finlay, from the University of British Columbia in Canada, said: “This research supports the hygiene hypothesis that we’re making our environment too clean. It shows that gut bacteria play a role in asthma, but it is early in life when the baby’s immune system is being established.”
The first three months of a baby’s life appear to be a critical “window” that allow gut bacteria to modify the immune system, said the researchers writing in the journal Science Translational Medicine.
Asthma rates have soared dramatically since the 1950s and the condition affects up to a fifth of all children in western countries.
Most babies naturally acquire the four bacteria, collectively known as FLVR (Faecalibacterium, Lachnospira, Veillonella, Rothia), from their environments, but some do not, scientists said.
Washing with antibacterial solutions, Caesarian births, use of antibiotics, and not having contact with animals were all cited as factors that might contribute to the “hygiene” effect.
Prof Finlay said an estimated 300m people worldwide suffered from asthma, but “ironically” disease rates had not increased in developing and poorer countries.
He added: “There’s a lot ... to indicate that the bacteria and other micro-organisms that inhabit us as part of our normal lives, may actually play a role in this because things such as a Caesarean section increases your risk of asthma, breastfeeding decreases it compared to bottle feeding, antibiotics in the first year of life significantly increases incidence, and having a pet or living on a farm decreases it.”
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