Treating mice with antibiotics during their first few days of life irreversibly hinders the development of their gut bacteria, leading to altered immune responses to common allergens. Writing in the journal Mucosal Immunology, researchers say that this discovery provides the first solid evidence for a causal link between childhood antibiotic use and the later development of asthma and allergies.

“The practical implication is simple: Avoid antibiotic use in young children whenever you can because it may elevate the risk of significant, long-term problems with allergy and/or asthma,” explained study author Martin Blaser in a statement.

In their experiment, the researchers exposed mouse pups to either plain water, azithromycin, or amoxicillin during the first nine days of their lives. In their write-up, the authors explain that they chose these particular medications as they are the “two most commonly prescribed antibiotics in pediatric practice.”

While monitoring the composition of the animals’ gut bacteria over their life course, the researchers found “sustained differences in the gastrointestinal microbiota of the antibiotic-exposed pups into adulthood.” Importantly, when the rodents were later exposed to house dust mites, those that had received the antibiotics as infants displayed elevated antibody and inflammatory cytokine production, indicating an allergic reaction. This effect was particularly noticeable in the mice that had been exposed to azithromycin.

To determine whether antibiotics affect young and old animals alike, the researchers then exposed adult mice that lacked any gut bacteria to fecal samples taken from the antibiotic-exposed pups. Results indicated that the immune response of these older rodents to common allergens did not differ from that of non-exposed mice.

However, pups born to these adult mice “acquired the antibiotic-perturbed microbiota at birth”, leading to elevated immune responses to dust mites as well as “altered airway reactivity”. According to Blaser, these findings “provide strong evidence that antibiotics cause unwanted immune responses to develop via their effect on gut bacteria, but only if gut bacteria are altered in early childhood.”

Previous studies have found that children with abnormal microbiota at the age of one have a higher chance of developing asthma by the time they are five. Until now, the causal mechanism behind this trend had remained unknown, yet the authors say their study “provides evidence for predisposition to altered lung physiology that is microbiome-dependent.”

Summarizing their findings, the researchers conclude that there exists a critical “window during early-life” during which the growing microbiome supports the healthy development of the immune system. The use of antibiotics during this period may therefore interrupt this process, potentially causing allergies and asthma later in life.