Some types of antibiotics may alter the gut biology of young children for up to two years after they have taken them, according to a study of Finnish children.

Key facts Study looked at long-term use of antibiotics and gut microbiome in children aged 2-7

Study looked at long-term use of antibiotics and gut microbiome in children aged 2-7 Antibiotics used for respiratory infections changed composition of gut flora

Antibiotics used for respiratory infections changed composition of gut flora Use of these antibiotics correlated with increased body weight and risk of asthma in the children

The use of these antibiotics, known as macrolides, was also associated with an increased risk of developing asthma and becoming overweight, reported a group of researchers in the journal Nature Communications.

"Our results ... indicate that macrolide use may have undesired effects on the developing microbiota [ecological community of microorganisms] of children, which may compromise the development of a healthy immune system and metabolism," they wrote.

Professor Willem de Vos of Wageningen University and his colleagues analysed the bacteria in faeces samples from 142 children aged between two and seven that attended a Finnish day care centre.

The samples were matched with details of any antibiotics that had been prescribed to the children for respiratory infections, their body mass index and allergy information.

The two most prescribed groups of antibiotics were penicillin and macrolides such as azithromycin.

The researchers found the use of macrolide antibiotics, but not penicillins, was associated with marked changes in the richness and balance of gut microbiota.

The gut flora of children treated with this type of antibiotic took between one to two years to fully recover — which was longer than the average time between courses of drugs prescribed to the children.

Type of antibiotic is important

Dr Laura Weyrich of the Australian Centre for Ancient DNA at Adelaide University said the study is the first to show that antibiotic use is associated with long-term health effects in children.

"There's been so much research over the past five years showing that antibiotic use has been correlated with disease but it's been done in mice," said Dr Weyrich, who studies how microbiomes change.

She said the study highlighted differences between penicillin and the more potent macrolides.

"So not only antibiotics in general are important but it's showing the type of antibiotics we are administering is important," she said.

Dr Weyrich said the difference between the two types of antibiotics may be due to resistance to penicillin, which has been used for a long time.

"It's really suggesting that we need to take a look at the specific antibiotic that we are administering to children, and think about the long-term impact that it plays in their health," she said.

"We can not simply go and continue to administer antibiotics ad hoc to any child that has an ear infection or a sore throat."

Long-lasting impact on risk of disease

Professor Mimi Tang of the Murdoch Childrens Research Institute said the study provided a detailed characterisation of the microbiota in early life and the impact of antibiotics.

"They show the use of antibiotics has quite dramatic changes on the microbiome in terms of its richness," said Professor Tang, who studies the gut microbiome and early life programming of immune responses.

She said the study did not prove that antibiotic use can cause disease, but it supports a hypothesis that the gut microbiome in early life plays a critical role in the body's immune and metabolic systems.

"Importantly even after some of these changes recover, what you're seeing is an association between antibiotic use and disease beyond the recovery of the microbiota," Professor Tang said.

"This is interesting because what it says is 'yes antibiotics disrupt the microbiota but even after the microbiome has recovered there are long-lasting effects on risk of disease'."

But, she said, while correlations to asthma and obesity were "highly interesting", the case numbers were small and the study would need to be replicated.

She also said it would be interesting to explore whether giving antibiotics later in life had the same effect.

"If we truly believe early life changes are the critical moment for programming the immune and metabolic systems, we need to be much more conservative with antibiotics in the first two years of life," Professor Tang said.