Siblings, a major determinant of microbiota development

From birth, the formation of an infant’s microbiota depends on its closest microbial sources, above all the mother, but also the rest of the family, particularly brothers and sisters. However, there have been few studies on the influence of siblings in this regard. To fill the gap, Danish researchers sequenced fecal (taken at 1 week, 1 month, 3 months, 1 year, 4 years, and 6 years) and pharyngeal samples (taken at 1 week, 1 month, and 3 months) from 686 children in the Copenhagen Prospective Studies on Asthma in Childhood 2010 (COPSAC₂₀₁₀) cohort. Data on younger and older siblings were collected or updated at each visit. In addition, fifteen covariates were identified, including birth weight, antibiotic use, diet, and the presence of pets. The researchers then assessed the relationship between the gut microbiota signature of having siblings and the presence of asthma, allergic rhinitis, and allergic sensitization at six years of age.

Differences in diversity and abundance in young children with older siblings 

The researchers found siblings to be one of the most important factors in the development of a child’s gut and pharyngeal microbiota. Significant differences in composition, both in terms of diversity and quantity of bacterial genera, were found. The impact of siblings was particularly evident in the first year of life, and a smaller age difference with respect to the older sibling had a greater impact than the number of older siblings.

The pharyngeal microbiota of children with older siblings had a reduced alpha diversity at three months compared with that of only children. Moraxella and Neisseria were more abundant, whereas Staphylococcus was less abundant. No other factor, including even major factors such as breastfeeding or antibiotic use, had a greater impact on the composition of the pharyngeal microbiota than siblings.

As for the gut microbiota, children with older siblings showed a higher alpha diversity and a significant difference in beta diversity up to four years of age. At one year, siblings were the most important determinant of beta diversity after mode of birth. Children with older siblings had a significantly lower abundance of Escherichia/Shigella, other Enterobacteriaceae, and Veillonella, but a significantly higher abundance of Prevotella. Notably, the increased abundance of Prevotella was even more pronounced at four years and persisted at six years. Lastly, a gut microbiota with the sibling signature at one year of age was associated with a reduced risk of asthma at age six.

Better integration of siblings into studies on microbiota development

The researchers therefore believe that the development of an infant’s microbiota is significantly influenced by siblings, with consequences for the child’s health. They suggest that studies on the development of the microbiota in children should take into account the presence of siblings, especially older siblings.