Off to a good start: current gaps and priorities in early-life microbiome research.

Early-life microbial exposures are essential for optimal development of human physiology. Yet, understanding of the human microbiome during pregnancy and childhood is still far from being complete. To identify knowledge gaps and establish research priorities, a multidisciplinary expert panel used the Delphi method for consensus development and conducted a literature search on early-life microbiome determinants. Responses from 55 researchers from an online survey were analyzed alongside keyword frequency from 20 501 publications. This approach enabled us to categorize existing evidence and highlight areas requiring investigation. While the main routes for mother-to-child bacterial transmission and their contributions to the newborn microbiome have been studied, many gaps remain. Priority areas include non-bacterial microbes, ecological principles of colonization, environmental and social influences, body sites beyond the gut, and factors affecting the maternal microbiome and its effects on the child's microbiome. Significance of factors such as hygiene habits, non-antibiotic medications, and pollution remains to be uncovered. Knowledge is also limited on postnatal microbial sharing via household contacts and shared environments (e.g. family members, peers) and the contribution of these pathways to microbiome assembly. We hope this report will guide and inspire future research into the early-life microbiome as a modifiable factor in reducing disease risk.