Mouse models have pointed to a role for dysbiosis of the gut microbiota during early life in the aetiology of asthma. Arrieta et al. surveyed 319 children from a 3-year longitudinal study and found that a similar phenomenon occurs in humans. 16S rRNA profiling of stool samples from 3-month and 1-year time points showed that an increased risk of asthma was associated with a reduced abundance of four bacterial genera — Lachnospira, Veillonella, Faecalibacterium and Rothia — during the first 100 days of life. Using a mouse model of asthma, inoculation of germ-free animals with one of the human dysbiotic stool samples confirmed that the severity of the inflammatory response could be significantly weakened by supplementation with these four genera. Finally, metabolic measurements of the dysbiotic stools revealed functional changes in the microbiome, including a decrease in the concentrations of LPS and acetate compared with healthy stools, although how these functional changes might relate to asthma is not known.