A study shows that preschool-aged infants acquire a greater proportion of gut microbial strains from preschool peers than from family members after 3 months of attendance.[1] Prior to kindergarten, the median mother-infant strain sharing was approximately 50%.[1] Within one month, the sharing of strains between babies in the same group increased significantly compared to babies from other nurseries.[1][2] By the end of the first school term, the average number of shared trunks had risen from 2.5 to 7.2, and same-group sharing rates averaged 20%.[1][3] The proportion of peer-acquired strains increased from 6.5% to 28% at 3 months, surpassing familial transmission, and in relative abundance nursery strains accounted for nearly 40% of the microbiome.[1] Babies showed high microbiome plasticity, where 44% of the conserved species genomes underwent strain switching in 5 months, as opposed to less than 11% in adults.[1] Antibiotic exposure decreased strain retention and increased acquisition of new ones, particularly in infants, while siblings increased basal microbiome diversity and decreased nursery acquisition.[1][2] Social interactions in the nursery thus significantly shape the development of the microbiome of babies.[1][2][3]