Fig. 4

β-lactam phylogenetic tree, annotated by study subject. Maternal subjects are marked with an “M”. Infant fecal samples are marked with a number; “1” indicating the first (baseline) sample collected at 1–2 months of age, “2” indicating the second sample collected at 6–7 months of age, and “3” indicating the third sample collected at 11 months of age. The antibiotic-naïve control family is colored green, the family with infants discordant for amoxicillin exposure at 8 months of age is colored purple, and the family with infants concordant for amoxicillin exposure at 8 months of age is colored brown. Infant twin A subjects are shaded darker; twin B subjects are shaded lighter. β-lactamases were commonly present in both members of a twin pair, and frequently persisted at more than one timepoint within a given subject. Many β-lactamases identified in the infant fecal microbiomes were not present in the maternal microbiome