Scientists do not yet know where infants get colibactin-producing bacteria, because these bacteria are rarely found in parents or in hospital environments, which suggests they may not be passed from mother to baby during birth as previously thought.

More than half of all infants carry bacteria that produce colibactin and have the pks gene cluster during their first two years of life. These bacteria are most commonly found between 6 and 12 months in healthy full-term babies, and between 12 and 24 months in babies cared for in neonatal intensive care units. This pattern suggests that such bacterial carriage is a typical part of early gut microbiome development.