Introduction:
Infant jaundice is a common condition which results from a high concentration of serum bilirubin. Phototherapy is a widely used treatment for bilirubin clearance. We analyzed the effect of phototherapy on intestinal flora and metabolism of newborns. The aim was to assess the benefit of treatment for hyperbilirubinemia with phototherapy.

Material and methods:
Fifty-three jaundiced infants hospitalized at our neonatal intensive care unit were treated with phototherapy. Of them, 29 were prescribed antibiotics during the hospitalization. Fecal samples were collected before and 24 h and 48 h after phototherapy. The bacterial species and relative abundance were identified with Macrogene sequencing. The bile acids in feces were identified using liquid chromatography-mass spectroscopy (LC-MS).

Results:
Differential microbial species/genera and secondary bile acids were found after phototherapy. There are significant differences in the changes of the microbial species/genera between infants who did not receive antibiotic treatment and those who were given antibiotic treatment. Secondary bile acids were also significantly altered. At the same time, the differential microbial species/genera and the differential secondary bile acids interacted with each other.

Conclusions:
This study identified several differential intestinal microbial species and secondary bile acids in fecal samples from infants with jaundice before and after phototherapy. Phototherapy can change the flora and its metabolism and its long-term impact needs further observation.