A study in Nature Microbiology showed that babies born very preterm who underwent antibiotic treatment for 21 months had reduced bacterial population diversity and increased antibiotic-resistant genes in their gut, compared with very preterm infants who were given antibiotics for less than a week and healthy late-preterm and full-term infants who didn't receive antibiotics. The findings also showed that those who received long-term antibiotic treatment had gut bacteria genes resistant to the antibiotics chloramphenicol and ciprofloxacin, which aren't usually given to newborns.