Objective To assess the impact of postnatal exposure to antibiotics on intestinal microbiome in preterm infants with 16S rDNA sequencing technology.Methods This study was conducted on 19 preterm infants admitted to the neonatal intensive care unit (NICU) at Tongji Hospital immediately after birth from September 2015 to February 2016.Two groups were set up according to the duration of antibiotic exposure (＜3 d,n=10;＞7 d,n=9).Fecal samples were collected from each infant within the first day and 2 or 3 weeks after bitrth.High-throughput sequencer (Hiseq 2500) was used for sequencing,from which information on composition and abundance of species,phylogenetic evolution and bacterial community diversity was obtained.Intergroup differences was analyzed with independent samples t-test or Fisher's exact test.Results (1) No statistically significant difference was found in general information about the infants between the two groups.(2) The intestinal flora in preterm infants was mainly composed of Lactococcus,Enterococcus and Bacillus for both groups before antibiotic treatment (36.41％,23.40％ and 14.98％).The proportions of Lactococcus and Bacillus were decreased significantly (1.73％ and 1.25％,P＜0.01) with Enterococcus becoming the predomiant genus (16.73％) after antibiotic treatment,while the relative proportions of Staphylococcus,Clostridium and Bifidobacterium were raised.(3) The Shannon index was decreased after antibiotic exposure [(2.34±0.84) vs (1.06±0.96) in ＜3 d group,and (2.64± 1.04) vs (0.35±0.36) in ＞7 d group,both P＜0.05],and the other three Alpha diversity indexes,including observed species,Chaol and PD whole tree indexes,were also decreased within each group (all P＜0.05).(4) Bacterial assemblages showed high beta diversity in both groups before the usage of antibiotics,but antibiotic therapy reduced the diversity.(5) Anoism analysis showed significant differences in the composition of intestinal flora within each group before and after antibiotic exposure (R=0.555and 0.733,both P=0.001),but no difference was found between the two groups after antibiotic exposure (R=0.060,P=0.138).Conclusions Antibiotic exposure,even short-term (＜3 d) administration,may significantly change the distribution of intestinal microbiota in preterm infants.Prolonged usage of antibiotics could have detrimental influence on intestinal flora.Therefore,for preterm babies,prescription of antibiotics should be cautious,even short-term empirical usage.