Impact of early life antibiotic and probiotic treatment on gut microbiome and resistome of very-low-birth-weight preterm infants.

Preterm infants (<37 weeks' gestation) are commonly given broad-spectrum antibiotics due to their risk of severe conditions like necrotising enterocolitis and sepsis. However, antibiotics can disrupt early-life gut microbiota development, potentially impairing gut immunity and colonisation resistance. Probiotics (e.g., certain Bifidobacterium strains) may help restore a healthy gut microbiota. In this study, we investigated the effects of probiotics and antibiotics on the gut microbiome and resistome in two unique cohorts of 34 very-low-birth-weight, human-milk-fed preterm infants - one of which received probiotics. Within each group, some infants received antibiotics (benzylpenicillin and/or gentamicin), while others did not. Using shotgun metagenomic sequencing on 92 longitudinal faecal samples, we reconstructed >300 metagenome-assembled genomes and obtained ~90 isolate genomes via targeted culturomics, allowing strain-level analysis. We also assessed ex vivo horizontal gene transfer (HGT) capacity of multidrug-resistant (MDR) Enterococcus using neonatal gut models. Here we show that probiotic supplementation significantly reduced antibiotic resistance gene prevalence, MDR pathogen load, and restored typical early-life microbiota profile. However, persistent MDR pathogens like Enterococcus, with high HGT potential, underscore the need for continued surveillance. Our findings underscore the complex interplay between antibiotics, probiotics, and HGT in shaping the neonatal microbiome and support further research into probiotics for antimicrobial stewardship in preterm populations.