Influence of microbiota on the development of the immune system and susceptibility to septic diseases in premature infants

Early-life host-microbe interactions are critical determinants of immune development, lifelong homeostasis, and disease susceptibility. The PRIMAL trial (Priming Immunity at the Beginning of Life) demonstrated that multistrain probiotics shifted the gut microbiota of very preterm infants toward eubiosis without affecting sepsis incidence; however, the immunological consequences of these microbiotal changes remained unresolved. To investigate the functional impact of probiotic-modified microbiota, we established a humanized mouse model using fecal microbiota transplantation from PRIMAL trial participants. Germ-free female mice (6-8 weeks old) were colonized via oral gavage with preterm infant fecal samples (1 × 107 CFU in sterile PBS) from either probiotic-treated or placebo-treated donors. Following a two-week engraftment period, colonized females were mated with germ-free males to assess transgenerational microbiota transmission. Microbiota composition and diversity were analyzed in both parental and F1 generations using 16S rRNA sequencing at three weeks post-birth. We achieved successful engraftment with approximately 64% of operational taxonomic units transferring from preterm donors to murine recipients at the genus level, though with notable variations in bacterial genus expansion within the murine gut environment. Importantly, we confirmed robust vertical transmission of gut microbiota from mothers to offspring, preserving both compositional structure and diversity patterns. This humanized model enables investigation for how probiotic-conditioned microbiota influences immune system development and infection susceptibility, offering insights into the mechanistic basis of early-life microbial programming.