Preterm infants display a disrupted and potentially pathogenic gut microbiome compared to infants born at full term. They are also more likely to be born by caesarean section, receive antibiotics and remain in hospital for an extended period, all which can contribute to gut microbiota disruptions. Probiotics are increasingly being given to preterm infants to counteract the disruptions; however, their effects are under researched in this cohort of individuals. This randomised control trial of 57 extremely premature infants aimed to determine the effects of a probiotic supplement on gut microbiota composition and their effects on gut immunity. The results showed that Bifidobacterium strains could colonise the premature infant gut but not Lactabacillus rhamnosus. Probiotics also accelerated the maturation of the gut microbiome in premature infants and Bifidobacterium were responsible for this resulting in an anti-inflammatory effect in the gut. It was concluded that probiotic supplementation with the right microbes can act to mature the gut microbiome of preterm infants resulting in its restoration and associated health benefits. This study could be used by healthcare professionals to understand that preterm infants may have a disordered gut microbiota, but a healthy community can be restored through using a probiotic containing Bifidobacterium.