Preterm birth complications represent a leading cause of mortality in children under five years old. To mitigate these complications, strategies such as the use of donor human milk, fortification with human milk fortifiers (protein supplements), and probiotic administration are employed. This thesis aimed to thoroughly understand the microbiota composition of preterm human milk and the influence of various factors on specific probiotics' properties.

The study began with analyzing human milk microbiota from healthy Argentinian women across different gestational ages, focusing on changes in microbiota from birth to term equivalent age. It was found that term milk exhibited greater microbial diversity than preterm milk, and by term equivalent age, the microbiota composition of human milk closely resembled that of term milk, suggesting breastfeeding's role in promoting microbial maturity. The research further assessed the stability of probiotics, specifically Lacticaseibacillus rhamnosus GG and Bifidobacterium animalis ssp. lactis Bb12, during cold storage of human milk. Results indicated that these probiotics maintained their viability for up to 72 hours. Additionally, this thesis examined the effect of liquid or powdered human milk fortifiers on the adhesion properties of these probiotics in donor human milk, finding that fortification did not adversely affect probiotic adhesion. The final study of the thesis explored the role of polyamines, in modulating the adhesion of probiotics and pathogens with intestinal mucus in infants. The presence of the polyamine spermidine significantly enhanced the adhesion of Bb12 in infant mucus under six months, whereas the polyamine spermine reduced the adhesion of Cronobacter sakazakii.

In conclusion, the thesis showed that the microbiota profiles of human milk significantly vary with gestational age. A key finding is that the microbiota composition of preterm human milk undergoes a transformation, eventually resembling that of term milk as the infant reaches term equivalent age. It was also found that probiotic adhesion properties are unaffected by cold storage or the fortification of donor human milk. However, the influence of polyamines on bacterial adhesion to mucus varies based on the bacterial strain and the mucus donor's age employed. These insights should be considered for developing optimized gut colonization strategies for infants, particularly for those born preterm.