A microbiota profiling of breast milk in relation to gestational diabetes mellitus status: a Greek pilot study

Gestational diabetes mellitus (GDM) is associated with metabolic alterations that may influence maternal and neonatal microbiota. While the impact of GDM on the infant gut microbiome has been increasingly studied, its effect on the breast milk microbiota remains poorly understood. In this pilot study, we compared the breast milk microbiota received from 25 women with GDM compared to that of 25 non-diabetic breastfeeding mothers to serve as controls using full-length 16 S rRNA gene sequencing on the Oxford Nanopore platform. While alpha-diversity showed no statistically significant differences between the two groups, beta-diversity analysis indicated a clear difference in overall breast milk microbiota composition depending on gestational diabetes status. These findings support a compositional shift rather than a broad change in diversity. Specifically, significant differences in microbial composition were observed between the two groups. Breast milk from GDM mothers showed increased abundance of Pseudomonadota (formerly Proteobacteria), including species such as Acinetobacter johnsonii and Bradyrhizobium mercantei , while beneficial Bacillota (e.g., Lacticaseibacillus paracasei ) were significantly reduced compared to non-GDM samples. These findings suggest that GDM may alter breast milk microbial composition in ways that could influence neonatal early-life microbial colonization and immune programming. Given the known links between dysbiosis and metabolic and immune-mediated diseases, our results underscore the need for longitudinal, multi-omic studies to elucidate the long-term health implications of GDM-associated shifts in the breast milk microbiome.