Background: Lactotypes are distinct patterns of microbial composition in human milk that may influence growth and brain development during infancy, in part, through colonization of the gut microbiome. It is unknown whether specific lactotypes are associated with these outcomes in very preterm infants.

Objective: To determine associations of human milk microbiome-derived lactotypes with growth and brain development in very preterm infants at term-corrected age.

Methods: In 134 infants <32 weeks’ gestation and their mothers, human milk samples collected at 2 weeks’ chronological age and 36 weeks’ postmenstrual age (PMA) were analyzed for microbial composition using 16S rRNA sequencing. Lactotypes were defined by grouping samples with similar microbial composition using K-means clustering and were labeled according to the two most abundant taxa within each cluster. Anthropometry, body composition using air displacement plethysmography, and total and regional brain volumes using magnetic resonance imaging were assessed at term-corrected age. Mixed linear regression was used to estimate mean differences in outcomes among lactotypes, adjusting for sex, gestational age at birth, birthweight z-score, and PMA at assessment.
Results: Three distinct lactotypes were identified at each timepoint. In adjusted models, there were no significant associations of lactotypes at 2 weeks’ chronological age with outcomes at term-corrected age. Compared with the Pseudomonas-Staphylococcus lactotype (reference group) at 36 weeks’ PMA, the Staphylococcus-Streptococcus lactotype was associated with greater fat mass (β=0.71, 95% CI: 0.14, 1.29) and body fat percent z-scores (β=0.75, 95% CI: 0.14, 1.35), whereas the Staphylococcus-mixed lactotype was associated with greater head circumference z-scores (β=0.40, 95% CI: 0.01, 0.80). The Staphylococcus-mixed lactotype was also associated with larger total brain (β=15.3, 95% CI: -1.13, 31.7) and cortical gray matter volumes (β=8.71, 95% CI: -1.48, 18.9) although these did not reach statistical significance (P≤0.09).

Conclusion: In very preterm infants, exposure to distinct human milk microbiome-derived lactotypes was linked to markers of adiposity and brain size at term-corrected age. These findings suggest that the human milk microbiome may contribute to shaping growth and neurodevelopmental trajectories in a vulnerable population of infants.