Human milk contains nutrients, bioactive compounds and commensal bacteria that may improve maternal and infant health outcomes. Milk’s microbial composition varies greatly between women but the factors that might explain this variation remain unknown.

A new longitudinal cohort investigation, led by Dr. Carolina de Weerth from Radboud University Medical Center in the Netherlands, has found for the first time associations between psychosocial distress and the milk microbiota in healthy breastfeeding women following full-term pregnancies.

Bacterial abundances and diversity in human milk samples from 51 healthy breastfeeding women were quantified through 16S ribosomal ribonucleic acid at weeks 2, 6, and 12 postpartum. Maternal psychological symptoms were also assessed via mood questionnaires at 6 weeks postpartum.

An analysis at the phylum level in all the samples revealed that the most frequent and abundant phyla were Firmicutes followed by Proteobacteria. But while Firmicutes decreased over time, Proteobacteria tended to increase across the study.

Moreover, a slight increase in milk bacterial diversity was observed during the first 3 months of breastfeeding, which overall consisted of an increase in Lactobacillus and a decrease in Staphylococcus.

Then, the authors explored whether maternal postnatal stress, anxiety and depressive symptoms might affect milk microbiota composition.

Changes in time at the phylum level (a fall in individual relative abundance in Firmicutes, and an increase in Proteobacteria and Bacteroidetes) and genera level (a rise in individual relative abundance in Acinetobacter, Flavobacterium, and Lactobacillus and a fall in the main bacterial genera Staphylococcus) were found in milk samples of women with low psychosocial distress. Apart from changes in Staphylococcus, no changes at the phylum and genus level were observed in women showing the highest levels of psychosocial distress at week 12.

As regards milk bacterial diversity, women with low levels of psychosocial distress showed a milk bacterial diversity that increased from week 2 to week 12, whereas diversity remained stable in women with high levels of psychosocial distress.

Altogether, these findings show that psychosocial distress is related to specific changes in the relative abundance of milk bacteria and to lower milk bacterial diversity.

In contrast, when scientists explored anxiety, stress, and depressive symptoms separately, they failed to find correlations with milk bacterial profiles.

The authors hypothesized that psychosocial distress could affect the microbial composition in milk via direct and indirect mechanisms. One way might consist of the altered maternal gastrointestinal microbiota secondary to psychosocial distress being transferred to breast milk via the entero-mammary pathway. A second possibility is that maternal psychosocial distress may affect milk’s nutrient content, which, in turn, may have an impact on milk microbiota.

On the whole, the study suggests that maternal postnatal psychosocial distress, which is a relatively common condition during the postnatal period, is a new factor that adds to the other maternal factors that contribute to explaining breast milk microbiota variation. As breast milk is considered a relevant source of commensal bacteria and other bioactive compounds for newborns, it is worth exploring whether these findings might have an impact on infant health outcomes. 


Browne PD, Aparicio M, Alba C, et al. Human milk microbiome and maternal postnatal psychosocial distress. Front Microbiol. 2019; 10:2333. doi: 10.3389/fmicb.2019.02333.