Exploring Molecular Interactions between Human Milk Hormone Insulin and Bifidobacteria

Abstract

Multiple millennia of human evolution have shaped the chemical composition of breast milk toward an optimal human body fluid for nutrition and protection and for shaping the early gut microbiota of newborns. This biological fluid is composed of water, lipids, simple and complex carbohydrates, proteins, immunoglobulins, and hormones. Potential interactions between hormones present in mother's milk and the microbial community of the newborn are a very fascinating yet unexplored topic. In this context, insulin, in addition to being one of the most prevalent hormones in breast milk, is also involved in a metabolic disease that affects many pregnant women, i.e., gestational diabetes mellitus (GDM). Analysis of 3,620 publicly available metagenomic data sets revealed that the bifidobacterial community varies in relation to the different concentrations of this hormone in breast milk of healthy and diabetic mothers. Starting from this assumption, in this study, we explored possible molecular interactions between this hormone and bifidobacterial strains that represent bifidobacterial species commonly occurring in the infant gut using 'omics' approaches. Our findings revealed that insulin modulates the bifidobacterial community by apparently improving the persistence of the Bifidobacterium bifidum taxon in the infant gut environment compared to other typical infant-associated bifidobacterial species. IMPORTANCE Breast milk is a key factor in modulating the infant's intestinal microbiota composition. Even though the interaction between human milk sugars and bifidobacteria has been extensively studied, there are other bioactive compounds in human milk that may influence the gut microbiota, such as hormones. In this article, the molecular interaction of the human milk hormone insulin and the bifidobacterial communities colonizing the human gut in the early stages of life has been explored. This molecular cross talk was assessed using an in vitro gut microbiota model and then analyzed by various omics approaches, allowing the identification of genes associated with bacterial cell adaptation/colonization in the human intestine. Our findings provide insights into the manner by which assembly of the early gut microbiota may be regulated by host factors such as hormones carried by human milk.

Keywords: Bifidobacterium; gestational diabetes mellitus; insulin; microbiota; milk hormones.

FIG 1

Bifidobacterial community of infant fecal samples from healthy and gestational diabetes mellitus (GDM) mothers. (a) Flow diagram showing the salient details regarding sample selection and analysis. (b) Average relative abundance of different bifidobacterial species found in infant fecal samples of healthy and GDM mothers. The right column displays the Student’s t test P value. Asterisks indicate statistically significant P values.

FIG 2

Transcriptional modulation of bifidobacterial strains when exposed to insulin. (a) Number of statistically significant upregulated genes of B. bifidum PRL2010, B. breve 31L, B. longum subsp. longum 1886B, and B. longum susp. infantis 1888B in contact with insulin. (b, c) Transcriptional modulation of genes of B. bifidum PRL2010 and B. bifidum 156B, expressed as the average of normalized count reads obtained from each independent biological triplicate, involved in the synthesis of teichoic acids. Each bar plot shows the average normalized count reads obtained. (d to f) Transcriptional modulation of upregulated orthologous genes of B. bifidum PRL2010, B. bifidum 156B and B. bifidum LMG 11582B, expressed as the average of normalized count reads obtained from each independent biological triplicate, involved in amino acid and carbohydrate metabolism. Each bar plot shows the average of the normalized count reads obtained. (g) The total number of statistically significant upregulated genes of B. bifidum PRL2010, B. bifidum 156B, and B. bifidum LMG 11582B involved in amino acid and carbohydrate metabolism.

FIG 3

Adhesion of B. bifidum PRL2010 cells to Caco-2 cells monolayers. (a, b) Light microscopic images of Caco-2 monolayer cells as observed with Giemsa staining of B. bifidum PRL2010 cells grown under standard conditions (a) and in the presence of insulin (b). (c) Quantification of adhesion ability of B. bifidum PRL2010 cells grown in the absence and presence of insulin, respectively. The vertical bars indicate standard deviations, and the three asterisks indicate t test P values < 0.001.