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Issue Date: June 2018 | PDF for this issue.

Milk Protein Decoys Fool Bacteria

  • Disease-causing bacteria often bind to complex sugar structures on the surface of mammalian gut cells and then disrupt gut cell functions.
  • Milk fat globule membrane proteins are heavily decorated by a variety of sugar structures like those present on the surface of gut cells.
  • Ingested milk fat globule membrane proteins act as a decoy in the gut by binding to some bacterial species and stopping them from attaching to gut cells, thereby preventing disease.
  • Food matrices made from dairy products are often used to extend the usefulness of bacteria present in health-promoting probiotics, but these dairy products may also alter the beneficial effects of the probiotic bacteria.

Decoys are commonly used in hunting, politics and warfare. Their attributes are deception and diversion.

The finer skills of a decoy are best exemplified in biology where millions of years of evolution have honed the occupation into a highly efficient art form. One example is a molecular decoy in milk that expertly plies its trade of deception and diversion to protect individuals from disease-causing bacteria.

A recent publication by Claire Gaiani and 11 colleagues highlights the important role of milk fat globule membrane proteins in preventing attachment of bacteria to gut cells [1]. The investigation, published in Colloids and Surfaces B: Biointerfaces, raises new questions about probiotics containing bacteria stabilized by dairy-based food matrices [2]. Probiotics contain bacteria that promote gut health. The investigators are located at the University of Lorraine (France), University of Antwerp (Belgium), and the Catholic University of Leuven (Belgium).

Gut Mucus Snares Bacteria

Many scientists report that large populations of different bacterial species are present in the human gut where they aid digestion of food [3, 4]. Preventing bacteria, especially disease-causing (pathogenic) bacteria, from crossing the mucosal cellular barrier of the gut, and similar barriers elsewhere in the body, is essential for survival. Once bacteria breach the mucosal cellular parapets, havoc ensues. The investigations of McGuckin and colleagues revealed a concentration of bacteria entrapped in the sticky mucus layer lining different regions of the gut [5-8]. The entrapment prevents the bacteria from attaching directly to the underlying cells and perturbing normal cell functions.

Mucins are characterized by a core protein heavily decorated by many sugar structures [5, 9, 10]. The molecular structure of mucin is like a bottle brush flower with its stem representing the mucin protein and the red flowers representing the dense array of sugars attached to the protein (see illustration). McGuckin and colleagues concluded that the sugar structures on gut mucins often resemble the sugar structures present on the surface of gut cells [5]. The latter structures are used by bacteria to bind to cells, which often then leads to cellular injury. Investigations led by Sando, Gaiani, and their colleagues demonstrate that milk also contains a mucin, MUC-1, that takes the fine art of deception and diversion to another level [1, 9, 10].

Milk Mucin Is a Decoy for Bacteria

Milk is exquisitely designed to suit the exacting nutritional requirements of rapidly developing infants, and additionally, it has broad nutritional and health benefits for adults. For the young in particular, milk also contains components designed to prevent bacterial infections [11]. Milk contains fat globules, which are small liquid fat droplets surrounded by a thin membrane containing a number of proteins decorated by sugars [10].

Sando and colleagues demonstrated that a principal milk fat globule membrane protein is the milk mucin named MUC-1, which is densely decorated with a variety of sugars that also resemble the sugar structures on proteins present on the surface of gut cells [10]. These investigators concluded that one of the functions of MUC-1 is to bind pathogenic bacteria, thereby diverting these bacteria from binding to gut cells [9]. This action diminishes the risk of infection of the gut in the suckling young. McGuckin and others revealed that MUC-1 is resistant to normal digestion, especially in infants, and the bacteria bound to MUC-1 are often shed in the feces [5, 9, 10]. Thus, MUC-1 has mastered the art of deception and diversion to protect the health of infants from harmful bacteria. However, what happens to good bacteria, especially bacteria used in probiotics designed to promote human health?

Milk Proteins Prevent Binding of Probiotic Bacteria to Gut Cells

Probiotics contain bacteria normally present in the gut. When ingested these bacteria help to establish a natural bacterial community in the gut that aids digestion and prevents growth of harmful bacteria. Gaiani and colleagues [1] investigated whether a probiotic bacterial species, Lactobacillus rhamnosus GG, binds to gut cells grown in the laboratory. They were also very interested in whether milk fat globule membrane proteins (containing MUC-1) or a purified pig gastric mucin interfered with the binding of bacteria to gut cells [1]. The extensive sugar structures present on the latter mucin are likely similar to those present on MUC-1 in milk and therefore, the pig gastric mucin is a good model for investigating the effects of MUC-1 on the binding of bacteria to gut cells [5, 6]. The laboratory grown gut cells develop cellular structures and functions typical of digestive gut cells in the intestine, hence they are very convenient and useful [12].

Gaiani and colleagues initially used the amazing power of atomic force microscopy (AFM) to directly investigate potential interactions of the bacteria with the milk fat globule membrane proteins and the purified mucin [1]. AFM measures the number, strength, and type of interactions between molecules and particles [13]. The AFM results were unambiguous. Gaiani and colleagues reported that the milk fat globule membrane proteins and purified mucin bound strongly to the probiotic bacteria. They concluded that the nature of the forces measured for the purified mucin indicated multiple interactions with the bacteria occurring through the mucin’s attached sugars. The investigators implied that MUC-1 in the milk fat globule membrane was acting in a similar way. The investigators also used mutants of L. rhamnosus GG to identify the protein on the bacterial surface responsible for binding to the dairy proteins and purified mucin. They discovered a specific protein in the bacterial pili that was crucial for this binding interaction. Pili are long and flexible protein filaments present on the surface of many bacterial species.

Gaiani and colleagues [1] next demonstrated that L. rhamnosus GG bound directly to gut cells grown in the laboratory. They incubated the gut cells with a known number of the bacteria, waited a while for binding to occur, and then counted the bacteria that remained firmly bound to the cells. The investigators additionally showed that the milk fat globule membrane proteins inhibited binding of L. rhamnosus GG bacteria to the gut cells. The L. rhamnosus GG mutant that lost the ability to bind to milk fat globule membrane proteins or bind the purified pig mucin also lost most of its ability to bind to the gut cells. Thus, Gaiani and colleagues [1] concluded that the sugars on the milk fat globule membrane proteins, in particular, the MUC-1 mucin, bound L. rhamnosus GG and prevented it from binding to the gut cells.

Implications

Lactobacillus is a naturally occurring bacterial species in the human gut that aids food digestion. The purpose of its use in oral probiotics is to promote normal gut function after its disruption by antibiotics, chemotherapy or hospitalization. To increase the survival of Lactobacillus as it passes through the acidic environment of the stomach, it is often packaged with a stabilizing food matrix, typically made from milk proteins including milk fat globule membrane proteins [2]. Many of the dairy proteins in this matrix are heavily decorated with sugars and designed by nature to be stable in the stomach so that they can pass into the intestine where some of these proteins help to prevent bacterial and viral infections. Guerin and colleagues speculated that the presence of milk fat globule membrane proteins, and particularly MUC-1, in food matrices, while helping passage of the probiotic bacteria through the stomach, may subsequently prevent binding of the probiotic bacteria to the gut cells. This action could disrupt the health-promoting activities of the probiotic. The investigators propose that additional research is required to assess the impact of food matrices on the claimed health benefits of probiotics.

References

1. Guerin J, Soligot C, Burgain J, Huguet M, Francius G, El-Kirat-Chatel S, et al. Adhesive interactions between milk fat globule membrane and Lactobacillus rhamnosus GG inhibit bacterial attachment to Caco-2 TC7 intestinal cell. Colloids Surf B Biointerfaces. 2018;167:44-53.
2. Guerin J, Burgain J, Gomand F, Scher J, Gaiani C. Milk fat globule membrane glycoproteins: Valuable ingredients for lactic acid bacteria encapsulation? Crit Rev Food Sci Nutr. 2017:1-13.
3. Consortium HMP. Structure, function and diversity of the healthy human microbiome. Nature. 2012;486(7402):207-14.
4. Sender R, Fuchs S, Milo R. Are we really vastly outnumbered? Revisiting the ratio of bacterial to host cells in humans. Cell. 2016;164(3):337-40.
5. Linden SK, Sutton P, Karlsson NG, Korolik V, McGuckin MA. Mucins in the mucosal barrier to infection. Mucosal Immunol. 2008;1(3):183-97.
6. Lindén SK, Sheng YH, Every AL, Miles KM, Skoog EC, Florin TH, et al. MUC1 limits Helicobacter pylori infection both by steric hindrance and by acting as a releasable decoy. PLoS Pathog. 2009;5(10):e1000617.
7. McAuley JL, Linden SK, Png CW, King RM, Pennington HL, Gendler SJ, et al. MUC1 cell surface mucin is a critical element of the mucosal barrier to infection. J Clin Invest. 2007;117(8):2313-24.
8. McGuckin MA, Every AL, Skene CD, Linden SK, Chionh YT, Swierczak A, et al. Muc1 mucin limits both Helicobacter pylori colonization of the murine gastric mucosa and associated gastritis. Gastroenterology. 2007;133(4):1210-8.
9. Parker P, Sando L, Pearson R, Kongsuwan K, Tellam RL, Smith S. Bovine Muc1 inhibits binding of enteric bacteria to Caco-2 cells. Glycoconj J. 2010;27(1):89-97.
10. Sando L, Pearson R, Gray C, Parker P, Hawken R, Thomson PC, et al. Bovine Muc1 is a highly polymorphic gene encoding an extensively glycosylated mucin that binds bacteria. J Dairy Sci. 2009;92(10):5276-91.
11. Ballard O, Morrow AL. Human milk composition: nutrients and bioactive factors. Pediatr Clin North Am. 2013;60(1):49-74.
12. Hidalgo IJ, Raub TJ, Borchardt RT. Characterization of the human colon carcinoma cell line (Caco-2) as a model system for intestinal epithelial permeability. Gastroenterology. 1989;96(3):736-49.
13. Patel AN, Kranz C. (Multi)functional Atomic Force Microscopy Imaging. Annu Rev Anal Chem (Palo Alto Calif). 2018.

Ice Age Milk: Did Low Ultra-Violet Light Influence Milk Vitamin D Content?

  • Human milk is low in vitamin D and infants must instead rely on ultraviolet light to help synthesize this important hormone.
  • A new evolutionary hypothesis argues that for human infants living at extremely high latitudes, natural selection favored a gene—EDAR V370A—that may have influenced human milk vitamin D content.
  • The relationship between the hypothesized changes in mammary duct density and milk composition is not supported by our current understanding of mammary gland physiology.
  • It is not clear if a change in mammary ducts would alter milk vitamin D composition, but the hypothesis highlights the need to understand evolutionary adaptations to low UVB light.

If human milk had a nutrition label, the concentration of one vitamin would really stand out. Human milk is quite low in vitamin D—lower, in fact, than the amount an infant actually needs for optimal growth and development [1, 2]. The discrepancy between the needs of the infant for this vitamin and its low content in milk can be reconciled, however, using an evolutionary perspective; human infants relied on ultraviolet B (UVB) radiation rather than diet to meet their vitamin D requirements. This explanation applies to populations that lived near the equator, who had ample UV radiation access throughout the year, as well as those that lived at higher latitudes with reduced UVB access. Indeed, the reliance on UVB for vitamin D synthesis was so vital that humans living at high latitudes evolved lighter skin pigmentation to increase their body’s ability to absorb UVB light [3].

But this evolutionary explanation comes up short when considering populations that live so far north (> 48°) little UVB light reaches the earth’s surface. At extremely high latitudes, even the lightest skin cannot absorb enough sunlight to make sufficient vitamin D. With UVB no longer an option, these populations must have had a dietary reliance on this essential nutrient to survive and successfully reproduce in these high latitude environments. Adults and children are able to eat vitamin D-rich fatty fish and marine mammals. But what’s an infant to do? In a new paper [4], UC Berkeley evolutionary biologist Leslea Hlusko and colleagues present a novel evolutionary hypothesis: at extremely high latitudes, natural selection favored increased maternal transfer of vitamin D in breast milk.

It All Started with Teeth

Milk does not survive in the archaeological record. But teeth are densely packed with minerals and are some of the most common elements archaeologists find from past populations. And luckily, some teeth may be able to tell us about milk production.

A variant of the ectodysplasin A receptor gene (EDAR V370A) is associated with shoveling of the incisors. (Shoveled incisors have a scooped out appearance on the inner surface). Among modern humans, shoveled incisors are exclusive to Native American and East Asian populations. Genetic data suggest that EDAR V370A underwent strong selection around 20,000 years ago during the last glacial maximum (LGM), when the ancestors of these modern populations were living in Northern Asia, including Beringia [4].

Why would natural selection favor shoveled incisors in cold, high latitude populations? One of the most common explanations was that because of their greater structural strength, shoveled incisors were used as tools [5]. But Hlusko did not find this explanation satisfactory. “The same gene that results in shoveled incisors also affects the development of other tissues in the body,” explains Hlusko. “It influences the embryonic development of teeth, hair, sweat glands, and breasts.”

Shoveled teeth are the only phenotype (or observed outcome) of EDAR V370A that is preserved in the archaeological record, but they may not be the phenotype targeted by natural selection. “We know natural selection is strongest on traits that influence fertility,” says Hlusko. “So we needed to shift our focus to look at the mammary tissue.”

Branching Out

In mice, EDAR V370A is associated with an increase in the branching density of mammary gland ducts during embryonic development [3]. Ducts connect the mammary lobes, where milk is synthesized, with tiny pores in the nipple. Hlusko and colleagues hypothesize that if EDAR V370A has the same phenotype in humans, more ducts would increase the maternal transfer of nutrients during lactation [4].

But EDAR may not be the only factor influencing mammary anatomy; vitamin D itself likely influences the way that the mammary gland develops during the postnatal period [5, 6]. Vitamin D is a hormone, which means it acts as a signal telling cells what to do (e.g., make more of a certain protein, stop making a protein). Cells in the mammary gland and mammary adipose tissue have vitamin D receptors (VDR) and are able to receive vitamin D’s message. Researchers used an ingenious way to discern what that message might be—they removed, or knocked out, the VDR on particular types of cells and observed what happened when that message was not received [6, 7].

Female VDR knock-out mice had normal development of their mammary fat pad, but they had an increase in the density of mammary ducts within that fat pad [7]. However, this was only true if these mice were consuming a high-fat diet. These findings suggest that in female mice, vitamin D may act to put the brakes on mammary duct development during the postnatal period [6]. However, in a state of vitamin D deficiency (and with a high-fat diet), the gas pedal stays down and the duct density increases.

Could this also be true in humans? Hlusko and colleagues are intrigued by this possibility. “The populations that lived in Beringia during the LGM would have consumed very high-fat diets from marine mammals and fish,” says Hlusko. They propose that there could be other genetic mechanisms that act similarly to those observed in mice to increase ductal branching in a diet- and environmental-specific manner (i.e., high-fat, low UVB light) [4]. “The embryonic changes [from EDAR V370A] mimic those that happen during puberty and pregnancy in the knock-out mouse model,” says Hlusko. “We think the embryonic changes may amplify those other mammary developmental changes that happen later in life.” Together, they argue, these changes may increase the transfer of vitamin D in milk [4].

Putting It to the Test

Rodents provide a very interesting animal model for understanding mammary duct development and are commonly used to understand the etiology of breast cancer. However, it is possible that mammary ducts in human females may not respond in a similar manner to the actions of EDAR V370A during fetal development. Moreover, the observed changes from VDR knock-out mice during puberty and pregnancy occurred in a state of vitamin D deficiency, but human mothers in the proposed evolutionary scenario were likely not vitamin D deficient. Additional studies are clearly needed to determine if there is a threshold level of vitamin D that elicits the observed response in increased branching of the mammary ducts.

Testing the hypothesis in humans presents another challenge. The changes in duct structure are genetic but also are diet- and environment-specific. Modern humans that carry the EDAR V370A genetic variant may not provide the best models for hypothesis testing because they do not specifically mimic the adaptive circumstances experienced during the LGM.

But perhaps the most important aspect of the hypothesis is the proposed connection between mammary duct density and milk composition. Do more branches actually result in increased nutrient transfer?

Peggy Neville, Professor Emerita in the Department of Physiology and Biophysics at the University of Denver, is an expert on mammary gland biology and milk synthesis and is not convinced that more highly-branched or extended ductal structure leads to more active mammary tissue. “Certainly there must be an adaptation that allows infants to get more vitamin D in low light environments like Beringia,” says Neville. “What would be needed to get more vitamin D to the infant would be an increase in the ability of the alveolar cells to transfer more vitamin D to the milk rather than more mammary tissue. While additional ducts could lead, in theory, to an increased concentration of alveoli, it is not at all clear that increased alveolar number is related to ductal growth.”

The genetic data point to a strong selective event on EDAR V370A in populations that occupied Beringia 20,000 years ago. But whether or not that selection was related to changes in mammary tissue is still far from resolved. Moreover, the relationship between the proposed changes in mammary duct density and milk composition is not supported by our current understanding of mammary gland physiology.

It is important to note that adipose tissue in the mammary gland is not the only tissue in the body with vitamin D receptors. “There are over 220 genes that are responsive to the actions of vitamin D,” explains Hlusko. “Adipose tissue needs vitamin D to function properly, and vitamin D is intertwined with immune function as well.” The focus tends to go straight to the skeleton when talking about sufficient vitamin D. The skeleton is most certainly important, but vitamin D deficiency is likely to impact multiple physiological functions that are critical for optimal growth and development. Although it is not yet clear how infants living in extremely high latitude environments obtained sufficient vitamin D, there would certainly have been strong selection for a mechanism (or mechanisms) to increase their access to this essential nutrient. Were moms better at storing vitamin D to transfer to offspring during pregnancy and lactation? Were infants better at converting vitamin D into its active form? It will take an evolutionary perspective on mothers and infants and more work across disciplines to find out.

References

1. Gartner, L.M. and Greer, F.R. 2003. Prevention of rickets and vitamin D deficiency: new guidelines for vitamin D intake. Pediatrics, 111(4): 908-910.
2. Institute of Medicine (US) Committee on the Evaluation of the Addition of Ingredients New to Infant Formula. 2004. Infant Formula: Evaluating the Safety of New Ingredients. Washington (DC): National Academies Press (US); 3, Comparing Infant Formulas with Human Milk. Available from: https://www.ncbi.nlm.nih.gov/books/NBK215837/
3. Norton, H.L., Kittles, R.A., Parra, E., McKeigue, P., Mao, X., Cheng, K., Canfield, V.A., Bradley, D.G., McEvoy, B. and Shriver, M.D. 2006. Genetic evidence for the convergent evolution of light skin in Europeans and East Asians. Molecular Biology and Evolution, 24(3): 710-722.
4. Hlusko, L.J., Carlson, J.P., Chaplin, G., Elias, S.A., Hoffecker, J.F., Huffman, M., Jablonski, N.G., Monson, T.A., O’Rourke, D.H., Pilloud, M.A. and Scott, G.R. 2018. Environmental selection during the last ice age on the mother-to-infant transmission of vitamin D and fatty acids through breast milk. Proceedings of the National Academy of Sciences, 115(19): E4426-E4432.
5. Kimura, R., Yamaguchi, T., Takeda, M., Kondo, O., Toma, T., Haneji, K., Hanihara, T., Matsukusa, H., Kawamura, S., Maki, K. and Osawa, M. 2009. A common variation in EDAR is a genetic determinant of shovel-shaped incisors. The American Journal of Human Genetics, 85(4): 528-535.
6. Zinser G., Packman L. and Welsh J. 2002. Vitamin D(3) receptor ablation alters mammary gland morphogenesis. Development, 129(13): 3067-3076.
7. Matthews D.G., D’Angelo J., Drelich J. and Welsh J. 2016. Adipose-specific Vdr deletion alters body fat and enhances mammary epithelial density. Journal of Steroid Biochemistry and Molecular Biology, 164: 299-308.

ACE Reasons for Consuming Sheep’s Milk

  • Sheep’s milk is widely used in cheese making, which results in large amounts of the waste product, sheep’s milk whey.
  • Researchers have shown that commercially available bacterial proteases successfully release bioactive peptides from sheep’s milk whey, and that these peptides control blood pressure.
  • These bioactive peptides may lower blood pressure by inhibiting ACE (angiotensin-converting enzyme).
  • Human digestion probably releases more of these blood pressure-controlling peptides from sheep’s milk than from cow’s, goat’s, or camel’s milks.

Sheep milk is not a regular feature on supermarket shelves, except in the form of cheese. In fact, many well-known cheeses—Feta, Manchego, and Roquefort among them—are made of sheep’s milk, often unbeknownst to consumers. It is the particular composition of sheep’s milk that makes it so good for cheese making. In short, sheep’s milk is very high in solids, containing quite a bit of fat and almost double the protein content of goat’s milk and cow’s milk [1]. But the process of making cheese leaves a lot of waste. And, according to recent studies, this leftover liquid (or whey) could find a use in the creation of novel products containing bioactive peptides. The bioactive peptides from sheep’s milk whey are of interest because they are unusually good at lowering blood pressure.

One group of researchers who have investigated the possibility of re-purposing sheep’s milk whey is based at the University of Otago in Dunedin, New Zealand. They wanted to know which of two kinds of commercially available, food-grade protease preparations (combinations of protein-digesting enzymes) would do a better job at breaking down the proteins in sheep’s milk whey and releasing bioactive peptides [2]. Their preparations were composed of either bacterial or fungal enzymes.

What it meant to “do a better job” in their 2017 study had several considerations. It meant to generate a soup of broken-down proteins with high antioxidant activity, strong ability to inhibit ACE (angiotensin-converting enzyme), to not show any signs of toxicity towards mammalian cells, and to remain stable through various chemical and physical treatments intended to simulate human digestion. The experiments showed that the bacterial protease preparation is the way to go; it was preferable to the fungal preparation by all four measures.

But what is ACE, and why would peptides able to inhibit it be of value? Among other roles in the body, this enzyme is involved in the regulation of blood pressure. It does this by converting a hormone called angiotensin I into the blood vessel-constricting angiotensin II [3]. Of course, when blood vessels constrict, the same amount of blood finds itself in a smaller overall space, so blood pressure rises. Because ACE inhibitors reduce the conversion of angiotensin I to angiotensin II, they keep blood pressure down. Unsurprisingly, for several decades now, there has been a huge market for medicines that have this effect.

Various studies have shown that sheep’s milk is a rich source of ACE-inhibitors [1]. But, as was well known to the researchers in Dunedin, the proteins in milk need to be broken down for the activity to be fully apparent. This makes the ACE-inhibitor activity of the milks of different mammals difficult to compare. It’s neither easy nor comfortable to extract partially digested material from a living human, and so the activity of the peptides that result from the real-life digestion of different mammal’s milks is not known.

In January 2018, however, a team led by Davide Tagliazucchi of the University of Modena and Reggio Emilia, in Modena, Italy, did the next best thing [4]. The researchers used a laboratory protocol to imitate the salivary, gastric, and intestinal stages of digestion, on one type of milk at a time. This way they could compare the peptides released from cow’s, camel’s, goat’s and sheep’s milks. After each stage of digestion, samples were taken. The team recorded how well the partially digested milky mixtures were able to quench free radicals, and for the peptide fractions of the post-pancreatic-digestion samples, they measured the ability to inhibit ACE.

Using mass spectrometry, this team showed that goat’s milk and sheep’s milk had the greatest diversity of peptides. The free-radical scavenging test found that undigested sheep’s milk performed better than the other milks, and that after digestion, it was equally as effective as goat’s milk. (Both of these were better free-radical scavengers than digested cow’s milk and digested camel’s milk.) But it was the ACE-inhibition test where sheep’s milk really stood out. For this test, activity was measured in units called “IC50,” indicating the concentration of peptide that was required to cut ACE activity by one half. Sheep’s milk was by far the most potent ACE inhibitor—about 626 micrograms of peptides per milliliter did the job of reducing ACE activity 50%; cow’s milk, meanwhile, required almost 2,400 micrograms of peptide per milliliter.

So, if digested sheep’s milk and digested sheep’s milk whey are such a good sources of blood pressure control, what kind of products might its bioactive peptides find their way into? All kinds of health foods are possibilities. In late February 2018, Senaka Ranadheera of the University of Melbourne, and his colleagues, proposed making ice cream out of sheep’s milk [5]. Their suggestion is motivated by the fact that probiotics such as Lactobacillus casei are thought to be well protected on their journey from mouth to intestines in ice cream made of sheep’s milk, but there is little reason why bioactive peptides could not be sprinkled in.

Imagine. Ice cream that lowers blood pressure.

References

1. C.F. Balthazar, T.C. Pimentel, L.L. Ferrão, C.N. Almada, A. Santillo, M. Albenzio, N. Mollakhalili, A.M. Mortazavian, J.S. Nascimento, M.C. Silva, M.Q. Freitas, A.S. Sant’Ana, D. Granato & A.G. Cruz. 2017. Sheep milk: Physicochemical characteristics and relevance for functional food development. Comprehensive Reviews in Food Science and Food Safety. 16(2), 247–262.
2. G. Welsh, K.Ryder, J. Brewster, C.Walker, S. Mros, A. E-D. A. Bekhit, M. McConnell & A. Carne. 2017. Comparison of bioactive peptides prepared from sheep cheese whey using a food-grade bacterial and a fungal protease preparation. International Journal of Food Science and Technology. 52, 1252–1259.
3. K. R. Acharya, E. D. Sturrock, J. F. Riordan & M. R. W. Ehlers. 2003. Ace revisited: A new target for structure-based drug design. Nature Reviews Drug Discovery. 2, 891–902.
4. D. Tagliazucchi, S. Martini, S. Shamsia, A. Helal & A. Conte. 2018. Biological activities and peptidomic profile of in vitro-digested cow, camel, goat and sheep milk. International Dairy Journal. doi: 10.1016/j.idairyj.2018.01.014.
5. C.S. Ranadheera, N. Naumovski & S. Ajlouni. 2018. Non-bovine milk products as emerging probiotic carriers: recent developments and innovations. Current Opinions in Food Science. doi:10.1016/j.cofs.2018.02.010

Maternal Bacteria Selectively Seed the Infant Gut

  • Transmission of bacteria from mothers to infants at birth is thought to set up the infant gut microbiome, but tracking the origin and persistence of the early colonizers has been a challenge.
  • A new study finds that selected maternal bacteria, particularly Actinobacteria and Bacteroidia strains that are essential components of the infant microbiome, seed the gut of vaginally-born infants and expand to form a stable community.
  • Selection for Actinobacteria and Bacteroidia likely occurs in the infant gut due to their ability to use human milk as a food source.
  • Infant gut bacteria are intermittently replaced in later childhood by strains from family members, and birth by C-section appears to prevent maternal seeding of the infant gut.

The gut microbes of infants play an important role in the early development of the infant immune system and may have long-term health effects [1]. These microbes are thought to be transmitted from the mother at birth [2-5]. However, studying where exactly the infant gut microbes originate and how long they persist has been a challenge.

In a new study, Professor Peer Bork and his colleagues at the European Molecular Biology Laboratory tracked the presence of maternal bacteria in the infant gut [6]. They found that selected maternal bacteria, particularly Actinobacteria and Bacteroidia strains that are essential components of the infant microbiome, seed the gut of vaginally-born infants and expand to form a stable community. However, the researchers did not find maternal transmission at birth in infants born via C-section. In addition, the study revealed that strains from the environment, including from family members, occasionally replaced existing strains in later childhood.

Previous studies suggest that microbes from the vagina or from breast milk are unlikely to account for the majority of infant gut species [7,8]. Infant gut microbes are most likely to originate in the mother’s gut, which contains most of the same species, although they are present at different relative abundances [5,8]. Some studies have shown that microbes present in the environment can colonize the infant gut after birth [9]. But it’s still unclear how long maternal microbes persist in the infant gut, and how the infant gut microbiome is affected by microbes from other family members and the environment.

In the new study, the researchers analyzed the genomes of gut bacteria from a cohort of family members in order to track strain transmission. They looked for rare single nucleotide variations (SNVs) shared between the gut bacterial strains of family members. SNVs have been shown to be able to track maternal transmission [8-10]. The researchers looked for rare SNVs shared exclusively between the gut bacteria of mothers and their babies, as this represented evidence of maternal transmission.

The researchers found that maternal strains were selectively transmitted to vaginally born infants and persisted in the infant gut. Strains of Actinobacteria and Bacteroidia were transmitted from the mother and persisted for at least one year. However, maternal strains of Clostridia, which are abundant in the mother’s gut microbiome, were not seen in the infant.

The researchers suggest that selection for Actinobacteria and Bacteroidia likely occurs in the infant gut and may arise due to the infants’ human milk-based diet. Actinobacteria and Bacteroidia species of bacteria are able to use human milk as a food source, and may thus gain a selective advantage over other bacteria in the gut of human milk-fed infants [11,12].

The study also showed that maternally transmitted strains were very stable during the first year of life, but later in childhood these strains were occasionally replaced by strains from the environment and from family members. Fathers appeared to account for most of the novel strains in the family environment, as they were more frequent donors of novel strains to other family members than recipients.

The researchers looked at maternal transmission in infants born via C-section and found that these infants did not show maternal transmission at birth. However, they gradually acquired maternal strains from the environment after birth. This suggests that vaginal birth may be the main transmission route for maternal gut microbes.

The researchers conclude that the transfer of gut bacteria from mothers to infants is a selective process and the maternal bacteria that colonize the infant gut persist and expand to form a stable microbiome. They suggest that the stability of maternal strains in the infant gut shows their importance relative to non-maternal strains, and hypothesize that maternal strains may have a protective effect by preventing the influx and growth of other potentially harmful strains in the infant gut. Future studies will need to further elucidate the benefits of maternal transmission and the consequences of transmission from the environment and family members.

References

1. Houghteling P.D., Walker W.A. Why is initial bacterial colonization of the intestine important to infants’ and children’s health? J Pediatr Gastroenterol Nutr. 2015 Mar;60(3):294-307.
2. Cho I., Blaser M.J. The human microbiome: at the interface of health and disease. Nat Rev Genet. 2012 Mar 13;13(4):260-70.
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