The microbiome

How does breastfeeding help my baby’s bacteria?

Humans carry trillions of microbes such as bacteria, viruses, and fungi that are grouped in communities called microbiomes. Some of these microbes make us sick, but others are essential for good health. A healthy gut microbiome contains a wide variety of health-promoting microbes and appears to lower the risk of rheumatoid arthritis, colorectal cancer, obesity, asthma, allergic diseases, diabetes, and a range of other ailments. Breast milk contains microbes that help babies develop a healthy gut microbiome and delivers large amounts of human milk oligosaccharides, a special kind of sugar that helps maintain that microbiome.

A) Describing the  microbiome 

A  microbiome is a group of microbes (bacteria, viruses, fungi) that live in a certain environment. The human microbiome consists of an estimated 40 trillion organisms hosting about 3 million different genes (Sender 2016; Ursell 2012). For every 10 human cells in the body, there are roughly 13 microbes (Sender 2016).

People have unique  types and balances of microbes in their microbiomes, just as they have unique fingerprints, and microbiomes can vary in their ability to protect us and shape our health. Microbiomes also vary based on their body location; these include the nose, mouth, lungs, stomach, small intestine, large intestine (colon), urogenital system, skin, breast and breast milk. Only some of the microbes found in breast milk are found in other microbiomes (Beghetti 2019).

The early life microbiome is one of the main contributors to short and long-term infant health (Arrieta 2014).

B) The  microbiome  of breast milk

Microbes that form the breast milk microbiome, predominantly Staphylococcus and Streptococcus species, are thought to have reached the breast by (Beghetti 2019; Latuga 2014; Urbaniak 2012): 

  • Passing from the mother’s skin into the breast.
  • Passing upward into the breast from the baby’s saliva.
  • Travelling in the blood after dental care.
  • Being transferred by the body’s own immune cells from the gut (entero-mammary pathway) or elsewhere.

The microbiome of breast milk has a large variety of different microbes including 400 to 800 species of bacteria, and there is a wide variety of types between mothers (Cabrera-Rubio 2012; Togo 2019; Toscano 2017). These differences can be based on (Boix-Amorós 2019; Moossavi 2018): 

  • How the baby was delivered.
  • The baby’s gender.
  • If the baby was delivered prematurely.
  • If antibiotics were given to the mother after delivery. 
  • Whether the mother is expressing breast milk or breastfeeding.
  • The mother’s weight (body mass index [BMI]).
  • The mother’s age.
  • Where the mother lives.
  • The mother’s diet (Padilha 2019).
  • The mother's use of disinfecting agents (Bever 2018).
  • The mother’s psycho-social distress (Browne 2019)

This maternal  microbiome is delivered to the baby from the first feed. Babies are estimated to take in about 100,000 to ten million bacteria per day through breast milk (Cabrera-Rubio 2012; Heikkilä 2003; Togo 2019).

The areola also has a microbiome. One study (Pannaraj 2017) reported that 30% of babies' gut bacteria  was  acquired from the mother’s breast milk and a further 10% from the areola.

Having a less healthy breast microbiome may increase the risk of the mother developing breast infections, subacute mastitis, and breast cancer (Parida 2020).

C) The baby’s gut microbiome

1) Describing the gut microbiome

The largest human microbiome is found in the gut. The adult gut microbiome contains more than 300 species of microbes and weighs about two kilograms (four pounds) (Rajilic-Stojanovic 2014).

2) How the baby’s gut microbiome protects the baby from disease

Researchers have shown that the gut microbiome plays a huge role in the development of the immune system and in overall health (Dinan 2017). Indeed, the gut  microbiome is like an organ with its own unique function and importance in human health.

The gut microbiome ensures the health of the baby by (Butel  2018; Urbaniak 2012):

  • Protecting the baby from disease-causing microbes by:
    • Competing for nutrients.
    • Producing agents that can harm them.
    • Helping the body recognize and kill them.
    • Supporting healthy development of the immune system. 
    • Keeping the gut healthy.
  • Preventing the baby’s immune system from overreacting (Feehley 2019).
  • Developing the blood vessels in the gut.
  • Making vitamins.
  • Digesting foods.

The gut microbiome does not live in isolation. There is constant communication between the gut, its microbiome, and the brain, using the nervous system, hormones, the immune system, and chemical signals (Lerner 2017). For example, the type of microbiome a baby acquires after birth affects how fat is stored, levels of hormones that control appetite (leptin), and how insulin is managed (Arrieta 2014).

3) The effect of breastfeeding on the baby’s gut microbiome

Breast milk not only gives the baby’s gut beneficial microbes but maintains them by providing the following: 

  • Special sugars (human milk oligosaccharides [HMOs]) (Duranti 2017)
  • Antibodies that coat good  bacteria and support their growth (Meyer 2018)
  • Small pieces of cells (exosomes) that contain nucleic acids and proteins that play a role in communication between human cells and the microbiome (Le Doare  2018)

In addition, the baby’s saliva and breast milk work together to regulate the growth of bacteria in the baby’s mouth and gut (Sweeney 2018).

4) Causes of a baby having a less healthy gut microbiome

The first two to three years of a person’s life are the most important in setting up a healthy gut  microbiome. After that, the gut  microbiome is relatively stable throughout childhood and adulthood. The change from a breastfed baby microbiome to an adult pattern is slowed by continued breastfeeding after six months and receiving breast milk is the most important factor in having a healthy microbiome during this time (Matsuyama 2018; Pannaraj 2017; Stewart 2018). The lack of breastfeeding can cause negative changes in the gut microbiome well into childhood (Cioffe 2020).

The gut  microbiome  tends to do less to support good health if the baby is (Dunn 2017):

  • Born by  Caesarean  section. This effect is mitigated by exclusive breastfeeding.
  • Born premature (Stiemsma   2018).  
  • Born to a mother who received antibiotics during delivery.
  • Infant formula-fed (O’Sullivan 2015).  
  • Hospitalized after delivery.
  • Sick after delivery.
  • Given antibiotic treatment (Penders 2006).  
  • Born in a higher-income country. 
  • Living in a city.

5) Consequences of a baby having a less healthy gut microbiome

Babies with a less healthy gut microbiome are at risk of diarrhea and sudden infant death syndrome (SIDS) (Praveen 2017). Premature babies are at risk of blood infection and a severe bowel disease called necrotizing enterocolitis (Ho 2018; Neu 2017; Stewart 2017).

A less healthy gut microbiome increases the risk of long-term health challenges including (Butel 2018; Ding 2019):

  • Asthma, eczema, and other allergic diseases (Galazzo 2020; Lee-Sarwar 2019; Oddy 2017)  
  • Obesity 
  • Autism (Bezawada 2020; Ho 2020; Mangiola  2016; Mulle 2013) 
  • Mental health issues 
  • Poorer brain development (Stiemsma  2018; Yang 2016)  
  • Crying (Loughman 2020)
  • Inflammatory bowel disease (Shreiner 2015)
  • Diabetes 
  • Rheumatoid arthritis (Horta-Baas 2017)  
  • Cardiovascular disease (blockages in the arteries leading to stroke and heart attack) 
  • High blood pressure
  • Obesity
  • Irritable bowel disease 
  • Parkinson’s disease 
  • Gout
  • Colorectal, prostate, gastric cancer 
  • Depression
  • Shorter lifespan

D) The effect of infant formula on the baby’s microbiomes

1) The gut microbiome

Breastfeeding or infant formula-feeding has a very large impact on the type of bacteria in a baby’s gut (Savage 2018). The gut microbiome of infant formula-fed babies have differences  that persist even after they start eating solid foods and up to six years of age (Ho 2018; Gschwendtner 2019; Panneraj  2017).

Babies fed both breast milk and infant formula (mixed feeding) have a  gut microbiome  profile that is more like that of an exclusively infant formula-fed baby (Bullen 1977; Madan 2016; Obermajer  2017).

The effect on the microbiome of small amounts of infant formula supplementation on newborns who have lost excess amounts of weight and are supplemented only until the mother’s milk comes in is uncertain. One study (Flaherman 2018) showed no effect on the microbiome and a larger one (Forbes 2018) showed subtle changes that could increase the risk of obesity. 

2) The respiratory system microbiome

The  microbiome  of the respiratory system (the nose, breathing tubes, and lungs) is important in the prevention of infection. It too is negatively affected by infant formula-feeding (Bosch 2017). 

E) Describing human milk oligosaccharides

Human milk oligosaccharides (HMOs) are a group of complex sugars present in breast milk that act in many ways to protect and improve the baby’s health. HMOs are different from oligosaccharides present in the milk of other mammals.

The amount of HMOs present in breast milk is greater than the amount of protein and HMOs are the third largest solid component in maternal milk after lactose and fat (Moukarzel 2017). The estimated concentration of HMOs is 5-15 grams per litre (Kunz 2000; Kunz 2017).

Compared with the milk of other mammals, breast milk has the highest concentration of HMOs. They are unique and often more complex than the ones found in other mammals’ milk (Ayechu-Muruzabal 2018; Smilowitz  2014). 

HMOs may support and protect the baby even before birth (Jantscher-Krenn 2018; Wise 2018). HMOs are present in the mother’s blood and urine by the end of the third month of pregnancy (Hallgren 1977). They are transferred from the mother to the baby through the placenta and are present in amniotic fluid (Hirschmugl 2019; Wise 2018).

HMO levels are higher in colostrum and decrease during lactation (Borewicz 2020; Smilowitz 2014). The breast milk of mothers of premature babies has higher levels of HMOs than term milk (Gabrielli 2011). 

There are more than 200 possible types of HMOs in breast milk, and they vary between mothers (Jost 2015). The breast milk of individual mothers has been reported to have as few as 24 types of HMOs and as many as 130 (German 2008). These differences can be based on various maternal factors including (Azad 2018; Bode 2015; Bode 2018): 

  • Genetics and how those genes function 
  • Ethnic group
  • Environment and season
  • Diet
  • Lifestyle, smoking, drug exposure 
  • Place of residence
  • Health and disease 
  • Age
  • Number and gender of children
  • Breastfeeding status (exclusive or not)
  • Amount of time after the baby’s birth

F) How HMOs work 

HMOs resist the acid in a baby’s stomach and are mostly not absorbed by the body (Smilowitz 2014).

1) Supporting the growth of beneficial bacteria

Different bacteria respond to different HMOs (Borewicz 2019). Some beneficial bacteria found in the baby’s gut have developed the ability to digest HMOs and use them for energy (Newburg 2015; Shani 2018). In exchange, these good bacteria keep their hosts healthy. This suggests that humans and good bacteria have a beneficial partnership that has evolved over time.

Cow’s milk, on which most infant formula is based, only contains small amounts of oligosaccharides. In addition, there are fewer types and the molecules are less complex when compared to HMOs.

2) Preventing the growth of harmful bacteria

HMOs prevent the growth of certain disease-causing microbes and keep them from attaching to the baby’s cells by acting as decoys and binding with them (Cacho 2017).

Some bacteria are able to grow better and protect themselves from antibiotics by the creation of a cover (biofilm). HMOs can prevent biofilms from forming.

3) Other effects in the gut

HMOs support normal gut development and maturation (Brandtzaeg 2010). 

HMOs may play a role in decreasing the risk of  necrotizing enterocolitis in premature babies (Bode 2018).  

4) Optimum development of the baby’s immune system

HMOs are able to (Cacho 2017):  

  • Support and stimulate the normal development of the newborn’s immune system (Jeong 2012; Morozov 2018).
  • Prevent abnormal inflammation (Xiao 2019).
  • Help communication between the baby’s cells.

5) Prevent infection and disease in other parts of the body

Roughly 1% of HMOs delivered to the baby’s gut are absorbed and travel through the blood system to target organs (Bode 2015). Some HMOs enter the baby’s  urinary system, where they  prevent kidney and bladder infections.

6) Nutrients

HMOs provide essential nutrients, such as scialic  acid, needed for brain development (Bode 2012; Wang 2013).  

G) Commercial interests

Recently, researchers and commercial interests have attempted to mimic the HMOs and bacteria in breast milk with prebiotics and probiotics. In both cases, commercial products cannot match the unique nature and variety of HMOs and bacteria in breast milk and evidence of benefits are limited, conflicting, or missing.

Pre- and probiotics have been marketed both as stand-alone products and added to infant formula.


Arrieta MC, Stiemsma LT, Amenyogbe N, et al. The intestinal microbiome in early life: health and disease. Front Immunol. 2014 Sep 5;5:427

Ayechu-Muruzabal V, van Stigt AH, Mank M, et al. Diversity of Human Milk Oligosaccharides and Effects on Early Life Immune Development.  Frontiers in Pediatrics. 2018;6:239
Azad MB, Robertson B, Atakora F, et al. Human Milk Oligosaccharide Concentrations Are Associated with Multiple Fixed and Modifiable Maternal Characteristics, Environmental Factors, and Feeding Practices. J Nutr. 2018 Sep 22

Beghetti I, Biagi E, Martini S, et al. Human Milk's Hidden Gift: Implications of the Milk Microbiome for Preterm Infants' Health. Nutrients. 2019 Dec 4;11(12):2944

Bever CS, Rand AA, Nording M, et al. Effects of triclosan in breast milk on the infant fecal microbiome. Chemosphere. 2018 Jul;203:467-473

Bezawada N, Phang TH, Hold GL, et al. Autism Spectrum Disorder and the Gut Microbiota in Children: A Systematic Review. Ann Nutr Metab. 2020;76(1):16-29 

Bode L. Human Milk Oligosaccharides at the Interface of Maternal-Infant Health. Breastfeed Med. 2018 Apr;13(S1):S7-S8 
Bode L. The functional biology of human milk oligosaccharides. Early Hum Dev. 2015 Nov;91(11):619-22
Bode L. Human milk oligosaccharides: every baby needs a sugar mama. Glycobiology 2012 Sep;22(9):1147-62 
Boix-Amorós A, Puente-Sánchez F, du Toit E, et al. Mycobiome Profiles in Breast Milk from Healthy Women Depend on Mode of Delivery, Geographic Location, and Interaction with Bacteria. Appl Environ Microbiol. 2019 Apr 18;85(9)
Borewicz K, Gu F, Saccenti E, et al. Correlating Infant Faecal Microbiota Composition and Human Milk Oligosaccharide Consumption by Microbiota of One-Month Old Breastfed Infants. Mol Nutr Food Res. 2019 Apr 24:e1801214

Borewicz K, Gu F, Saccenti E, et al. The association between breastmilk oligosaccharides and faecal microbiota in healthy breastfed infants at two, six, and twelve weeks of age. Sci Rep. 2020 Mar 6;10(1):4270  
Bosch AATM,  Piters  WAAS, van  Houten  MA, et al. Maturation of the Infant Respiratory Microbiota, Environmental Drivers, and Health Consequences. A Prospective Cohort Study. Am J  Respir  Crit  Care Med. 2017 Dec 15;196(12):1582-1590 
Brandtzaeg  P.  The mucosal immune system and its integration with the mammary glands.  J  Pediatr. 2010 Feb;156(2 Suppl):S8-15 
Browne PD, Aparicio M, Alba C, et al. Human Milk Microbiome and Maternal Postnatal Psychosocial Distress. Front Microbiol. 2019;10:2333
Bullen CL, Tearle PV, Stewart MG. The effect of "humanised" milks and supplemented breast feeding on the faecal flora of infants. J Med  Microbiol. 1977 Nov;10(4):403-13
Butel MJ, Waligora-Dupriet  AJ, Wydau-Dematteis S. The developing gut microbiota and its consequences for health. J Dev  Orig  Health Dis. 2018 Mar 22:1-8 
Cabrera-Rubio R, Collado MC, Laitinen K, et al. The human milk microbiome changes over lactation and is shaped by maternal weight and mode of delivery. Am J Clin Nutr. 2012 Sep;96(3):544-51 
Cacho NT, Lawrence RM. Innate Immunity and Breast Milk.  Frontiers in Immunology.  2017;8:584 

Cioffi CC, Tavalire HF, Neiderhiser JM, et al. History of breastfeeding but not mode of delivery shapes the gut microbiome in childhood. PLoS One. 2020 Jul 2;15(7):e0235223

Dinan TG, Cryan JF. Gut instincts: microbiota as a key regulator of brain development, ageing and neurodegeneration. J Physiol. 2017 Jan 15;595(2):489-503 
Ding R, Goh W, Wu R et al. Revisit gut microbiota and its impact on human health and disease. J Food and Drug Analysis 2018;12.012
Dunn AB, Jordan S, Baker BJ, et al. The Maternal Infant Microbiome: Considerations for Labor and Birth. MCN Am J Matern Child Nurs. 2017;42(6):318–325
Duranti S, Lugli GA, Mancabelli L, et al. Maternal inheritance of bifidobacterial communities and bifidophages in infants through vertical transmission.  Microbiome. 2017 Jun 26;5(1):66 
Feehley T, Plunkett CH, Bao R, et al. Healthy infants harbor intestinal bacteria that protect against food allergy. Nat Med. 2019 Mar;25(3):448-453
Flaherman VJ, Narayan NR, Hartigan-O'Connor D, et al. The Effect of Early Limited Formula on Breastfeeding, Readmission, and Intestinal Microbiota: A Randomized Clinical Trial. J  Pediatr. 2018 Mar 13. pii: S0022-3476(17)31770-5 
Forbes JD, Azad MB, Vehling L, et al. Association of Exposure to Formula in the Hospital and Subsequent Infant Feeding Practices With Gut Microbiota and Risk of Overweight in the First Year of Life. JAMA Pediatr. 2018 Jun 4:e181161
Gabrielli O, Zampini L, Galeazzi T, et al. Preterm milk oligosaccharides during the first month of lactation. Pediatrics. 2011 Dec;128(6):e1520-31

Galazzo G, van Best N, Bervoets L, et al. Development of the Microbiota and Associations With Birth Mode, Diet, and Atopic Disorders in a Longitudinal Analysis of Stool Samples, Collected From Infancy Through Early Childhood. Gastroenterology. 2020 May;158(6):1584-1596

German JB, Freeman SL, Lebrilla CB, et al. Human milk oligosaccharides: evolution, structures and bioselectivity as substrates for intestinal bacteria. Nestle Nutr Workshop Ser Pediatr Program. 2008;62:205-18; discussion 218-22
Gschwendtner S, Kang H, Thiering E, et al. Early life determinants induce sustainable changes in the gut microbiome of six-year-old children. Sci Rep. 2019 Sep 3;9(1):12675
Hallgren P, Lindberg BS, Lundblad A. Quantitation of some urinary oligosaccharides during pregnancy and lactation. J Biol Chem. 1977 Feb 10;252(3):1034-40
Heikkilä MP, Saris PE. Inhibition of Staphylococcus aureus by the commensal bacteria of human milk. J Appl Microbiol. 2003;95(3):471-8
Hirschmugl B, Brandl W, Csapo B, et al. Evidence of Human Milk Oligosaccharides in Cord Blood and Maternal-to-Fetal Transport across the Placenta. Nutrients. 2019;11(11):E2640. Published 2019 Nov 4

Ho LKH, Tong VJW, Syn N, et al. Gut microbiota changes in children with autism spectrum disorder: a systematic review. Gut Pathog. 2020 Feb 3;12:6  

Ho NT, Li F, Lee-Sarwar KA, et al. Meta-analysis of effects of exclusive breastfeeding on infant gut microbiota across populations. Nat Commun. 2018 Oct 9;9(1):4169
Horta-Baas G, Romero-Figueroa M del S, Montiel-Jarquín  AJ, et al. Intestinal Dysbiosis and Rheumatoid Arthritis: A Link between Gut Microbiota and the Pathogenesis of Rheumatoid Arthritis.  Journal of Immunology Research 2017; 2017:4835189
Jantscher-Krenn E, Aigner J, Reiter B, et al. Evidence of human milk oligosaccharides in maternal circulation already during pregnancy: a pilot study. Am J Physiol Endocrinol Metab. 2018 Nov 13
Jeong K, Nguyen V,  Kim J. Human milk oligosaccharides: the novel modulator of intestinal microbiota. BMB Rep. 2012 Aug;45(8):433-41 
Jost T, Lacroix C,  Braegger  C, et al. Impact of human milk bacteria and oligosaccharides on neonatal gut microbiota establishment and gut health.  Nutr  Rev. 2015 Jul;73(7):426-37 
Kunz C, Meyer C, Collado MC, et al. Influence of Gestational Age, Secretor, and Lewis Blood Group Status on the Oligosaccharide Content of Human Milk. J Pediatr Gastroenterol Nutr. 2017 May;64(5):789-798
Kunz C, Rudloff S, Baier W, et al. Oligosaccharides in human milk: structural, functional, and metabolic aspects. Annu Rev Nutr. 2000;20:699-722
Latuga MS,  Stuebe A, Seed PC.  A review of the source and function of microbiota in breast milk.  Semin Reprod Med. 2014 Jan;32(1):68-73 
Le Doare K, Holder B, Bassett A, et al. Mother's Milk: A Purposeful Contribution to the Development of the Infant Microbiota and Immunity.  Front. Immunol 2018: 2(28)
Lee-Sarwar KA, Kelly RS, Lasky-Su J, et al. Integrative analysis of the intestinal metabolome of childhood asthma. J Allergy Clin Immunol. 2019 Mar 23
Lerner A,  Neidhöfer  S, Matthias T. The Gut Microbiome Feelings of the Brain: A Perspective for Non-Microbiologists. Microorganisms. 2017 Oct 12;5(4). pii: E66 
Loughman A, Quinn T, Nation ML, et al. Infant microbiota in colic: predictive associations with problem crying and subsequent child behavior [published online ahead of print, 2020 Apr 13]. J Dev Orig Health Dis. 2020;1‐11

Madan JC, Hoen AG, Lundgren SN, et al. Association of  Cesarean  Delivery and Formula Supplementation With the Intestinal  Microbiome  of 6-Week-Old Infant.  JAMA  Pediatr. 2016 Mar;170(3):212-9
Mangiola  F,  Ianiro  G, Franceschi F, et al. Gut microbiota in autism and mood disorders. World Journal of Gastroenterology. 2016;22(1):361-368
Matsuyama M, Gomez-Arango LF, Fukuma NM, et al. Breastfeeding: a key modulator of gut microbiota characteristics in late infancy. J Dev Orig Health Dis. 2018 Nov 19:1-8
Meyer KM, Prince A, Boggan B, et al. Maternal IgA targets distinct communities of bacteria in the breast milk, maternal gut, and infant gut microbiomes. AJOC 2018: 2018(1S); S371-S372 
Moossavi S, Miliku K, Sepehri S, et al. The Prebiotic and Probiotic Properties of Human Milk: Implications for Infant Immune Development and Pediatric Asthma. Front Pediatr. 2018; Jul 24
Morozov V, Hansman G, Hanisch F-G, et al. Human Milk Oligosaccharides as Promising Antivirals. Mol.  Nutr. Food Res 2018;1(16) 
Moukarzel  S, Bode L. Human Milk Oligosaccharides and the Preterm Infant: A Journey in Sickness and in Health.  Clin  Perinatol. 2017 Mar;44(1):193-207 
Mulle JG, Sharp WG, Cubells JF. The gut microbiome: a new frontier in autism research. Curr Psychiatry Rep. 2013 Feb;15(2):337
Neu J, Pammi M. Pathogenesis of NEC: Impact of an altered intestinal microbiome. Semin Perinatol. 2017 Feb;41(1):29-35
Newburg DS, Morelli L. Human milk and infant intestinal mucosal glycans guide succession of the neonatal intestinal microbiota. Pediatr Res. 2015 Jan;77(1-2):115-20 
O’Sullivan A, Farver M, Smilowitz  JT. The Influence of Early Infant-Feeding Practices on the Intestinal Microbiome and Body Composition in Infants. Nutr Metab Insights. 2015;8(Suppl 1):1-9
Obermajer T, Grabnar I, Benedik E, et al. Microbes in Infant Gut Development: Placing Abundance Within Environmental, Clinical and Growth Parameters. Sci Rep. 2017 Sep 11;7(1):11230 
Oddy  WH. Breastfeeding, Childhood Asthma, and Allergic Disease. Ann  Nutr  Metab. 2017;70  Suppl 2:26-36 
Padilha M, Danneskiold-Samsøe NB, Brejnrod A, et al. The Human Milk Microbiota is Modulated by Maternal Diet. Microorganisms. 2019 Oct 29;7(11). pii: E502
Pannaraj  PS, Li F,  Cerini  C, et al. Association Between Breast Milk Bacterial Communities and Establishment and Development of the Infant Gut  Microbiome. JAMA 2017 May 8

Parida S, Sharma D. Microbial Alterations and Risk Factors of Breast Cancer: Connections and Mechanistic Insights. Cells. 2020;9(5):E1091. Published 2020 Apr 28

Praveen V, Praveen S. Microbiome-Gut-Brain Axis: A Pathway for Improving Brainstem Serotonin Homeostasis and Successful Autoresuscitation in SIDS-A Novel Hypothesis. Front Pediatr (2017) 4:136.10.3389/fped.2016.00136 
Penders J, Thijs C, Vink C, et al.  Factors influencing the composition of the intestinal microbiota in early infancy.  Pediatrics. 2006 Aug;118(2):511-21
Rajilic-Stojanovic M, de Vos WM. The first 1000 cultured species of the human gastrointestinal microbiota.  FEMS  Microbiol Rev. 2014;38:996–1047 
Savage JH, Lee-Sarwar KA, Sordillo JE, et al. Diet during Pregnancy and Infancy and the Infant Intestinal Microbiome. J Pediatr. 2018 Aug 30. pii: S0022-3476(18)30975-2
Sender R, Fuchs S, Milo R. Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans. Cell. 2016 Jan 28;164(3):337-40 
Shani GI, Lewis ZT, Robinson AM, et al. Interactions Between  Bifidobacteria, Milk Oligosaccharides, and Neonate Hosts. In The Bifidobacteria and Related Organisms. London: Elsevier; 2018. p165-175 

Shreiner AB, Kao JY, Young VB. The gut microbiome in health and in disease. Current opinion in gastroenterology. 2015;31(1):69-75
Smilowitz  JT,  Lebrilla  CB, Mills DA, et al. Breast milk oligosaccharides: structure-function relationships in the neonate.  Annual review of nutrition. 2014;34:143-169 
Stewart CJ, Ajami NJ, O'Brien JL, et al. Temporal development of the gut microbiome in early childhood from the TEDDY study. Nature. 2018 Oct;562(7728):583-588
Stewart CJ, Embleton ND, Marrs ECL, et al. Longitudinal development of the gut microbiome and metabolome in preterm neonates with late onset sepsis and healthy controls. Microbiome. 2017 Jul 12;5(1):75
Stiemsma  LT, Michels KB. The Role of the  Microbiome in the Developmental Origins of Health and Disease.  Pediatrics. 2018 Mar 8. pii: e20172437 
Sweeney EL, Al-Shehri SS, Cowley DM, et al. The effect of breastmilk and saliva combinations on the in vitro growth of oral pathogenic and commensal microorganisms. Sci Rep. 2018 Oct 11;8(1):15112
Togo A, Dufour JC, Lagier JC, et al. Repertoire of human breast and milk microbiota: a systematic review. Future Microbiol. 2019 May;14:623-641
Toscano M, De Grandi R, Grossi E, et al. Role of the Human Breast Milk-Associated Microbiota on the Newborns' Immune System: A Mini Review. Front Microbiol. 2017 Oct 25;8:2100 
Urbaniak C, Burton JP, Reid G. Breast, milk and microbes: a complex relationship that does not end with lactation. Womens Health (Lond). 2012 Jul;8(4):385-9
Ursell LK, Metcalf JL, Parfrey LW, et al. Defining the human microbiome. Nutrition reviews. 2012;70(Suppl 1):S38-S44 
Wang B, McVeagh P, Petocz P, et al. Brain ganglioside and glycoprotein sialic acid in breastfed compared with formula-fed infants. Am J Clin Nutr. 2003 Nov;78(5):1024-9
Wise A, Robertson B, Choudhury B, et al. Infants Are Exposed to Human Milk Oligosaccharides Already in utero. Front Pediatr. 2018 Oct 2;6:270
Xiao L, van De Worp WR, Stassen R, et al. Human milk oligosaccharides promote immune tolerance via direct interactions with human dendritic cells. Eur J Immunol. 2019 Mar 22
Yang I, Corwin EJ, Brennan PA, et al. The Infant Microbiome: Implications for Infant Health and Neurocognitive Development. Nurs Res. 2016;65(1):76-88