Microscopically small organisms, consisting of bacteria, viruses, fungi, and other single cell organisms, live in and on our bodies. Some are found on the skin, in the mouth, the nose, the urogenital system, but the vast majority are found in the gastrointestinal tract (known as the gut), particularly in the large intestine (colon).
This body wide collection of microorganisms, including their genetic material, is known as the microbiome. The term “microbiota” is used to refer to the community of microorganisms themselves, usually in a specific environment, for example the gut microbiota. These microorganisms play a vital role in the functioning of our bodies and scientists view the microbiome as an “essential organ” of the human body. More than 1 000 different species of microorganisms live in a delicately balanced symbiotic relationship with the human host. Although many species are commonly found, every person has a unique microbiome, which contributes to how the individual digests food, fights disease, and it even affects the brain and mood.
The microbiome is deemed to be established at birth. The healthy fetus is suspected to have no microbial organisms until birth, when the newly born infant’s gastrointestinal tract becomes colonized with a multitude of microbes. Along with the development of the growing infant, the gut microbiota develops into a complex microbial community during the first year of life. The birth method as well as the infant’s diet play a major role in the type of the microorganisms that are established in the gut.
Effects of the type of birth on the infant’s gut microbiota:
Colonization of the intestine starts at birth and babies born by caesarean section have different gut bacteria to babies born vaginally, as the mode of delivery plays a role in the initial bacterial composition in the infant’s gut.
- The gut microbiota of infants born by vaginal delivery resembles that of the maternal vagina and intestinal tract.
- The gut microbiota of infants born by caesarean section delivery shares characteristics with that of the maternal skin.
- By the time babies are weaned (usually 6-9 months), the differences in gut microbiome composition between C-section and vaginal births have mostly disappeared.
Effects of feeding type on the infant’s gut microbiota:
There are major differences in the bacterial composition the infant’s gut according to the different feeding types – breastfeeding and formula feeding – with complimentary feeding following at a later stage. Breastfeeding versus formula feeding strongly affects the composition of the gut microbiome during early infancy.
Breastfeeding: Breast milk is the most complete food imaginable, cleverly adapting to the needs of the growing infant. Apart from the optimal macronutrients (fats, carbohydrates, and proteins) to support baby development and growth, breast milk also offers protection as it is full of essential vitamins, minerals, and vital antibodies to strengthen the baby’s immune system. Breastfeeding also provides stem cells, the building blocks for growth and development.
Breast milk also contains a diverse microbial community that is tailored to meet the baby’s nutritional needs. Breastfeeding results in the colonization of an abundance of highly beneficial microbes of the Lactobacillus and Bifidobacterium species in the infant’s gut. Scientists suspect that the breastmilk microbiota may have two origins – firstly the maternal skin or the infant’s mouth may provide the milk with some bacteria, or, secondly, the bacterial strains may get transferred from the maternal intestine to the mammary ducts and milk via the lymph/blood circulation system. Breastfeeding for at least six months is important for the development of beneficial bacteria in the infant’s gut microbiome.
The Bifidobacterium species are of particular importance, as they are specialized to degrade a type of sugar in breastmilk called human milk oligosaccharides (HMO), the third most abundant component in breastmilk. (Interestingly, the most abundant component is lactose, the major carbohydrate constituent of breastmilk, followed by fat, with protein the fourth most abundant macronutrient.)
The human milk oligosaccharides are not digested in the infant’s upper digestive tract, but are fermented in the colon by the bifidobacteria, contributing to the protection of infants against infections and immune-related diseases.
Bottle-fed: Infant formula was developed as a substitute for breast milk, as not all mothers are able to breastfeed, or able to breastfeed for the recommended minimum term of six months. Cow’s milk or soy milk are commonly used as the base for infant formula, or else a specialized formula is used for infants who cannot tolerate cow of soy milk, with nutritional ingredients added to closely resemble human breast milk. Studies have shown that different formulas may have different effects on the microbiota.
In contrast to breast-fed infants, infants that are bottle-fed with milk formula develop a more diverse gut microbiota, resembling the gut microbiota of older children, with a higher abundance of potentially harmful bacteria, which feed on proteins as a preferred fuel, rather than carbohydrates. Formula feeding lacks human milk oligosaccharides and has a higher protein content. Some of the excess proteins are incompletely digested and absorbed in the upper gastrointestinal tract and therefore not metabolized by gut microbes.
The gut microbiota of formula fed infants are more frequently colonized with bacterial species that may be toxin carrying, including bacterial species being capable of causing disease (pathogenic). This may increase the risk of gastrointestinal infections, as well as metabolic and immune-related diseases.
Complementary feeding: The gradual cessation of milk-based feeding and the introduction of the complementary feeding period (6-24 months of life) means the infant is exposed to an increased consumption of dietary fibers and proteins, which results in increased diversification of the bacterial species and a more mature gut microbiota. Poor growth and development in early life is associated with inadequate maturation of the gut microbiota during complimentary feeding. The complimentary feeding period, with a shift away from a milk-based diet, is a critical period in the development and composition of the gut microbiota. Early complimentary feeding before three months of age has been associated with an increased risk of gastrointestinal tract and respiratory infections, obesity, and allergies. However, the shorter duration of breastfeeding may also play a role in this regard.
The milk-based component of the infant’s diet is gradually replaced during complimentary feeding by other foods such as fruit, vegetables, diary, meat, cereals, and bread. The protein and dietary fibers in these foods modulate the infant’s gut microbiota by increasing the diversity and abundance of important bacterial families.
Conclusions:
Although diet is the most influential factor that plays a role in the development of the infant’s gut microbiota, many other variables also play a role, starting with the mother’s health, the use of antibiotics, term of birth, mode of birth, local environment, geographical location, as well as the maternal diet.
Although the role that bifidobacterial species play in the gut health and immune system of infants have been widely studied, scientists still have an incomplete understanding of the details. What is known is that the right balance of Bifidobacterium strains helps in educating the immune system and preventing inflammatory responses.
Cesarean sections, bottle feeding and exposure to antibiotics reduce the abundance of the Bifidobacterium strains, resulting in a higher proportion of protein degrading microorganisms, which are potentially disease-causing.
The establishment of a healthy gut microbiota in early human life is a critical period that plays a significant role in the individual’s health later in life, with breast milk the main influence factor of a healthy infant gut.
References:
What is bifidobacteria’s role in establishing a healthy infant gut microbiome and immune system? Published 20 June 2022. Gut Microbiota for Health. (www.gutmicrobiotaforhealth.com)
Gut microbiota development: Influence of diet from infancy to toddlerhood. Published in the journal: Annals of Nutrition and Metabolism 2021; 77 (suppl 3): 21-34. Karger. (Publishing House). (www.karger.com)
Comparison of gut microbiota in exclusively breast-fed and formula-fed babies: a study of 91 term infants. Published 25 September 2020. Scientific Reports 10, Article number: 15712 (2020). Nature. (www.nature.com)
Why are bifidobacteria important for infants? Published 25 January 2022 in the journal: Microorganisms 2022, 10(2) 278. MDPI Academic Open Access Publishing. (Publisher of fully peer-reviewed, open access journals.) (www.mdpi.com)
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