Breastfeeding as a Gold Standard: How does the “Gold Standard” Benefit the Microbiota of Newborns? A Narrative Review
By: Nicole Knebusch, student of Clinical Nutrition at the Francisco Marroquin University. Correspondence: firstname.lastname@example.org
Newborns have a critical period of time in which their microbiota is colonized. During this time, an alteration in the microbiome may affect their healthy development due to dysbiosis. Human breastmilk has been established as the preferred feeding method in infants for many reasons. However, there are many times when the mother’s own breastmilk cannot be utilized, and it is necessary to use donor human milk or formula, which has the potential to aggravate the infant’s intestinal microbiota composition. The purpose of this review is to evaluate the benefits of human breastmilk for childrens’ microbiome and to evaluate if modifications in breastmilk or substitutes can alter the microbiome of children in the short and long term.
Los recién nacidos tienen un periodo de tiempo crítico en el cual se coloniza su microbiota. Durante este tiempo, una alteración del microbioma puede afectar a su desarrollo saludable debido a la disbiosis. La lactancia materna se ha establecido como el método de alimentación preferido en los bebés por muchas razones. Sin embargo, hay diversas ocasiones en las que no se puede utilizar la lactancia materna de la madre y es necesario utilizar leche humana de donante o fórmula, que tiene el potencial de agravar la composición de la microbiota intestinal del lactante. El objetivo de esta revisión es evaluar los beneficios obtenidos de la lactancia materna y valorar si las modificaciones en la leche materna o los sustitutos pueden alterar el microbioma de los niños a corto y largo plazo.
Key Words: Microbiota, Microbiome, Human Breastmilk, Infant Formula, Donor Breastmilk
Neonatal microbiota is shaped by many factors, including transmission from the mother, type of delivery, and the environment surrounding them (1). One of the most important factors contributing to the initial colonization and development of the gut microbiome is breastfeeding. Evidence suggests that by 6 days of life, 67.7% of microorganisms found in neonatal stool are combined with human breastmilk (HBM) (2).
Historically, individuals believed that birth would result in a sterile gastrointestinal tract, and colonization would occur afterward, but research indicates the uterus is not a sterile environment. New research has found that amniotic fluid and placenta present microbial populations, such as Proteobacteria, which is also present in infant’s meconium in a low abundance (3,4). Newborn’s gut is highly populated with Bifidobacterium as a result of HBM, making it an important component of their health. Bifidobacterium is a type of lactic acid-forming bacteria, which are passed from mothers to their infants. These bacteria maintain the intestinal barrier, increase the maturation and modulation of the immune system, and influence some metabolic pathways. Table 1 describes how the microbial population affects metabolic pathways. (5,6). Moreover, this genus of bacteria has been associated with a significant reduction in endotoxemia (p < 0.05), as well as a reduction in the incidence of diarrhea in both types of delivered infants (p < 0.001) (7,8).
For many years, HBM has been established as the main and exclusive form of feeding in infants, and it is still being researched to determine whether these benefits are related to its role in establishing the microbiota (9). In this review, we aim to determine whether breastfeeding plays a significant role in preventing and treating dysbiosis, how it protects the microbiome development of newborns, and how HBM substitutes alter the neonatal microbiome.
Causes of Microbiome Variation and Dysbiosis
The development and establishment of an infant’s gut microbiome can be disturbed by many aspects, including the delivery mode, the feeding method, the antibiotic treatment, and the gestational age (10). In addition to these factors, exposure to the hospital environment can impact the microbiome of an infant. It was revealed by Combellick et al. that infants born in a hospital have alterations in their microbiota during their first month of life compared to those born at home, in which infants born at the hospital had lower alpha fecal diversity (Phylogenetic diversity, p = 0.036) (11). Similarly, as demonstrated in a prospective cohort study of 47 preterm infants by D’Agata et al., stress exposure at 1 week of age, as measured by the NISS scale, significantly increases Proteus and Veillonella (p < 0.001 and p < 0.033, respectively) (12). Studies have shown that Proteus can lead to immunosuppression in preterm infants and Veillonella has been linked by other research with increased vulnerability to intestinal disease (12,13).
Many factors can be avoided but many others are inevitable like gestational age. In a prospective cohort study, fecal samples from 34 very low birth weight (VLBW) preterm infants were collected and compared with fecal samples from 10 healthy term infants. It was found that 10 VLBW developed invasive infections and that these infants had a significantly lower alpha diversity according to the Chao1 and Shannon index since birth and persisted through 7 weeks of age (p < 0.05). When VLBW and healthy term infants, were compared, the healthy term infants had and maintained a higher alpha diversity in the following weeks (p < 0.01) (10).
Consequences of Dysbiosis
Dysbiosis has been linked to many disease states of pediatric and infant patients. Some of them include necrotizing enterocolitis (NEC), sepsis, allergies, and others (10,14,15). As for NEC, Olm Et al. studied 1163 fecal metagenomes collected from 34 preterm infants who developed NEC and 126 preterm infants without NEC in a prospective cohort study. Compared to infants who were not affected by NEC, the bacteria found in the microbiomes of NEC patients were less abundant in Firmicutes and more abundant in Enterobacteriaceae (16). These results are similar to those obtained by Pammi et al.’s metanalysis of 8 studies. According to this study, NEC patients had fewer species in their microbiota (p = 0.05, negative binomial regression model) and Proteobacteria tended to be more abundant, with a lower abundance of Firmicutes and Bacteroidetes preceding NEC than control infants (14). Among 34 very low birth weight preterm infants evaluated for the possible association between an infant’s microbiota and neonatal sepsis, it was demonstrated that infants that developed an invasive infection had a lower alpha diversity from birth that continued into their second month of life compared to those without an invasive infection (p < 0.05, Shannon index) (10).
Interesting results were demonstrated by Arrieta et al. in the ECUAVIDA birth cohort study. They compared their microbiome analysis from 1066 stool samples to the results from the CHILD study and their association to atopic wheezing in children (15,17). Children from the ECUAVIDA study originated from El Ecuador, whereas kids from the CHILD study originated from Canada. However, even though the microbiome analysis in age-matched kids varied and gut dysbiosis associated with atopic wheezing differed in taxa, both groups showed a decrease in fecal acetate in atopic wheezing (15,17). These results suggest that it is not only the taxon, species, or microorganism itself that may influence the risk of atopic wheezing, NEC, or sepsis, but also how much and what kind of short-chain fatty acids the microbiota produces or contributes to the production of those acids.
Breastmilk as a Gold Standard
Benefits of breastmilk for the infant’s gut microbiota
As previously noted, breastfeeding influences an infant’s gut microbiota, and there is a close relationship between the infant’s microbiota and human milk oligosaccharides (HMO) (6). Exclusive breastfeeding for the first six months of life has been proven to benefit the child in many different ways and several studies are evaluating whether these benefits are associated with the effects of breastfeeding on the infant’s gut microbiota (6,9). Human milk contains a wide variety of microorganisms and HMOs. Together, these factors carry out the colonization of a strong, healthy microbiota (Figure 1) (6,18).
The effects of human breastmilk were shown to be dose-dependent by a longitudinal study that evaluated infant stool samples, breast milk, and areolar skin of 107 healthy mother-infant pairs. In this study, it was demonstrated that the percentage of daily milk intake was associated with an alteration in microbiota composition (P =0.002) when comparing infants with < 75% of breastmilk consumption to those who were breastfed more than 75% (19). These results can be compared with those of a meta-analysis of 8 studies performed by Ho et al. Based on the results of this meta-analysis, it was found that infants who received less than two months of exclusive HBM had a higher abundance of Streptococcaceae and a decrease in Bifidobacteriaceae and Coriobacteriaceae (p < 0.05) when compared to infants who received more than two months of exclusive HBM, who had no alterations at the time of diarrhea (6). In addition to emphasizing the importance of HBM exclusivity as a means to prevent dysbiosis, these findings also demonstrate how substitutes for HBM can alter the microbiome of children with even a small introduction. It is important to emphasize that non-exclusive breastfed infants have a reduced amount of Bifidobacteriaceae, which further indicates that the type of feeding can cause substantial changes in the newborn’s gut functionality.
Breastmilk microbiota composition
The composition of HBM’s microbiota is described as a “dynamic community represented mostly by members of Proteobacteria, Firmicutes and Actinobacteria phyla” (20). Additionally, the microbiota of HBM is not only composed of beneficial bacteria but also archaea, such as Methanobrevibacter smithii and Methanobrevibacter oralis, which is known to be depleted in patients with severe acute malnutrition. Its presence in human colostrum and its function at producing nonenzymatic antioxidants was evidenced by Togo et al (21).
The microbiota of HBM changes over time, with significant differences in bacterial concentration between colostrum, transitional milk, and mature milk, and are also dependent on the amount and type of HMOs (6,22). According to the results of a longitudinal study that evaluated 96 HBM samples, the total count of Bifidobacterium spp. and Enterococcus spp. increase throughout the lactation period, with the lowest concentration of Bifidobacterium spp. being in colostrum milk when compared to transitional (p = 0.001) and mature milk (p = 0.001) (22). As a dynamic fluid, HBM will change depending on the needs of the child, adapting to what is required at any given time, explaining the total variations in different types of HBM.
The microbiota community in HBM can be influenced by maternal diet as demonstrated by Cortes-Macías et al. in a cross-sectional study that assessed 120 samples from healthy mothers by 16sRNA gene sequencing. Based on the results of this cohort study, there is an association between a higher intake of carbohydrates and polyphenols with Bifidobacterium abundance (p = 0.031; p = 0.042) and higher microbial diversity, according to the Shannon index, when the mother’s intake of fiber was higher (p = 0.055) (23). As a dynamic fluid, human breast milk is exceptionally important to the development of the neonatal gut microbiota and the maturation of the immune system (24).
The type of delivery can also alter the gut’s microbiota, but HBM has the potential of shifting a cesarean delivery infant into a vaginally delivered infant as demonstrated by Guo Et al. in a prospective study. In comparison with vaginally delivered infants, HBM-receiving cesarean infants had a microbiota composition that was more similar to that of formula-receiving infants (8). The consumption of food by the mother will not be the only factor to affects the microbiota in the HBM, but it can have an effect. With these results, it can be argued that a healthier HBM microbiota will benefit the newborn. Furthermore, these results demonstrate that HBM can improve the microbiome in newborns who had cesarean sections, therefore demonstrating its role in the treatment of variations and dysbiosis, in this case, as well showing its potential role in the treatment of other forms of dysbiosis.
Human milk oligosaccharides: why are they important?
HBM contains nondigestible oligosaccharides called human milk oligosaccharides (HMOs), and their role in the infant’s health is to establish the gut microbiota, enhance the immune system’s development and increase colonization resistance to non-appropriate microbiota (25). As each mother’s HBM is unique, so it’s the content and type of HMO composition (18). In a randomized controlled trial (RCT) that assessed the effect of probiotic modulation of HBM, it was evidenced that the total HMO concentration has a positive correlation with Bifidobacterium spp. and B. breve (ρ=0.63, P=0.036 and ρ=0.66, P=0.027, respectively) (18).
These results and those obtained from different studies have opened up the research for HMO supplementation in infants’ formula. For example, in a RCT performed by Berger et al., it was demonstrated that formulas supplemented with HMO modulated the abundance of alpha diversity measured by the Faith’s diversity index when compared to infants receiving formula without HMOs (p < 0.05), putting them closer to the breastfeeding group. In this same study, it was demonstrated that HMOs increased Bifidobacteriaceae and that as this type of bacterial composition increased, the likelihood of requiring antibiotic treatment at 12 months of age decreased (OR: 0.4; 95% CI (0.17 – 0.93); p = 0.033) (25). Additionally, in a systematic review, it was revealed that the high content of Bifidobacterium and Lactobacillus prevents gastrointestinal, respiratory, sepsis, and infections in infants. Bifidobacterium hasbeen shown to help maintain the ideal production of lactate and acetate, needed for a healthy gut and educate the immune system(26). As well, a lower abundance of Bifidobacterium is associated with an increased likelihood of developing type one diabetes (p < 0.001) (27). A significant amount of evidence suggests that Bifidobacterium has both short and long-term advantages, with it being incredibly common in newborns who are breastfed and of significant importance in the colonization of an immature gastrointestinal tract.
Can altering human breastmilk alter the microbiome of infants?
There are many different ways in which HBM can be altered or manipulated such as storing, freezing, skimming, and pasteurization. In situations where the infant is separated from the mother, such as in the NICU or when forced enteral nutrition is needed for the infant, expressed HBM is commonly used (28). Consequently, the mother may need to pump or manually express milk to provide HMB for the infant, and in a small study where 10 samples of HMB were evaluated to determine if differences developed depending on the modality of collection. It was established that there are no significant differences in the composition of bacteria between pumping and manual expression according to the Jaccard similarity (29). An important finding of another study is that mothers’ pumping equipment and poor hygiene habits can lead to a higher concentration of Enterobacteria, Pseudomonas spp., and Gram-negative bacteria (24).
These results illustrate the importance of good practices when it comes to HBM in order to fully utilize its health benefits in newborns.
Another type of treatment that HBM typically receives is pasteurization for when it is used for donation. Donor Human Milk (DHM) is preferred over formula and it is recommended by the American Academy of pediatrics for situations in which the supply of HBM is insufficient (30). The processing of DHM requires the Holder Pasteurization method to diminish the infectious agents (31). In a prospective observational study, 27 samples of HBM from the mother and DHM, were compared to analyze the differences in microbial patterns. It was demonstrated that alpha diversity indexes did not differ between mothers’ own HBM (MO-HBM) and DHM. However, beta diversity was significantly different in DHM when compared to MO-HBM (PCoA, PERMANOVA; p = 0.04) (31). These results can be compared to those obtained by Arboleya Et al., in a larger study evaluating the fecal samples from 42 preterm infants. The results showed that alpha diversity remained similar between DHM and MO-HBM for the first two days of life. However, it was demonstrated that infants receiving MO-HBM showed a decline in alpha diversity with increased age and that it became higher in the DHM group at one month of life (p < 0.01; Shannon Index) (32). Lastly, in a study that was sequenced by 16S rRNA from 61 fecal samples from infants receiving MO-HBM and HDM, it was found that infants receiving DHM had a lower microbial richness (Chao 1 index) than infants receiving MO-HBM (33). Results show that DHM and MO-HBM differ slightly in composition. These results can also be explained by the fact that DHM is obtained from mature milk normally, and not from colostrum, or transitional milk, suggesting that this type of HBM will not be as dynamic as MO-HBM (31).
Numerous studies have evaluated how to restore the HBM microbiota by inoculating DHM with fresh MO-HBM because of the concerns derived from alterations in DHM. In a small study, samples from 11 mothers were obtained to inoculate DHM with MO-HBM at 30% or 10%. In this study, it was observed that DHM inoculated MO-HBM showed a similar alpha diversity, highlighting the possibility of personalization of HDM when some MO-HBM is available (20). Comparing these results to those obtained by Mallardi Et al. in which alpha diversity showed differences among samples from inoculated DHM and MO-HBM (34). The importance of alpha diversity was demonstrated by Roswall et al, in which 471 children were followed for their first 5 years of life. It was demonstrated that children with a lower diversity had a slower than expected weight gain than children with a higher diversity (OR = 1.00, 95% CI (1.03 – 4.04); p = 0.028) (1). The evidence points to an excellent potential solution for mothers who have a small MO-HBM production, however, bigger studies are needed to evaluate the efficacy of this practice, as stated previously, manipulation of HBM can increase the risk of contamination with other microorganisms.
How does formula change the microbiome and what risks does it impose?
When there is no HBM or DHM available, infants will require supplementation with formula products. It is well known that HBM is more beneficial to the gut microbiota than formula feedings (8). In the meta-analysis by Ho et al., it was demonstrated that alpha microbial diversity in non-exclusive HBM is increased when compared to HBM infants (p < 0.001, Shannon index) and that there is an increase in abundance of Bacteroidetes and Firmicutes in non-exclusive HBM (p < 0.05) (6). These results have significant importance when examining the long term effects, as the longitudinal study evaluating early microbiota composition and risk of obesity at 5 – 6 years, in which it was evidenced that among children with minimal treatment with antibiotics, there was a positive association between Bacteroidetes and in children with a history of antibiotic usage a higher abundance of Firmicutes at 3 months of age with a high BMI at age 5 – 6 (Correlation = 0.3 [0.01 – 0.05]; p= 0.04 and correlation = 0.21 [0.03–0.39], p = 0.02; respectively) (35). The significance of this observation is that it provides an insight into how these mechanisms and alterations in early life can increase the risk of metabolic diseases (6). Similarly, a higher abundance of Firmicutes was associated with an increased risk of type 1 diabetes (27). These studies indicate that alterations in the critical period can persist for many years, emphasizing the importance of microbiota development and establishment in infancy and that HBM may be beneficial for infants in the very early months of their lives.
The intestinal microbiota
entails the coexistence of many different microorganisms with the host and
modulates a beneficial system and functionality when it is a “healthy
microbiota”. There are many factors that can negatively affect
the microbiota composition, leading to dysbiosis and subsequent insidious
problems in the newborn infant such as NEC, infections, diarrhea, allergies,
and inadequate growth. This disorder can be prevented and treated in children
with HBM, which shows to be one of the most important factors. When HBM is not available,
DHM can be used in infants, but extraction needs to be safe to decrease any
potential contamination with microorganisms that could have an effect on the
newborns’ immature gut. This being the
case, there are numerous studies demonstrating that MO-HBM can restore the
content of DHM when the supply is insufficient, which will result in the
repairing of the microbiota of newborns, which is a better choice than formula
if it is available.
In studies, the main challenge is the small size
of the sample of infants who are fed HBM compared to those who are fed formula,
which tends to change the results. Despite this,
the HBM has been associated with a more stable microbial composition, meaning
that it can contribute to a neonate’s maturity and stability, both of which are
crucial for healthy development and a
reduction in the risk of short and long-term consequences, such as dysbiosis
and the risks that it entails.
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