INSERT FOR PAEDIATRICIANS CONTENT Human milk: what is normal? Modulation by diet Maternal and perinatal factors Shaping the infant gastrointestinal microbiome © Ramona Heim / Fotolia Human milk research group The Human Milk Microbiome – Possible Implications for Infant Health The human body contains and carries a large number of microorganisms and, surprisingly, the mammary gland and mother’s milk also contain their own microbiota. Very little is known, however, about the function of the human milk microbiome or the factors responsible for the marked variation observed within and among populations all over the world. It is hypothesized that the human milk microbiome plays an important role in the development of an infant‘s gastrointestinal microbiota and thus for its health in the future. Scientific exploration of the human milk microbiome and possible strategies for modification has just began but appears to be very promising. The first studies investigating the quantitative, and only 30 % of bacte- Entero-mammary pathway bacterial content of human milk were un- ria can be cultured. On the other hand, The presence of obligate anaerobic bacteria dertaken in the 1970s and 1980s because culture-independent methods such as was not compatible with the traditional view of concerns that bacteria might affect pre- 16S rRNA gene sequencing are ex- that human milk is sterile by nature. Bacteria term babies fed with human milk. At that pensive but may provide in-depth detected in milk were thought to stem exclu- time, the only bacteria that had been de- information about the microbi- sively from the breast skin or from the baby’s tected in human milk were Staphylococci al composition in human milk. mouth colonizing the mammary gland, said and Streptococci, reported Dr. Esther Jimé- 16S rRNA sequencing data indicatedthat Jiménez. nez, Madrid (Spain). It was not until 2003 the bacterial communities of human milk In another study the same lactic acid that lactic acid bacteria were isolated from are complex and often stable over time bacteria were found in locally fermen- human milk [1]; these were regarded as within an individual [3]. In addition, sub- ted food, maternal milk, and fecal sam- beneficial because they were able to sup- stantial inter-individual variability was ob- ples of breastfeeding mothers and their press the growth of Staphylococcus aureus served. “Each woman’s milk has its own babies [5]. This observation supports the hy- in the laboratory [2]. bacterial profile,“ concluded Jiménez. pothesis that bacteria are transferred from In particular, 16S rRNA sequencing the mother’s gastrointestinal tract to her milk Culture-dependent and culture- has been very useful in identifying bacte- and through the milk to the infant’s gastroin- independent techniques ria that cannot be cultured, such as those testinal tract. According to Jiménez, about 50 gene- that cannot survive in the presence of oxy- As Jiménez reported, evidence has in- ra and 200 species have been detected gen (anaerobes), in different body fluids. creased that a mother’s gastrointestinal bac- in human milk using culture techniques. For instance, a Swiss group detected such teria are able to reach the mammary gland The normal concentration of bacteria in obligate anaerobic genera like Bifidobac- via an endogenous route. For this bacterial milk from healthy women was shown to terium, Veillonella, and Bacteroides in the be about 103 colony-forming units (cfu) feces of healthy breastfed neonates using a per milliliter. Culture techniques, how- comprehensive approach with culture and ever, have limitations. They are semi- molecular methods [4]. INSERT FOR PAEDIATRICIANS translocation, dendritic cells appear to play those of non-secretor mothers [10]. Factors se diets contained more Bacteroides. Micro- an important role. These specialized cells influencing these variations are widely un- nu-trient intake correlated inversely with the were shown to open the tight junctions be- known. In McGuire’s opinion, “we need to re- relative abundance of Firmicutes, while diets tween epithelial cells in the digestive tract. think the idea of there being a normal range rich in eicosapentaenoic acid (EPA) or doco- By sending projections into the intestine, of milk compounds.“ sahexaenoic acid (DHA) were associated with dendritic cells were able to sample bacteria more Firmicutes. Diets rich in amino acids residing there and take them to the lamina Marked variations positively correlated with higher counts of propria [6]. From there, bacteria likely tra- Furthermore, the INSPIRE project revealed Proteobacteria and Fusobacteria. vel with immune cells through the body; in- clear population differences and variation In McGuire’s view, “the only way to verify if deed, DNA signatures of intestinal bacteria within populations in the maternal milk mi- we can modulate the milk microbiome with were found in immune cells in both human crobiome [11]. For instance, in a Hispanic diet is to do controlled intervention studies.“ milk and blood of lactating mothers [7]. population from California (USA) more Sta- McGuire differentiated between three phylococcus and Escherichia/Shigella were potential routes whereby maternal diet could Human milk: what is normal? found as compared with a population from influence maternal milk microbiota: Little is known about what is normal in terms Washington/Idaho (USA), in which Lactoba- — Bacteria consumed with probiotic food of human milk composition, including its mi- cillus didn’t even belong to the 30 most fre- translocate from the gastrointestinal crobiome. Therefore, the INSPIRE project was quently encountered genera. In populations tract to the mammary gland. from Peru or Spain, Streptococcus was pre- — Ingestion of micronutrients influences dominant; in Sweden, however, Staphylococ- the growth of bacteria present in the cus was most frequently encountered. While gut, from where these bacteria get to milk microbiomes in urban and rural popu- the mammary gland. lations in Gambia were similar, a remarkable — Nutrients reach the mammary gland (via dissimilarity was observed between an urban circulation) where they affect local bac- and a rural population from Ethiopia. teria differently. McGuire reported that Staphylococcus But things may not be that simple, because and Streptococcus (present in 99 % and 98 % bacteria could contribute to milk nutrients of the samples) were universal core bacteria and thereby impact the milk microbiome. [11]. The next-frequent genera were Propio- “So, to look at the influence of diet, we need nibacterium (in 78 % of the samples) and Co- to assess diet, the human milk, and fecal mi- rynebacterium (in 74 % of the samples). The crobes all at the same time,“ said McGuire. data showed, however, that there are differ- The INSPIRE Project also found a large va- ent core bacteria in different regions. riability in healthy infant fecal microbiomes “At the time being we don’t understand within and among populations [11]. In or- © Anatomy Insider / stock.adobe.com © Anatomy what is a normal or healthy human milk mi- der to explain the variability, it is not enough crobiome,“ said McGuire. What is regarded as to differentiate between “breastfed“ and “not normal in one population may be subopti- breastfed,“ said McGuire. In her opinion, it is started to determine (using standardized data mal for another. In McGuire‘s opinion, how- necessary to account for the variability in the collection methods) how human milk compo- ever, “understanding this is a must before we human milk composition. sition and microbiome vary in different parts can understand what is unhealthy or inter- of the world. As Dr. Michelle McGuire, Pull- vene.“ And “normal“ or “healthy,“ she said, is Maternal and perinatal factors man (USA), reported, 413 healthy breastfeed- clearly not a one-size-fits-all construct. In addition to geographic location and diet, ing women with healthy babies belonging to several other maternal and perinatal factors populations from Africa, Europe, South Amer- Modulation by diet have been shown to be related to the human ica, and the USA were included in this study. In another 21 healthy breastfeeding women, milk microbiome, reported Dr. Maria Carmen Substantial population differences were the McGuire research group performed de- Collado, Valencia (Spain). found regarding the content of milk prote- tailed dietary assessment over a period of 6 Cabrera-Rubio et al. observed human in, immune factors, human milk oligosaccha- months postpartum to investigate the in- milk microbiome changes during the course rides (HMOs) such as 2‘-fucosyllactose (2’FL), fluence of diet on the human milk microbi- of lactation [13]. Weissella, Leuconostoc, and even lactose, which is regarded as ome [12]. As McGuire reported, multiple re- Staphylococcus, Streptococcus and Lac- beeing the least variable component of hu- lationships were found. For instance, high- tococcus were predominant in colostrum, man milk [8–10]. HMO profiles in milk pro- calorie diets were associated with a higher whereas in mature human milk lactic acid duced by mothers categorized as secretors abundance of Firmicutes in milk, whereas bacteria prevailed. Furthermore, in mature (2’FL producers) differed substantially from milk of mothers consuming less-energy-den- milk an increased abundance of such oral 2 Springer Medizin | Insert for Paediatricians bacteria as Veillonella, Leptotrichia and relationship was evident. According to Mc- of life, breastfed infants obtained 27.7 % of Prevotella