The human milk microbiota, also known as human milk probiotics (HMP), refers to the microbiota (community of microorganisms) residing in the human mammary glands and breast milk. Human breast milk has been traditionally assumed to be sterile, but more recently both microbial culture and culture-independent techniques have confirmed that human milk contains diverse communities of bacteria which are distinct from other microbial communities inhabiting the human body.
The human milk microbiota which could be source of commensal, mutualistic, and potentially probiotic bacteria to the infant gut microbiota. The World Health Organization (WHO) defines "probiotics" as "living organisms which when administered in adequate amounts confer a health benefit on the host".
Occurrence
Breast milk is a natural source of lactic acid bacteria for the newborn through breastfeeding, and may be considered a symbiotic food. The normal concentration of bacteria in milk from healthy women was about 103 colony-forming units (CFU) per milliliter. The milk's bacterial communities were generally complex. Among the hundreds of operational taxonomic units detected in the milk of every woman, only nine (Streptococcus, Staphylococcus, Serratia, Pseudomonas, Corynebacterium, Ralstonia, Propionibacterium, Sphingomonas, and Nitrobacteraceae) were present in every sample from every woman, but an individual's milk bacterial community was generally stable over time. Human milk is a source of live Staphylococci, Streptococci, lactic acid bacteria, Bifidobacteria, Propionibacteria, Corynebacteria, and closely related Gram-positive bacteria for the infant gut.
Composition
Breast milk was considered to be free of bacteria until about the early 2000s, when lactic acid bacteria were first described in human milk hygienically collected from healthy women. Several studies have shown that there is a mother-to-infant transfer of bacterial strains belonging, at least, to the genera Lactobacillus, Staphylococcus, Enterococcus, and Bifidobacterium through breastfeeding, thus accounting for the close relationship of bacterial composition of the gut microbiota of breastfed infants with that found in the breast milk of their respective mothers. Research has also found that there are similarities between human milk and infant gut microbial flora, suggesting that dietary exposure, such as human milk probiotics, may have a contribution in supporting infant gut microbiota and immune development.
Bacteria commonly isolated in human milk samples include Bifidobacterium, Lactobacillus, Staphylococcus, Streptococcus, Bacteroides, Clostridium, Micrococcus, Enterococcus, and Escherichia. Metagenomic analyses of human milk find it is dominated by Staphylococcus, Pseudomonas, and Edwardsiella. The human milk microbiome likely varies by population and between individual women, however, a study based on a group of U.S. women observed the same nine bacterial taxa in all samples from all of their participants, suggesting a common "core" of the milk microbiome, at least in that population. Bacterial communities of human colostrum have been reported as being more diverse than those found in mature milk.
The three strains of Lactobacilli with probiotic properties that were isolated from breast milk were L. fermentum CECT5716, L. gasseri CECT5714, and L. salivarius CECT5713, with L. fermentum being one of the most abundant strains. Early administration of L. fermentum CECT5716 in infant formula is claimed to be safe and well tolerated for infants one to six months of age, and safe for long term use.
Origin
While the origins of the human milk microbiome are not exactly known, several hypotheses for its establishment have been proposed. Bacteria present in human milk may be derived from the surrounding breast skin flora, or the infant's oral cavity microbiota. Retrograde backflow during nursing or suckling may also lead to bacterial establishment in the mammary ducts, supported by the observation that a certain degree of flowback has been shown to occur during nursing using infrared photography. Alternatively, bacteria may be translocated to the mammary duct from the maternal gastrointestinal tract via an entero-mammary pathway, facilitated by dendritic cells.
Environmental factors
Several factors may influence the composition of human milk probiotics, such as maternal body mass index (BMI), infant sex, birth modality, and mode of breastfeeding. A study done by Soto et al also revealed that Lactobacilli and Bifidobacteria are more commonly found in the human milk of women who did not receive any antibiotics during pregnancy and lactation.