# An Insight into Probiotics Bio-Route: Translocation from the Mother's Gut to the Mammary Gland
## 3 Microbial Partners of Human Milk 'Factory'
### Predominance of Skin Microflora  
[PERSON] et al. (2014) reported that _Staphylococcus_ and _Streptococcus_ strains were commonly found in almost all culture-independent quantification studies of HMB. The members of _Staphylococcus_ and _Streptococcus_ genera are dominant skin microflora including _Propionibacterium_, which was also reported to inhabit the infant gut ecosystem. Therefore, their predominance in human milk indicates that they may be derived from maternal skin or from gut microbes ([PERSON] et al., 2014). A study revealed that the microbial load in the milk samples of healthy mothers originates from maternal aeolar skin. It was predicted that the skin bacteria from the breast surfaces of the nipple or areola could gain access through the ducts of the mammary glands during breastfeeding ([PERSON] et al., 2015; [PERSON] et al., 2015). Still, the question arises here, why does breast milk contain an abundance of opportunistic pathogenic genera such as _Staphylococcus_ and _Streptococcus_? Traditionally, greater amount of _Staphylococcus aureus_ and _Staphylococcus epidermidis_ in the HBM samples, along  
Figure 2: 16S rRNA gene-based phylogenetic tree of the human milk core microbiota. The tree was constructed based on the information provided by [PERSON] et al. (2014) and [PERSON] et al. (2015) with the inclusion of additional species, of which the genome sequence data have been available since 2015. The analysis involved 49 nucleotide sequences. All the evolutionary analyses were performed by using MEGA (version 7.0). All positions comprising gaps and missing data were removed. The final dataset observed 1266 positions. with other genera including _Streptococci_ and _Corynebacterium_, were considered as the main agents causing mastitis [25]. Mastitis is a condition of inflammation in lactating mammary glands caused by microbial dysbiosis in the human milk microbiome and leads to opportunistic pathogenic bacteria outgrowth and a decline in healthy and stable commensal bacteria [26]. Opportunistic pathogenic bacteria are the normal microflora that outgrow in numbers due to several conditions including the compromised host's immunity [15]. [PERSON] et al. [21] reported that individuals with subacute mastitis and acute mastitis had higher abundance of _Proteobacteria_ than _Firmicutes_. In addition, there have been several studies that reported that commensal skin microflora including _Staphylococcus_ and _Streptococcus_ were also found to be predominant in the breast milk samples collected from healthy mothers [5; 20; 27]. [PERSON] et al. [28] reported that skin originated coagulase-negative _Staphylococci_ was able to produce bacteriocin, which inhibits the growth of pathogenic bacteria. In addition, HBM contains various antimicrobial proteins and peptides such as lactoferrin, beta-defensins, and alpha-defensin [1; 29]. Antimicrobial proteins and peptides present in breast milk show broad inhibitory activity against a variety of pathogens like _Streptococcus epidermidis_, _S. aureus_, _E. coli_, and _Streptococcus agalactiae_[26]. Previously, [PERSON] et al. [30] described the survivability of these skin microflora in human milk. The relative abundance of _Staphylococci_, especially _S. aureus_ and _S. epidermidis_ strains, was expected due to the influence of human milk oligosaccharides (HMO). The HMO are well-known for promoting the growth of various commensal bacteria in HBM including _Bifidobacterium_ spp. [31; 32]. However, these HMOs were also found to stimulate the growth and proliferation of _S. aureus_ and _S. epidermidis_ strains. Another interesting fact is that these two strains fail to metabolize HMOs available in breast milk [25]. Hence, conjectures can be made that this (above stated reason) could be the main factor for the abundance of skin microflora in the breast milk, especially _Staphylococci_ and _Streptococci_. After breaching into the mammary glands from the areolar skin, these skin microflora could be stimulated to multiply by breast milk components.