# An Insight into Probiotics Bio-Route: Translocation from the Mother's Gut to the Mammary Gland
## 4 Microbial Transmission: Solving the Labyrinth Path
### Cross-Contamination from Infants' Oral Cavity  
The work done by [PERSON] et al. reported the involvement of areolar skin microbiota in the breast milk [23]. The study also found that about 60% of infant gut microbiota came from other sources than breast milk and areolar skin. However, the study did not further characterize the sources [23]. One of the possible causes could be translocation from the newborn oral environment. When an infant is born by the natural method or C-sections it's oral cavity is exposed to the surrounding microbiota [43]. [PERSON] et al. reported that oral microbiota differs between infants delivered by natural and C-section [72]. The naturally delivered infants contained an abundance of _Lactobacillus_, _Prevotella_ and _Gardnerella_ genera than the C-section infants who were more likely to have skin microflora such as _Staphylococcus_, _Pseudomonas_, _Desulfovibrio_, and _Petrimonas_[68]. Naturally born infants could have placental or vaginal microflora as they might take up some of the amniotic fluid during delivery. This was confirmed with the discovery of various microorganisms found in the meconium (first stool of infants) [78]. Therefore, some of the bacteria from the infants' oral cavity could cross-contaminate and move into the mammary glands during suckling. [PERSON] et al. found that infants' oral microbiota were the least diverse as _Streptococcaceae_ are the most common with an average relative abundance of 69.8 percent. The study also reported that the baby's mouth microbiota had similar identity between dominant _Streptococcus_ detected in their mothers' milk. This finding suggests that infants' oral microbiota can produce a seeding consequence on the mammary gland microbial community during nursing [43]. However, this factor does not clarify the presence of some bacteria in the pre-colostrum, which are secreted in some women before delivery [3; 37; 43]. Entero-Mammary Pathway: The 'Silk Route' to Trade Microbiota from Maternal Gut to Infant 4.3.1. Link between Maternal Gut and Breast Milk  
The human gastrointestinal environment contains the highest and most diverse microbial population compared to other body parts. Starting from the oral cavity to the rectum, the gastrointestinal tract includes more than 800 species of microflora [79; 80]. The gut microbiota is generally dominated by _Firmicutes_ and _Bacteroidetes_ phyla where their presence and abundance are crucial in regulating body metabolism and energy harvesting [81; 82]. However, the gut microbial population often changes over time with age, besides additional factors such as type of diet, ingestion of alive microorganisms, host genotype, and health status [80; 81]. The gestation cycle is a structural transition in the female body that includes hormonal, immunological, and metabolic modifications to promote fetal development and growth. During this period, endocrine secretions (especially progesterone and estrogens) are higher and there is major alteration in the immune response [9; 79; 81]. In addition, there are also noticeable changes in the maternal gut microbiota during pregnancy [7; 83]. A healthy gestation period is described with increased bacterial loading and changed gut microbial composition [9]. [PERSON] et al. first reported the gradual change of gut microbiota during early pregnancy [83]. The feces of 91 pregnant women in their first to third trimesters were analyzed by amplifying the V1-V2 variable region of the 16S rRNA gene. Microbial composition of first trimester women was similar to healthy non-pregnant women. However, gut microbiota composition evolved dramatically during pregnancy with amplified loads of _Actinobacteria_ and _Proteobacteria_, but their individual richness was reduced [83]. Similarly, [PERSON] et al. also described that the abundance of maternal gut microbiota was reduced, and their variety and consistency were increased from early pregnancy (\(\leq\)20 weeks) to the end of the third trimester [7].