Initial acquisition of microbiota

Early hypotheses assumed that human babies are born sterile and that any bacterial presence in the uterus would be harmful to the fetus.[7] Some believed that both the womb and maternal milk were sterile, and that bacteria did not enter an infant’s intestinal tract until supplementary food was provided.[9] In 1900, the French pediatrician Henry Tissier isolated Bifidobacterium from the stool of healthy, breast-fed infants.[10][11] He concluded that breast milk was not sterile and suggested that diarrhea caused by an imbalance of intestinal flora could be treated by supplementing food with Bifidobacterium.[12] However, Tissier still claimed that the womb was sterile and that infants did not come into contact with bacteria until entering the birth canal.[11]

Over the last few decades, research on the perinatal acquisition of microbiota in humans has expanded as a result of developments in DNA sequencing technology.[7] Bacteria have been detected in umbilical cord blood,[13] amniotic fluid,[14] and fetal membranes[15] of healthy, term babies. The meconium, an infant’s first bowel movement of digested amniotic fluid, has also been shown to contain a diverse community of microbes.[13] These microbial communities consist of genera commonly found in the mouth and intestines, which may be transmitted to the uterus via the blood stream, and in the vagina, which may ascend through the cervix.[7][13]

In one experiment, pregnant mice were given food containing genetically labeled Enterococcus faecium.[16] The meconium of term offspring delivered by these mice via sterile C-section was found to contain labeled E. faecium, while pups from control mice given non-inoculated food did not contain E. faecium. This evidence supports the possibility of vertical microbial transmission in mammals.

Most research on vertical transmission in non-mammalian vertebrates focuses on pathogens in agricultural animals (e.g. chicken, fish).[7][17][18] It is not known whether these species also incorporate commensal flora into eggs.

Marine sponges are host to many sponge-specific microbe species that are found across several sponge lineages.[19] These microbes are detected in divergent populations without overlapping ranges but are not found in the sponges' immediate environment. As a result, it is thought that the symbionts were established by a colonization event before sponges diversified and are maintained through vertical (and, to a lesser extent, horizontal) transmission.[20] The presence of microorganisms in both the oocytes and in the embryos of sponges has been confirmed.[20][21]

Many insects depend on microbial symbionts to obtain amino acids and other nutrients that are not available from their primary food source.[7] Microbiota may be passed on to offspring via bacteriocytes associated with the ovaries or developing embryo,[5][22][23] by feeding larvae with microbe-fortified food,[24] or by smearing eggs with a medium containing microbes during oviposition.[25][26] Alternatively, in other instances, microbiota composition can also be determined by the environment, as is the case for mosquito larvae, living in the water.[27]

• Human microbiome project
• Human microbiota
• Gut flora
• Vaginal flora