The vaginal microbiome changes during and after pregnancy

The vaginal microbiome is critically important to a healthy pregnancy, and studies have shown that vaginal dysbiosis during pregnancy can lead to infection and preterm birth.  In order to help understand what the microbiome looks like throughout and just after pregnancy, researchers from England performed longitudinal studies on 42 pregnant women.  They published their results last week in Nature Scientific Reports.

The scientists sequenced the microbiomes of the 42 women throughout their pregnancies, and then for the 6 weeks afterwards for some of the women.  They discovered, in agreement with other literature on the subject, that the vaginal microbiome becomes dominated by Lactobacilli species during pregnancy.  The Lactobacilli are thought to prevent pathogens from colonizing the vagina because they produce lactic acid which decreases the overall pH of the vagina, and they secrete antibacterial toxins.  These Lactobacilli are also important as they are normally the first to colonize the new infants' guts after they pass through the birth canal. 

The researchers also learned that the microbiome shifts away from Lactobacilli and towards a more diverse microbiome in the period immediately following birth.  The new bacteria that colonize are often associated with vaginosis, and these can lead to inflammation and infection of the birth canal in some women.  The scientists suspect this shift occurs because there is a sudden drop in estrogen production upon removal of the placenta.  The increase in circulating estrogen is thought to be important for Lactobacilli colonization, so it makes sense that the rapid decrease in estrogen decreases Lactobacilli abundance.

Finally, this study showed that there were geographic and ethnic variations to the pregnant microbiome.  While each microbiome was associated with a healthy pregnancy, there were important differences, especially on the species level.  For example, Asian and Caucasian women’s pregnant microbiomes were dominated by Lactobacillus gasseri, while this species was absent in black women’s pregnant microbiomes.

This paper helps show the normal progression of the microbiome during and after pregnancy.  With the mounting evidence that the microbiome is often a contributing factor to preterm birth and some post-partum diseases, papers like this one are important to some day discovering the mechanistic basis for our microbiome's association with these issues. 

The infant microbiome changes before the onset of type 1 diabetes

Type 1 diabetes (T1D) is a disease in which your immune system attacks and destroys your insulin-producing cells.  There is a known genetic risk factor in developing T1D, but there are also significant non-genetic components to getting the disease.  Previous research in mice has established the microbiome's connection with the development of diabetes, but the link in humans has not been studied as closely.  Researchers from various institutions in the U.S. and Finland recently assembled a cohort of infants genetically at-risk for diabetes, and tracked the changes in their microbiomes.  They discovered that the microbiomes of those individuals that were eventually diagnosed with diabetes underwent characteristic shifts leading up to diagnosis, and that these changes were not observed in healthy infants.  They published the results of their study in Cell Host and Microbe.  

The researchers sampled the stools of 33 infants in Finland and Estonia that were genetically at-risk for diabetes.  Their first major discovery was that even though the bacterial composition of the microbiome grew, changed, and became more diverse with age, the types and number of genetic pathways that were expressed by the microbiome, as well as the metabolites produced by the microbiome remained stable.  They also found many similar bacterial species between infants, however these infants usually had different strains of said bacterial species.  In most of these cases, once a particular strain established itself in the gut it remained stable and would not be displaced.

The scientists tracked the microbiome changes that occurred with diet as well.  During breast feeding Bifidobacterium and lactobacillus predominated, and Lachnospiraceae decreased.  After cessation of breast feeding the addition of eggs barley and soy seemed to have a direct influence on the microbiome.  One of the biggest factors in the developing microbiome was actually geography, as the Estonian infants had significantly higher levels of Bacteroides and Streptococcus species.

The researchers then compared the microbiome samples between those infants that were eventually diagnosed with diabetes and those that were not.  They discovered that a few bacterial species were much more abundant in those infants that got diabetes: Blautia, the Rikenellaceae, and the Ruminococcus and Streptococcus genera, including Ruminococcus gnavus and Streptococcus infantarius.  Interestingly, each of these bacteria are ‘pathobionts’, or bacteria which exist in many healthy peoples’ microbiomes but have the potential to become pathogenic.  Also, certain bacteria such as Coprococcus eutactus and Dialister invisus were non-existent in the diabetics' guts.  In addition, the researchers discovered that the expression of specific genes, like those associated with sugar transport and the biosynthesis of amino acids, underwent shifts prior to the onset of diabetes.  Finally, many of these bacteria that were associated with T1D appeared right before the onset of the disease, and these bacteria were linked to the presence and absence of certain metabolites in the stool.

These results provide exhaustive evidence for an association between the microbiome and diabetes.  It links specific bacteria in the microbiome and the expression of certain genes by the microbiome to the disease.  The next step is to study the mechanisms by which the microbiome induces diabetes, and then therapeutics can be developed.