One thing that becomes more clear with each piece of research is that the human body is a hive of mostly harmless bacteria that live in any crevice they can reach while affecting their human host as little as possible. In some cases these bacteria can be very beneficial - preventing more dangerous bacteria from taking up residence in places like the stomach and throat. In some cases they can occasionally go rogue, get into places they shouldn't be, and cause havoc.
Despite the relatively safe and prosperous homes that the human body provides, living on it does have it's downsides. As the body changes, grows and falls ill the bacteria are subject to seismological shifts in their living quarters. And for bacteria that live around the female reproductive areas, pregnancy represents a pretty major shift indeed.
Just to clarify, bacteria do not actually live in the womb (if they get inside there that can go very badly wrong) but they do set up camp along the vagina and basically any areas of the reproductive system (and indeed the body) that can be reached safely from the outside. As well as protecting against fungal and other pathogenic infections simply by colonising the available space, the vaginal bacteria also actively help to decrease the pH inside the vagina, making it harder for other infections to set up. Killing off these bacteria (as anyone who has ever taken antibiotics to deal with cystitis will be aware) can lead to the fungal infection thrush, as without the bacteria in place the fungus can take over.
In order to explore how the vaginal bacteria might change during pregnancy, researchers took 68 samples from 24 healthy pregnant women (between 18-40 confirmed weeks) and compared them to 310 samples from 60 non-pregnant controls. They isolated DNA from the vagina, sequenced it, and then organised it into different bacterial taxonomic groups. All the control women were non-menstruating at the time, and around half of them were taking hormonal contraceptives. Full details can be found in reference 1.
Overall, pregnant women showed much less diversity in bacterial species and fewer colonies present, particularly in areas up near the uterus which were very sparse indeed. In both pregnant and non-pregnant women, colonies varied throughout the vaginal area, and the dominant species were Lactobacillus spp. Some particular bacterial species were found to be more prevalent in pregnancy, although the overall diversity was reduced. This snapshot of the vaginal bacteria flora as a changing landscape; affected by internal pH and decimated by oncoming pregnancy, is a fascinating one, showing a wonderful interaction between humanity and the bacteria that live inside them.
But pregnancy doesn't just involve changes in bacteria, it also comes with a baby! The womb where the foetus grows is a comfortable and sterile environment, and suddenly in a short space of time it is required to be forcibly pushed down a narrow bacteria-laden tube into a cold unforgiving world. In a fascinating (although rather underpowered) study of infants born by either by vaginal birth or C-section, researchers (reference 2) did indeed find a difference in the bacterial communities. Babies born through vaginal delivery had typical vaginal species such as Lactobacillus, Prevotella, or Sneathia on their skin, throat and gut while babies born through Cesarean had picked up species of skin bacteria (Staphylococcus, Corynebacterium, and Propionibacterium). Furthermore while in adults there are different bacterial species in different places (skin, gut, vagina, etc.) in the newborns it was the same bacterial species everywhere. Given that they'd only just entered the world from a sterile environment this isn't surprising, but it does raise interesting questions about how the different species find their way into the infant and start to flourish in their own little niches.
Speaking as one of the many, many C-section babies out there I can't say my method of birth had a huge impact on overall health, or indeed my relationship with bacteria! But what is clear is that bacteria are with us throughout our life, changing to match our bodies, and growing and fluctuating as we go through different stages of health.
Ref 1:Kjersti Aagaard, Kevin Riehle, Jun Ma, Nicola Segata4, Toni-Ann Mistretta, Cristian Coarfa, Sabeen Raza, Sean Rosenbaum, Ignatia Van den Veyver, Aleksandar Milosavljevic, Dirk Gevers, Curtis Huttenhower, Joseph Petrosino, James Versalovic (2012). A Metagenomic Approach to Characterization of the Vaginal Microbiome Signature in Pregnancy PloS one, 7 (6) DOI: 10.1016/j.ajog.2010.10.087
Ref 2: Maria G. Dominguez-Bello, & et al. (2010). Delivery mode shapes the acquisition and structure of the initial microbiota across multiple body habitats in newborns Proceedings of the National Academy of Sciences, 107 (26) DOI: 10.1073/pnas.1002601107