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My classes have been watching a video produced by the Discovery Channel on bacteria. At one point in the video, the narrator says that human babies are born "bacteria free" but within a few hours hundred of kinds of bacteria colonize them. I can't believe that we are born bacteria free. Is this true or is this a mistake in the video?
Question Date: 2002-04-28
Answer 1:

When I first read your e-mail, I thought the statement in the video had to be a mistake as well. Human bodies are infected with probably hundreds (if not thousands) of different species of bacteria, and I assumed that it would be pretty hard for a mother to prevent passing some of these bacteria on to the developing fetus within her body. After doing some reading on the internet and thinking about it a bit more, however, I think the statement you heard could be correct.

The fetus spends most of the 9 months of its development in a placenta, bathed in amniotic fluid. This is all produced by the mother, and it would make sense if safeguards were taken by the mother's immune system to keep bacteria out. Mother's milk is sterile, so why can't amniotic fluid within the placenta also be sterile?

Scientists will probably never know for sure if the inside of the placenta and its amniotic fluid are truly bacteria free, since our ability to detect bacteria is often limited by the crude methods we have to use, which include checking for bacteria growth in the fluid after sampling and incubating it, looking for bacteria under a microscope with special fluorescent DNA stains or looking for bacterial DNA in the fluid. Most microbiologists would probably agree that we are missing (can't see) some fraction of bacteria using these methods. That said, tests of amniotic fluid and placental membranes show that they are "bacteria-free", or sterile, in healthy mothers. Infections in the amniotic fluid CAN occur if bacteria are present in the uterus when the placental membranes seal. Some of the strains of bacteria that can infect a mother's uterus during pregnancy include Ureaplasma urealyticum, Mycoplasma hominus, Bacteroides, and Gardnerella vaginalis. In fact, a recent study suggests that pregnancies that end in very premature labor (before 20-30 weeks) may be because the amniotic fluid has become infected. Early delivery, in this case, could stem from the body's reaction to the infection. Most babies are born free of infection by HARMFUL bacteria, in any case.

Occasionally babies ARE born with harmful infections. Infections may develop if bacteria enter the amniotic fluid, if the mother herself has a blood-born infection which is then passed on to the developing baby through her bloodstream, or if the baby is exposed to bacteria in the mother's birth canal after the amniotic sac ruptures (i.e. the mother's water has broken).

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