|MadSci Network: Medicine|
Dear Ashley -
As you may know, the myriad of bacteria living in our intestines, on our skin, and in our urinary and upper respiratory systems colonize the body *after* birth. Inside the womb the fetus devlops in a completely sterile environment (or so one hopes).
Escherichia coli enters the baby's digestive system as most things do, through the mouth. Via suckling, placing things in the mouth, even breathing, it acquires bacteria from the surrounding environment. E. coli also enters from the opposite end, through the anus, to colonize the gut.
Like many other enteric ("living in the gut") bacteria, E. coli has a variety of features that help it colonize and survive in the deepest recesses of our bowels. The bacterium is "motile"; once it enters the gut it can swim to colonize distant locations. Many strains of E. coli produce sticky projections called "pili" and "curli" which help the microbes stick to the surfaces lining the gut. E. coli also possesses a diverse array of enzymes that help it use leftover wastes and complex carbohydrates produced by our intestine for food. Without a source of food, bacteria entering the gut would not be able to survive, let alone divide to produce new bacteria.
I wrote an answer to a related question: Development of E. coli in human intestines that should provide some further information about the process of microbial colonization in the gastrointestinal tract.
I hope this answers your question..
Be nice to your flora :).
Lynn Bry, MD/PhD
Department of Clinical Pathology
Brigham & Women's Hospital
Harvard Medical School
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