|MadSci Network: Development|
Thanks for your interesting question. You have asked one that touches on a number of topics. Though you may not realize it, you have a highly complex and diverse microbial ecosystem living in the deepest recesses of your bowels.
The adult human body contains an estimated 1 * 10^14 cells (a 1 followed by 14 zeros..), 10% of which actually belong to us!! Thus, we have roughly 10 times more cells living in us than we do cells that make up our own bodies. Most of these 'outside' cells are bacteria living in the gastrointestinal tract. More than 400 species of microbes live in the colon alone, and can reach a density of 1 * 10^11 organism per milliliter of 'intestinal lumenal contents.'
Escherichia coli is one of the many organisms that lives in the human gut. However, in spite of the media coverage it receives, it is a relatively minor component of the gut flora. Appoximately one third of all bacteria living in the intestine belong to the genera Bacteroides. These bacteria are anaerobic (they can't live in the presence of oxygen). Other common genera found in the gut include the Bifidobacteria, and Eubacteria. Like Bacteroides, these microbes are also anaerobes, but have a different cellular structure from the Bacteroides. E. coli differs from the above organisms in that it is a facultative anaerobe - it can live in the presence or absence of oxygen. In the adult intestine it comprises less than 1% of the total number of organisms.
How bacteria come to live in the intestine is a very interesting question. We are all born essentially sterile, with no bacteria in out gut, respiratory or genitourinary systems. Microbial colonization begins at birth as we come in contact with bacteria in the air, on all surfaces, in milk and foodstuffs. Aerobes (microbes that need oxygen to live) and facultative anaerobes are usually the first colonizers of the gut. There is evidence that microbial colonization progresses through a series of stages, starting first with the aerobes and facultative anaerobes. As these initial colonizers consume oxygen, they create environmental niches that can be more readily colonized by the anaerobes. As E. coli is a facultative anaerobe, you would find higher numbers of this organism in a young infant's bowels. Colonization occurs primarily through ingestion of the organism. I should point out that the E coli commonly found in your GI tract is usually of the non-pathogenic variety, unlike the strains of E. coli that you hear about in the news media.
Given that we and all other multicellular organisms evolved in a world populated with microbes, it should perhaps not seem so surprising that we have come to adapt to them as well as they have managed to adapt to our intestinal environment. The bacteria living in the gut play a number of vital roles. Ruminants including goats and cows need bacteria to help them digest their food. The microbes living in the gut provide us with a source of vitamin K - a cofactor needed for the proper clotting of blood. All too often we only consider that 'bad' role of microbes in disease, without consider the many benefits we receive from them.
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