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How do antibiotics affect the normal flora of the large intestine?

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There are estimated to be billions, if not trillions of bacteria of varying type living in the average human digestive system, especially in the intestinal tract.  Many of these bacteria are essential for the proper normal functioning of the digestive system. For example, bacteria are essential for the absorption of nutrients and the maintenance of the immune system.  So-called "good bacteria" are also essential for combating destructive bacteria, which can be dangerously potent.  When antibiotics are consumed, these medications usually cannot differentiate between good and bad bacteria; they simply kill both types, leaving the body to replenish its supply of good bacteria.  As with intensive chemotherapy intended to destroy cancer cells in the blood, however, wiping out the good with the bad leaves the body dangerously vulnerable to regeneration of the bad bacteria.  Not all the good bacteria regenerates, while the bad bacteria rapidly fills the vacuum.  The bad bacteria that regenerates, or new forms of bad bacteria that develop, become resistant to antibiotics and facilitate the development of disease and potentially-debilitating conditions like gastroenteritis.  Such pathogens as salmonella and others that cause inflammation of the large intestine can prove fatal if not treated expeditiously, and can kill even if treated quickly in cases where the strain of bacteria is resistant to current antibiotics -- an increasing problem with the over-prescription of antibiotics and proliferation of "anti-bacterial" household products.  

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