Welcome to JA and thanks for this question. Actually while it is true that you have bacteria lining the urethra, when we geta urine sample we always ask for a mid stream sample, the reason being the initial part of the urine sample can be contaminated and once those contaminants are passed in the urine, the mid stream sample should represent the true bacteria which are present inside the urinary tract.
Secondly, the small amount of bacteria which are present on the urinary tract usually do NOT grow into a large population which can cause an infection and usually e.coli is not found in that sample. The e.coli which could be present normally is a variety which does not cause an infection as there are thousands and even hundreds of thousands of strains of each bacterium, some of whom can cause an infection while others may not.
The urine left out of the body would NOT grow bacteria unless the conditions are ideal, the temperature needs to be correct and they need a host of factors in order to propagate and grow and that is why most bacteria would not grow on a simple urine sample left outside.
Lastly, the recommendations to treat a UTI is when you actually have symptoms OR if the colony count of the bacteria is greater than 50,000 CFU/ml. The reason for this is because it has been seen that if the colony count is less than this number and there are no symptoms, the bacteria would usually be cleared by the body itself however if symptoms occur then that would indicate that the body has failed to clear out these bacteria and thus would need treatment. Numerous studies have indicated that a count of less than 50,000 would be more consistent with simple colonization and that would not lead to any issue in most patients. Only in diabetics or patients who are immunosuppressed would treatment be necessary in this case.
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