Can somebody tell me one helpful and one harmful bacteria in or on the human body?

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Escherichia coli (E. coli) is a bacterium that can be helpful or harmful to humans. How E. coli affects humans depends on where the bacteria are located in the human body.  

When E. coli is helpful:

 E. coli is helpful to humans when it is located in the large intestine.  Here. E. coli aids in the production of vitamin K2 and helps humans digest and absorb food. E. coli may even prevent other harmful bacteria from becoming established in the human body.  

When E. coli is harmful:

If ingested (eaten), E. coli can be harmful and cause infection within humans. Infection caused by the ingestion of E. coli may cause severe abdominal cramps, fever, and diarrhea that usually becomes bloody within the first 24 hours.

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