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

A Bacteria that is helpful:

E.Coli is a type of bacteria that is helpful to humans. It is a bacteria that is found in the intestines, and helps  with the breakdown and digestion of the food that a person consumes.

A bacteria that is harmful:

An example of harmful bacteria is Staphylococci. It affects humans by causing problems with their skin. Problems include boils, pustules and abscesses. This bacteria further causes other problems such as infections in the bones, joints and wounds - which are very painful. This bacteria can also enter the joints and heart valves (seldomly occurs though) during an operation (surgery) can be life threatening due to the severe infection it may cause. 

SUMMARY:

Bacteria helpful to humans: E.Coli

Bacteria that is harmful to humans: Staphylococci