Question

in which case bacteria are harmful and usrful

Answer 1

useful
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1)they are used in antibiotics 
2)they are used in medical for preparation of various kinds of medicines
3)it is used for experimental purposes 
4)they fix nitrogen into the soil
eg :- to study how the evolution , reproduction takes place

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harm

1)they cause diseases
2)they attack food and make it contaminated , which will not be fit for consumption
3)they damage our health , it kills white blood cells etc