Firsty i tell that all bacteria are not useful for us.some of bacteria are useful in breaking milk and making curd, making cheese etc.
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isnt the second sentence showing that bacteria are useful for us?
Bacteria is useful in preparing antibiotics, some industrial chemicals. Bacteria are used for decomposing and cleaning oil spills on big water bodies, and decomposing wastes.
 there are some bacteria which are useful like rizobium it is persent in roots of legamas plants which converts the nitrogen in the air independently to nitrogen compounds
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he economic importance of bacteria derives from the fact that bacteria are exploited by humans in a number of beneficial ways. Despite the fact that some bacteria play harmful roles, such as causing disease and spoiling food, the economic importance of bacteria includes both their useful and harmful aspects.
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