Following reasons:

1. To get actively growing bacterial cell population

2. In same phase of growth

3. to achieve near accurate cell density for inoculating main culture for reproducible protein expression

Gene expression and hence protein expression depends is an orchestrated event between the nucleus and the cytoplasm for which all the cells need to be in same phase for optimal protein expression. Therefore, starter culture is needed before main culture in Molecular microbiological assays (protein expression studies)

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