Microbial limit test

Sir , I have a dout about microbial limit test.

Whenever we are going for analysis of any water samples or finished product for pathogen like e coli or any other is there positive control plates are required for analysis or not… if not why or if yes why?.

Positive controls are required every time you analyze anything to prove that growth factors and conditions support the microbial growth.

What are acceptances criteria for BU and CU as per IP for process validation batches in OSD