Is it reasonable to test purified water for the presence of E. coli?

For drinking water E. coli is already defined as an exclusion germ. Thus, one could expect (in case of a negative result) that the feed water of the PW system is free of E. coli. Nevertheless, it may be useful to test purified water for E. coli at production tapping points to monitor the general hygiene status in the manufacturing plant.

[GMP Compliance Adviser, Chapter 5.A]