There's two occasions when I'll reach for the sterilising tablets:
1) if bottles have been left half-full for a few days and develop a funky bacterial film on the inside. Can't see it, but you can
feel it (slippery)
2) if I cycle through some foul-smelling run-off from a drain or farm yard ; then I worry about 'spatter' on bottle nozzles
Where did this fear of bacteria come from? People don't understand that they are ingesting all kinds of bacteria in their food and drink at every meal and the stomach acid deals with 99.99% of them. It's only when you get a big dose of a nasty like E Coli that the bacterium makes it through to the gut, if the stomach doesn't reject it first.
Agree- the human immune system is incredible at dealing with these attacks.
But since you mention E-Coli specifically, the fear is about the bad strains which have only been identified since the 1980s; they're incredibly infectious, sometimes fatal (particularly in children and the elderly), and there's still no antibiotic; you just have to let the infection run it's course.
Thankfully outbreaks are still pretty rare, and there's no way it should ever be present in treated water.