Tap water does contains some bacteria, the water treatment plant puts in just enough anti-bacterial agent that the water is going to be within their specification only for the time it takes to get to your tap. They aim to put the minimum amount of treatment agent in because it costs money and people don't like it.

Bottled water is generally just tap water filtered to remove some minerals, it's not generally treated to remove any more bacteria and the extra handling steps could introduce more.
On the other hand it is sealed in a reasonably clean factory - unlike you filling a 50 gallon drum with a hose pipe and your dirty hands. Gallon jug bottles of water are also easier to handle and are consumed faster when opened than a 50Gal drum.

The advice used to be to add 1/2 teaspoon/5 gallon of chlorine bleach to stored water. Probably can't do any harm.

After a few years the stored tap water might have some bugs in it, but it isn't likely to suddenly contain cholera, polio, typhoid or a bunch of parasites so it's still better than the water supply for most of the world.