Yuk! I assume you wash it every time before you fill it up? Otherwise, I can confidently say that you're raising a potentially dangerous culture of bacteria on the neck of the bottle.

Every time you put your mouth to the bottle you transfer some spit to it.

Your spit contains a number of yummy proteins, carbohydrates, minerals and other nutrients, and also germs that are adapted to digesting those charitable contributions from your food, your salivary glands, and from dead cells sloughed from  mouth, gums and tongue.

The microbes include both germs and fungi, some from between your teeth, some from under the collars of gum tissue around the teeth, some from snot that made a detour as you were swallowing it as it travelled down the back of your throat, and lots and lots of it from the mat of fungi and detritus between the papillae on your tongue.

As you can tell, your mouth is a fascinating place, a vibrant ecology with an amazing number of niches. You could practically go about with your mouth open for all your friends to admire. What else are friends for?  For other spectators and visitors you might levy a small charge.

Of course, everything comes at a cost, and one of the costs in this case is that when these things accumulate on your increasingly nutritious bottle of water, it becomes increasingly fragrant as well. Some of your bacteria, probably in combination with some of your fungi, form what is called a biofilm on the bottle and some of their metabolic products become progressively more obvious as they accumulate. Such biofilms are ubiquitous in nature and in technology. One finds them in the films of slime where water runs over rock, or in equipment in sewage dams, where they are of great value in breaking down unwanted substances, sometimes actually converting them into valuable materials.

Now, whether you want to conserve your biofilm on your bottle or not, is up to you. As Pete said, it is possible that you might build up a dangerous bacterial culture, but frankly I am sceptical. I think it is just a fragrant biofilm. The chances are that if you do not interfere with it, it will do no worse than occasionally flaking off and continuing to produce that smell, which when you come down to it is very much like the process that produces the aromas that are so expensive in fancy cheeses. It is all a matter of taste, and when it comes to your own bottle of water, who is anyone to criticise yours?

And anyway, would you have the heart to disturb such a beautiful community by periodically washing it with peroxide or bleach?

I think this is a build up bacteria from your mouth. Remember a bite from a human is far more dangerouse than from a dog.

The bottle will have been sterilised at the plant and the water was probably passed through UV light to kill anything off without effecting its mineral content and flavour

J

Our mouths are home to around 700 types of bacteria. As well as harmful organisms, which can cause tooth decay, gum disease and permanent bad breath, there are "good" bacteria, which promote oral health by stopping the harmful ones proliferating.

When you drink directly from a bottle, you leave some of your oral bacteria and saliva on its neck. The saliva contains food debris and dead cells on which oral bacteria can thrive. If you don't wash the neck after you have drunk from the bottle, the bacteria left on the plastic will break down nutrients in the debris and release the unpleasant stale smell your correspondent noticed. The smell is always the same because your bacterial flora stays the same.

This is similar to the situation that causes "morning breath". During the night, your saliva flow slows and is less effective at washing out food particles and delivering oxygen to the bacterial flora. This stimulates the growth of anaerobic microbes, which are particularly smelly - hence bad breath in the morning.

Bad breath is likely to be more pronounced if you have been breathing through your mouth, as this will dry out the saliva, further cutting the chances of a good wash-out. One reason for drinking is to wash out a dry mouth, making it particularly likely that material left on the bottle's neck contains problem-causing bacteria and debris.

Joanna Jastrzebska, Auckland, New Zealand