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
