There is an old saying, out of sight, out of mind. Sometimes there isn't wisdom in this...
I live in an area which is often rainy and humid (I think much of humanity does) and so mold and algae grows where it can (often where you don't want it). I often wonder about what grows inside things which are out of sight.
Today I broke open the toothbrush head to have a good look.
Here it is
As you can see there is quite an amount of stuff which is essentially green in there.
That is most likely mold, but it could be a kind of algae.
I often rinse it in mouthwash just to prevent it from having a perfect environment to live in, but perhaps molds are not effected by mouthwash? If its an algae these things can quickly form biofilms (which resist attacks from nasty chemicals that may kill the organism) so I wonder just how effective that is. Clearly it isn't very effective is it.
I also pathologically dry my toothbrush after use by spin drying it using a tool I made (don't ask). Yet still it harbors this within a month of use.
So while it may be out of sight, its not out of your mouth ;-)
I'm now wondering about leaving it immersed in a mouthwash solution when not in use ...
PS: I am now going back to dropping my toothbrush head into alcohol. I'm using Gin at the moment, but OP Vodka could also do the trick. Metho will work but taste bad.