J.G.
I don't have an answer, but a suggestion for when you get one.....
WASH with hydrogen peroxide fairly often. The brushes I've bought my son (whatever character he liked. I don't know what brand).... well they have an oblong little hole towards the top behind the bristles. After a while - that hole collects yucky bacteria or what-not. I pour hydrogen peroxide in the hole and on the bristles and it starts to foam up quickly and brown yuckies comeo out of the hole.
Needless to say - if you get an electric brush and it has this oblong hole (not sure why it's there or what it's function is), clean often or trash and replace often. I have always trashed a toothrbrush even before the battery runs out.