Plaque is a form of biofilm. It's a slime nest that bacteria develop to protect themselves. We have to brush our teeth to remove plaque. We can't rely on using an alcohol based mouth wash to kill off the harmful bacteria within the biofilm on our teeth, as the mouth wash can't break down the plaque biofilm. So, post brushing, those bacteria, often within biofilm cover our toothbrush. We therefore have to clean our toothbrushes.