Your old brush though looks naive, it’s a great magnet for bacteria. The toothbrush can contain up to 10 million bacteria, including E. coli and staphylococci, a study by the Manchester University.
If you drop the brush on the floor, the five-second rule does not apply.

Here’s how to keep your toothbrush:

  1. Change the brush regularly
    Changing brush dentists recommend on every three months, but after they recovered from flu, bacterial infection or a cold.
  2. Use the right toothpaste
    Some toothpaste is better because they kill germs. Buy one containing a triclosan or copolymer. They are better than regular fluoride toothpaste at killing oral bacteria.
  3. Do not share your brush with someone
    Take care of your bacteria. When pouring the toothpaste on the brush, do not place the opener on the brush itself. It is better not to get a physical contact between the toothpick and the brush itself.
  4. Clean your brush thoroughly
    Thoroughly rinse your toothbrush after use. From time to time, rinse it with a mouthwash or hydrogen peroxide.
  5. Keep the toilet lid closed
    Always close the lid on the toilet before you flush the water.
  6. Store it properly
    Most people holding a toothbrush on the sink in the bathroom, but this is the worst place to store! The reason for this is the immediate proximity of the toilet, from which every flush of water is transmitted countless bacteria, but also washing the hands over the sink.
    Dentists say the brush should not be closed in an airless place. It is best to keep the brush in the cabinet – where it is air enough, but it is also protected from the external influence of the bacteria. If you hold two brushes together, do not touch their “heads”.

Source: https://www.b92.net/zdravlje/prevencija.php?yyyy=2014&mm=04&nav_id=841070