If you've switched to using reusable bottles and thermal flasks, it's really important to give the insides of your bottles a good scrub once in a while with hot soapy water to prevent mold from lining the insides. We've found that swirling hot soapy water isn't enough as the mold still seems to be able to form, but a good scrub takes care of that!
The bristles of our bottle brushes are made of coconut fibres and the handles are made of wood - great if you're looking to go plastic-free in your kitchen!