Water Bottles!

My son has been using these water bottles. He gets on absolutely great with them he got used to the straw straight away. I’ve started to notice they are going mouldy through the tube. We hand wash them (don’t have a dishwasher) well with soap and sponge and rinse them of soap then leave them on the rack to dry but I just don’t know where I’m going wrong for them to collect mould? Has anyone got any tips? Or can anyone give suggestions on good bottles?