Some drink bottles come with very small opening making it hard to get a sponge or a brush in there for a good clean. Over time if a drink bottle is not cleaned properly or left for a period of time in the car or bag it can become quite slimy.
A simple trick to clean slimy drink bottles is fill it up with water and drop in a denture tablet, these can be found in the toothpaste section at the supermarket. Let the bottle soak for a few hours and give it a good rinse and clean with warm soapy water.