

I’ve been using the same few Hydrocell bottles for years. (They’re a stainless steel vacuum insulated bottle, basically a significantly less expensive Hydroflask knockoff.) I use them mostly for water, but occasionally other drinks like unsweetened tea, sugar free electrolyte drinks, etc.
I have never had a mold problem with these. I just disassemble them and run in the dishwasher every few days. (Before I had a dishwasher, I would wash them with hot water and dish soap, lightly scrub, and air dry.)
This really sounds like a cleaning problem to me. I’d recommend washing water bottles every 3-4 days, and if you use a bottle for anything other than water (especially anything with sugar) wash it that same day.
Mold doesn’t magically grow out of nowhere, it needs “food”, a nutritious medium. This could be sugar in a sugary drink, protein in a protein drink, or just contamination from having been in contact with a mouth too many times without proper cleaning.
Also, the quality of the water you’re putting into the bottle makes a difference too. If you’re filling up at the water cooler at the gym or at work, for example, it could be that the water cooler isn’t being cleaned properly and has mold growing within. That would certainly accelerate things in your bottle.
It took me a minute to realize that “MVP” means “most valuable player” to most people.
Apparently I’ve been in tech far too long, and care about sports far too little.