Need help with a glass bottle?

So, I picked up this glass bottle recently, but after using it for a bit, I’ve got mold growing inside—just these little black spots sitting at the bottom. I need a way to kill them off. I can't exactly scrub them out manually because the bottle is way too deep for any brush I own. Does anyone have a better tactic for getting rid of this stuff?

grab that little brush they use to scrub out those baby bottles

Thanks, but what I'm really looking for is whether anyone knows of a specific solution (or something similar) that I can use to wash out a bottle.

Give that descaler for water and coffee machines a shot

and btw, you're posting in the wrong subforum for this

Citric acid...

Man, I think this ended up in the wrong subforum.