Does anybody know how to get mold out of the bottom of a water bottle?

I just recently discovered that both of my water bottles have mold growing in the bottom of them. I don’t have access to a dishwasher and I tried washing them out with normal dish soap and it didn’t work at all. I even soaked it over night to see if that would work, but it didn’t at all. Has anybody had to deal with this before, or does anybody just have any ideas?