I made the cute mistake of leaving water in my desk water bottle—my beloved one that’s infused with shungite crystals—for a couple of days, and then realized I had unknowingly harvested mold in it. Whoops. Growing mold in your water bottle is a way, way too easy of a thing to do. This is something I’ve realized over the years of owning dozens of reusable bottles—you might say I’m not as diligent as I should be about properly cleaning them. But if you do as I do and simply neglect your precious vessel—just how do you deal?

“It doesn’t matter if it’s a bacterium or fungus—it just needs three things [in order to grow]”, says Jason Tetro, microbiologist and author of The Germ Files. “One is a nice, warm environment to grow, which is over 60 degrees Fahrenheit. And the warmer it gets, the more likely you’ll have growth. Two is water. And three is where it gets a little gross—sugar and salt are good. But other types of organic matter, such as the backwash from your cheeks and from your sinus cavities, will provide an ample amount of food for bacteria and fungi to grow. And so all three of those things are going to be in a water bottle.” Cool.

“[Some] types of organic matter, such as the backwash from your cheeks and from your sinus cavities, will provide an ample amount of food for bacteria and fungi to grow.” —Jason Tetro

It doesn’t take long for the trifecta to join forces to create mold, either—Tetro notes that it can just be a matter of days. “It takes a few days, most likely—unless it’s sugar water, at which point it’ll grow within 48 hours,” he says. “Usually it’s over the course of five days, which is usually how often people wash their water bottles. At that point there’s as very good likelihood you’ll have some kind of growth.”

Your mold can be one of two types, one of which is a biofilm. “One method of growth is on surfaces,” says Tetro. “The inner surface of a water bottle can essentially harbor biofilms, and these will allow numerous different species—including bacteria and fungi—to grow, and they’ll be able to stick there because biofilms are very sticky. So even rinsing and shaking your water bottle may not be enough.” Then he mentions “floaties,” a term he coined for the mold that floats on the top of the water’s surface. “These are the bacteria and fungi in the water itself,” he explains. “This is more rare, because you need to have a high level of food in there.” If you have a sports drink in a bottle for a few days, you may find floaties, but mainly he says you should be concerned about the biofilm growing.

So how can you avoid this mold, you may ask? The answer is twofold. “You’re going to have to clean it manually,” says Tetro, who highly recommends using a brush. “Use a brush, some soap, and hot water. What that will do is break up the biofilms so it’s easier for you to clean it.” BTW, if you have a bottle with a very small mouth opening, he advises to use your fingers to get underneath the lip—otherwise, ya can get mold there, too.

Then it’s key to disinfect the bottle, which really just requires boiling hot water. “If water is over about 160 degrees Fahrenheit, the bacteria and fungi won’t grow—they die,” he says. “So the best thing for you to do is boil water and put it directly into your water bottle and fill it straight up to the top. Add a little vinegar if you want because as an acid it helps to break up some of the stuff. Then let it sit for about three minutes. That’ll pretty much kill everything. And if you’ve gotten rid of the biofilms with a brush and on the lip with a towel, you’re pretty much going to have a clean water bottle.”

Do this at least once a week, but it’s better to do it every five days. But that’s not it (sorry). Tetro says you should also be washing the outside of the water bottle. “You’ve got to make sure you’re using some kind of friction, whether a sponge or a wet paper towel—just make sure the outside is treated,” he says. Okay, fine—off to the sink I go.

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