So, this made me think: How gross is it really not to wash your reusable water bottles after each use? To get to the bottom of it, I caught up with Trevor Craig, a food safety expert and corporate director of technical training and consulting at Microbac Laboratories, to learn more about what’s happening inside reusable water bottles (on the microscopic level). As it turns out, it’s a feeding ground for bacteria, which is why Craig recommends washing your water bottles daily. Gulp.

Ahead, Craig explains why washing your water bottles should be a daily practice. Meanwhile, Julianne Ryan, director of product management at YETI, shares the best tips for cleaning your water bottles to remove all that icky grit and grime.

How often should you wash your reusable water bottle?

According to Craig, it’s plain and simple: You should wash your reusable water bottles daily since potentially harmful pathogens can spawn quickly and spread rapidly. “Some bacteria—like E.coli—in the right conditions can duplicate in under an hour, meaning if you cross contaminate your bottle with bacteria—which is very easy to do—those counts could get very high in just a day,” Craig says.

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Now, as for the areas of most (bacterial) concern, Craig says it’s the straws, rubber seals, and spouts. “The areas that would grow bacteria the fastest would be ones that are wet but can get semi-dry occasionally, like the straw or rubber seals of the lids. The spout could also be a concern,” Craig says.

What happens if you don’t wash your water bottle regularly?

Even though it’s not abnormal for a certain degree of bacteria to be present in water bottles, the longer a bottle goes unwashed, the more bacteria can grow. “Bacteria are always going to be in your water bottle to some level. We have it naturally in our mouth, face, skin, nose, etcetera, and it’s easy for that bacteria to get inside your bottle and water, which you then drink. While that bacteria may be naturally occurring, when it’s given the options of overgrowth or getting to areas in your body where it can really thrive, it can cause other issues,” Craig says.

Although the bacteria may not be visible to the naked eye, Craig says it can still make you ill. “What’s worse is these bacteria can form films or clumps that you might not see or even taste as you’re consuming them,” he says. By that point, the bacteria count is likely in the millions. “It only takes a few harmful bacteria that could make you sick,” Craig adds.

Plus, Craig says flavorings or other ingredients can also promote bacterial growth. “Flavoring, powders, proteins, and other additives in your water can add more variables and nutrients to feed those bacteria,” he says. In which case, plain water might be slightly less likely to promote bacterial growth than flavored water.

The good news? The type of bacteria you’re dealing with isn’t as harmful as other common foodborne pathogens, comparatively speaking. “Luckily, you’ll likely not have too many dangerous pathogens in your water unless you’re being really unsafe, but the possibility is always there. It’s more likely that the bacteria you consume could give you an upset stomach, but not going to give you full-blown foodborne illnesses, like salmonella, listeria, or E.coli,” Craig says.

What’s the best way to clean a reusable water bottle?

According to Craig, hot, soapy water is all you need to wash your reusable bottles. “Make sure to get all the edges and clean the hard-to-reach areas like straws, lids, and spouts,” he adds. Meanwhile, Ryan agrees mild dish soap and warm water will do the trick when washing YETI’s products, like its Rambler and Yonder drinkware. The key? Getting all the nooks and crannies squeaky clean, which is why Ryan recommends washing the body, caps or lids, and any additional accessories separately.

Ryan also says YETI’s drinkware are all dishwasher safe, to help ensure they’re as easy to clean as possible. But even in the dishwasher, she recommends that each component should be washed separately. “The drinkware body should be placed standing, upside down on the top or bottom rack, depending on where it fits best,” Ryan says. As for the lids and bottle caps, she recommends the top rack of the dishwasher away from the heat. Meanwhile, the gaskets or the magnetic slider (MagSlider) should go in the silverware tray on the bottom rack of the dishwasher.

Once it’s all nice and clean, Ryan recommends “reinstalling the gasket by placing the larger blade around the gasket above the smaller blade and ensuring all parts of the cap are dry before reinstalling and storing it.”

As for what not to use: anything too abrasive that may damage the lining of the bottle. “Do not use bleach or abrasive cleaners and note that stiff bristle brushes may scratch the Yonder bottle, so we recommend avoiding those,” Ryan says. “I recommend not using a metal scrubber as it can damage the inside of the container which can cause areas for bacteria to get into, and make sure to also turn your bottle upside down to drain all the water,” Craig adds.

Can storing the water bottle in the fridge help with bacteria?

According to Craig, cold temperatures can help with bacterial growth only slightly. “Cold temperatures can slow down the growth of bacteria, but it’s just slowing it down. It doesn’t kill or stop the bacteria, and they will still grow,” he says. To put it into context, he says it’s not different than leftovers that turn moldy or go bad, even when stored in the refrigerator.