This story is part of Home Tips, CNET's collection of practical advice for getting the most out of your home, inside and out.

How often do you clean out your reusable water bottle? Daily, weekly or monthly? Turns out, your water bottle could be harvesting harmful bacteria. 

I'll admit it. I'm guilty of it, as I'm sure so many of us are. I forget to clean my reusable water bottle regularly. I'm so preoccupied with trying to meet my daily water intake goal that I don't take time to consider the water I'm drinking. How bad is it to continue to drink out of a dirty water bottle? Am I putting my health at risk?

Dirtier than your kitchen sink and dog's water bowl?

Imagine how many germs are in your kitchen sink right now. Now think about your dog's water dish. According to a recent study conducted by New Jersey-based EmLab P&K on behalf of Treadmillreviews.net, an average reusable water bottle contains 313,499 colony-forming units of germs per square centimeter. That's more than a kitchen sink (3,191 CFU) and a dog bowl (47,383 CFU) combined. 

The study broke down reusable water bottles into four categories: squeeze top, straw top, slide top and screw top. Three different bottles in each category were swapped and tested, for 12 bottles total. Each water bottle had been used for a week without being washed. 

Of all the categories, the squeeze-top bottles were the germiest, with 99% harmful bacteria and 1% of bacteria that causes strep and staph. Screw-top bottles were the second grossest, with 98% harmful bacteria and about 2% of harmless bacteria. Slide-top bottles had the most variety of bacteria, with 33% harmful bacteria, 17% of bacteria that causes strep and staph, 17% of harmless bacteria found in nature and 33% of harmless bacteria. Lastly, straw tops had 8% of bacteria causing strep and staph and 92% of harmless bacteria. The study didn't specify if different bottle materials (metal, glass or plastic) were tested.  

It's important to point out that this particular study has yet to be peer reviewed. However, microbiology and immunology professor Charles Gerba, "Dr. Germ," from the University of Arizona, has supported the results of this study. He told Shape that "while the germs that come from your own mouth aren't likely to make you sick, the ones that are transferred to bottles from your hands are."

Factors in the amount of germs

While the EmLab P&K study had reliable results on the average CFU of germs living in our reusable water bottles, the results failed to take into account what the bottle was made of and how often it was refilled. A peer-reviewed study by the International Association for Food Protection took into account more factors.

The study collected and tested 90 water bottles; of which 65 were hard-plastic bottles, 12 were squeezable bottles, 10 were metal bottles and three were glass bottles. The exterior and interior of each bottle was then swabbed. 

Bottle material 

Four bottle materials were tested: hard plastic, soft plastic, metal and glass. Of those materials, the exterior of the glass bottles had the least amount of germs, while metal had the most germs. The hard and soft plastic bottles had around the same amount, in the middle between glass and metal. Glass bottles might carry the least amount of germs because glass is less porous than plastic and metal. It's also easier to see if the inside is dirty.   

Cleaning method

Participants were asked how often they clean their water bottle: never, just rinse, just wash or rinse and wash. Those who said they never clean and only rinse their bottles had a higher level of bacteria detected than those who rinsed and washed their bottles. 

Refill frequency

The results found that the bottles that were frequently refilled throughout the day had more contamination than those that weren't refilled as often. This may be because a bottle has to be touched more frequently when it's refilled and the water might spill or drip down the sides, causing a moist source for more bacteria to grow.  

Beverage type

The study asked its participants what the water bottle contained in the last seven days. 72 of the 90 water bottles contained just water, but 16 contained other beverages (coffee, tea, juice, sports drinks or soda). The bottles that had beverages other than water in them were significantly germier than the bottles used only for water. 

How to properly clean your water bottle

It's obvious that there's a clear link between an unwashed water bottle and the growth of bacteria. Here's how to keep you and your family healthy by cleaning your water bottles.

It's recommended that you wash your water bottle daily, or at the very least, every few days. Make sure to not only clean the inside but the outside as well.  

If your water bottle can go in the dishwasher, great! Most metal and glass reusable water bottles are dishwasher safe. Place on the top rack and make sure to take the cap off. You might need to wash the straw separately with a straw brush. 

If you can't put it in the dishwasher, don't panic. There's a safe way to hand wash these water bottles.

What you'll need

• Liquid dishwashing soap
  
• A bottle brush
  
• A straw brush
  
• A paper towel or a clean and dry dish towel
  

Fill your water bottle with warm water and a little bit of liquid soap. Using a clean bottle brush (or a brush small enough to fit inside), scrub the inside and the cap. Rinse and repeat. Clean the outside of the bottle as well. Always completely dry the bottle after cleaning. If your bottle comes with a reusable straw, consider buying a straw brush to clean the inside. 

Bottom line

After finishing this article, go clean your reusable water bottle -- especially if you haven't in the last week. You might just be carrying around harmful bacteria in your metal, glass or plastic water bottle. 

However, I'm in no way suggesting you should turn to one-use plastic water bottles or bottled water. We want to save the environment as much as you do. But for your own health, consider washing your water bottle if not every day, every other day. Don't share your water bottle with anyone else, and wash it especially if you're sick. If you're refilling it frequently throughout the day, dry any excess water on the outside. When you do wash it, use warm water, soap and a bottle brush. Your immune system will thank you. 

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.