Karl Benke, a food safety specialist at Purdue University in the U.S., wanted to know how clean his reusable water bottle really was. One day, after wiping the inside of the bottle with paper towels, he got a shock.
“The towels were white until I pulled them out. I realized that the slimy feeling inside the bottle was caused not by the material, but by a buildup of bacteria,” the expert recounted.
Benke’s team then stopped people in a university hallway and asked for their water bottles to include them in a study checking cleanliness.
Many people were reluctant to see the results, Benke said — probably because they suspected their bottles weren’t as clean as they thought. The tests confirmed the bottles were teeming with bacteria.
The global market for reusable water bottles was estimated to be around $10 billion in 2024. They help us stay hydrated, so it’s unlikely we’re ready to give them up. Maybe we just need to use them correctly.
What Lies Within a Reusable Bottle?
Primrose Freestone, an associate professor of clinical microbiology at the University of Leicester in the UK, says bacteria will start to multiply if water is left in a bottle for a few days. Bacteria that can infect humans do best at about 37 °C, she notes, but many will multiply even at around 20 °C.
“The longer water sits in a bottle at room temperature, the more bacteria will grow in it,” the scientist said.
Researchers in Singapore ran a study using boiled tap water, where most bacteria should have been killed. Still, microbial populations expanded quickly inside bottles over the course of a day. If there were an average of 75,000 bacteria per milliliter in the morning, within 24 hours that number rose to one to two million per milliliter.
Freestone says one way to slow bacterial growth is to store the bottle in the refrigerator between uses, but that doesn’t stop bacteria entirely.
The outside of a bottle is also covered in microbes. They transfer inside along with bacteria from your mouth every time you take a sip, the expert explains.
Freestone warns that people who don’t wash their hands regularly may find bacteria such as E. coli building up on their bottles.
The human mouth typically harbors 500–600 different species of bacteria. “What may not necessarily be harmful to you can be harmful to others,” Freestone added.
Pouring something other than water into a bottle encourages bacterial growth. Sugary drinks promote the growth of bacteria and mold.
“Everything except water is a paradise for bacteria and fungi, especially protein shakes,” Freestone said.
How Do These Bacteria Affect Us?
Bacteria are everywhere: in the soil, in the air, and on our bodies. Most of them are harmless or even beneficial.
Water contaminated with bacteria like E. coli can cause diarrhea and vomiting, but not always. Many E. coli strains are normal residents of the human gut. Only when bacteria acquire traits that make them pathogenic do they cause disease.
Freestone says most microbes are not dangerous to healthy people, but people with weakened immune systems may be more susceptible to infections. In some cases, gastrointestinal illness can also lead to long-term changes in the gut.
People who recently took antibiotics that affected their gut microbiome are also more vulnerable to other infections. An analysis of a reusable water bottle taken from the editorial office of a British newspaper showed that these containers can be a breeding ground for new strains of antibiotic-resistant bacteria. Researchers found a type called Klebsiella grimontii, which can form biofilms on sterile surfaces. Although this bacterium is normally part of the gut flora, it can cause severe diarrhea in people who recently took antibiotics.
If mold or fungus proliferates in a bottle, it can trigger symptoms in people with allergies.
How to Clean a Reusable Bottle?
After Benke’s paper-towel experiment revealed how dirty a bottle could be, his team dug deeper into people’s bottle habits.
Benke found that more than half of the 90 participants admitted to sharing their bottles with others, and 15 percent said they never cleaned their bottles, according to the BBC.
Participants who washed their bottles with a brush or used a dishwasher typically had the least bacteria inside. Using a dishwasher with a sanitizing cycle may be the most effective option.
However, the results could be skewed because the team relied on self-reports from participants, which may have led some people to change their answers to seem more socially acceptable.
The study also found that bottles used for tea, coffee, or juice were more contaminated than those used only for water.
Regular, proper cleaning is the only reliable way to avoid consuming harmful bacteria with your water. Even if the water is sterile, Freestone says saliva and leftover nutrients enter the bottle and feed bacteria.
Rinsing a bottle with cold water is not enough to remove the slimy bacterial layer from the inner surface that creates an ideal environment for growth, she warns.
Wash reusable bottles with hot water (above 60 °C). Use detergent, shake the bottle and leave the soap inside for 10 minutes. Rinse thoroughly with hot water.
Then let the bottle air-dry completely — microbes prefer moist environments.
Clean the bottle this way after every use, or at least several times a week, Freestone advises. Don’t wait until it starts to smell. If the bottle begins to give off an odor, toss it.
Also, after you have a new clean bottle, wash your hands before handling it.
Are There Water Bottles to Avoid?
Some studies have shown that the bacterial load on plastic bottles can be higher than on stainless steel bottles. But the most important factor is not the material — it’s how you clean the container.
The most hygienic bottles are the ones that are easiest to clean, Freestone says. Be sure to clean every part: the outer surface, the lid, and any straw.
There may be another reason to choose metal over plastic. “Plastics typically contain chemical additives that provide advantages such as flexibility, durability, heat resistance, and light weight. These additives are physically bound to the plastic, meaning they can leach into the water,” said Amit Abraham, a researcher at Weill Cornell Medicine in Qatar.
Some studies have linked additives such as bisphenol A (BPA) to hormonal effects and a higher risk of heart disease, stroke, diabetes, and other ailments.
Plastic can also break down, increasing microplastics in bottled water. Glass or stainless steel bottles are a safer alternative.
No matter which bottle you choose, your hygiene habits are the crucial factor. Those habits determine whether the water you drink is teeming with harmful bacteria.
