Researchers have long pondered this question. For instance, Karl Benke, a food safety specialist at Purdue University in the U.S., has always been curious about how clean his reusable water bottle really is. One day, after wiping the inside of the bottle with paper towels, he was in for 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 the university hallway and asked if they could provide their water bottles for a study to check how clean they were.
Interestingly, many people were reluctant to know the results, Benke noted. They likely suspected their bottles weren’t clean enough. The test results confirmed that they were teeming with bacteria.
The global market for reusable water bottles was estimated to be around $10 billion in 2024. These bottles help our bodies maintain hydration. So, it’s unlikely we’re ready to give them up. Perhaps we just need to use them correctly?
What Lies Within a Reusable Bottle?
If water is left in a bottle for a few days, bacteria will start to multiply, noted Primrose Freestone, an associate professor of clinical microbiology at the University of Leicester in the UK. According to her, bacteria that cause infections in humans thrive at around 37 °C, but often they can multiply even at 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 conducted a study using boiled tap water, where theoretically most bacteria had been killed. However, it turned out that populations of microorganisms quickly expanded inside the bottles over the course of a day. If there were an average of 75,000 bacteria per milliliter in the bottle in the morning, within 24 hours that number skyrocketed to one to two million per milliliter.
Freestone claims that one way to slow bacterial growth is to store the bottle in the refrigerator between uses. However, this doesn’t completely stop bacterial multiplication.
Moreover, the outside of the bottle is always covered in microbes. They easily transfer inside along with bacteria from your mouth every time you take a sip, the expert explains.
According to Freestone, users of bottles who don’t wash their hands regularly may find that their bottles have accumulated bacteria such as E. coli.
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.
Another way to encourage bacterial growth in a bottle is to pour in something other than water. For example, 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 surround us everywhere: in the soil, 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. Members of this large group are common natural residents of the human gut. Only when bacteria become pathogenic—acquiring certain traits that make them harmful—do they cause disease.
According to Freestone, most microbes are not dangerous to humans, but people with weakened immune systems may be more susceptible to infections. Additionally, in some cases, gastrointestinal illness can lead to long-term changes in the gut.
People who have recently taken 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 also showed that these containers can be a breeding ground for new strains of antibiotic-resistant bacteria. Researchers discovered a type of bacteria called Klebsiella grimontii, which can form biofilms on sterile surfaces. Although this bacterium is considered part of the normal gut flora, it can cause severe diarrhea in those who have recently taken antibiotics.
If mold or fungus proliferates in the bottle, it can trigger symptoms in people who suffer from allergies.
How to Clean a Reusable Bottle?
After Benke’s experiment with paper towels revealed just how dirty the water bottle was, his team decided to delve deeper into people’s habits regarding these bottles.
Benke found that over 50 percent of the 90 participants surveyed admitted to sharing their bottles with others. Meanwhile, 15 percent stated they never clean their bottles, according to BBC.
Benke’s team discovered that participants who washed their bottles with a brush or used a dishwasher typically encountered the least amount of bacteria inside. The scientists also stated that the most effective solution might be using a dishwasher with a sanitizing cycle.
However, as the researchers noted, their results could have been skewed because the team relied on self-reports from participants. This means respondents might have altered their answers to appear more socially acceptable.
The study also showed that bottles used for tea, coffee, or juice were more contaminated than those used solely for water.
Regular and proper cleaning of our water bottles is the only way to ensure that you’re not consuming harmful bacteria along with your water. Even if the water inside is sterile, Freestone says, your saliva enters the bottle along with leftover nutrients that bacteria thrive on.
According to her, rinsing the bottle with cold water is not enough, as it won’t remove the slimy layer of bacteria from the inner surface that creates an ideal environment for their multiplication.
Freestone recommended washing reusable bottles with hot water (above 60 °C). High temperatures kill most pathogens. You should also use detergent, shaking it up and leaving it in the bottle for 10 minutes. After that, it needs to be thoroughly rinsed with hot water.
Then, the bottle should air dry well—this is the best way to avoid bacterial buildup, as microorganisms prefer moist environments.
You should clean the bottle this way after every use—or at least several times a week, Freestone advises. And never wait until it starts to smell. If the bottle begins to emit an odor, it’s time to toss it, the scientist emphasized.
She also added: once you have a new clean bottle, don’t forget to wash your hands before touching 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. However, scientists emphasize that the most important factor is not the material, but the cleaning strategy for the container.
The most hygienic bottles are those that are easiest to clean, according to Freestone. It’s also essential to ensure that you clean every part—the outer surface, the lid, and the straw if there is one.
However, there may be another reason to choose metal over plastic. “Plastics typically contain chemical additives that provide them with 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,” emphasized Amit Abraham, a researcher at Weill Cornell Medicine in Qatar (India).
Some studies have shown that these additives, such as bisphenol A (BPA), affect hormonal function and may be linked to a higher risk of heart disease, stroke, diabetes, and other ailments.
Additionally, plastic material can break down, increasing the concentration of microplastics in bottled water. Therefore, glass or stainless steel bottles are a safer alternative.
No matter which bottle you choose, your hygiene habits will be the most crucial factor. It’s these habits that determine whether the water you drink is teeming with harmful bacteria.