Reusable water bottles are a breeding ground for strep and fecal bacteria, study warns

Carrying a reusable water bottle is a smart habit. It saves money. It cuts down on single-use plastic. It keeps you hydrated. There’s a catch, though.

Many of us don’t clean our bottles nearly as well or as often as we think we do, and the germs that hitch a ride on our hands, rims, and lids can stick around.

In 2024, the global market for reusable water bottles was valued at about $10 billion, with surveys of university students reporting usage rates ranging from 50% to 81%.

A study on the campus of Purdue University put everyday bottles under the microscope to see how clean they really are and which habits help or hurt. The takeaway is a reality check with simple fixes that anyone can put into practice.

Studying reusable water bottles

The research team kept the question simple: “How clean are the bottles that students actually use all day?”

To get real-world answers, they collected 90 bottles directly from students. They asked owners how they used and cleaned their bottles, then tested the bottles in two ways.

First, they swabbed the outer surfaces using a rapid hygiene test called ATP that lights up when organic residue remains. This signal is widely used in food safety as a quick screen for whether a surface is truly clean after washing.

Second, they rinsed the insides and counted bacteria clinging to water-contact surfaces. They measured common water-loving bacteria and checked for coliforms, an indicator group that can signal when conditions could support more concerning contamination.

The study was led by Carl Behnke from Purdue University with a cohort of academics and scientists. Later in the process, Dr. Yuriko Fukuta, an assistant professor of medicine (infectious diseases) at Baylor College of Medicine, added clinical context.

Their goal wasn’t to name and shame anyone’s bottle. It was to map daily habits to measurable cleanliness, so people could tweak what they do and keep their bottles safer.

What’s on the outside

The outsides of bottles were dirtier than they looked. ATP readings indicated that the exterior surfaces of all the water bottles collected (including two new bottles that were purchased from a local retailer as controls) were considered “dirty” and failed the ATP test.

That’s not surprising in daily life, where hands move from phones to doorknobs to laptop keyboards to bottle caps. Residue left behind gives microbes places to hang on to.

More frequent refills were associated with dirtier outer surfaces. Each refill adds touchpoints: your hands, the cap, counter edges, and faucet areas. The rim matters here because it is where hands and mouths meet.

What’s on the inside

Scientists tested the inside of people’s reusable bottles to see how many live bacteria were inside. Think of the number they report – CFU per milliliter – as how many tiny bacterial “colonies” can grow from a small sip of liquid rinsed off the bottle’s inner walls.

Many places set the bar for drinking water at below 100–500 CFU/mL. In this study, about 7 out of 10 bottles cleared 100, about 2 out of 3 topped 200, and 3 out of 5 exceeded 500. In plain English: most bottles carried more bacteria than you’d want in clean drinking water.

Researchers also checked for “coliforms,” the bacteria we use as a red flag for sanitation. For bottled water, the rule of thumb is basically “none detected” (no more than 1 per 100 mL). Here, roughly 1 in 4 bottles broke that limit, and a few were sky-high.

Bacteria need three things: moisture, nutrients, and time. A bottle provides moisture by design. Nutrients show up when anything other than water goes in, and time accumulates when a bottle sits in a warm car, on a desk, or by the bed overnight.

One clear pattern stood out. Bottles used strictly for water tended to have lower bacterial counts. Bottles that also held sports drinks, soda, or sweetened tea or coffee with cream often had much higher counts.

Sugars feed bacteria. Proteins and fats can leave a film that helps cells attach and form biofilms, which are much harder to remove than free-floating cells.

Cleaning reusable water bottles

Material played a limited role. Glass tended to look cleaner on the quick outside test, likely because its surface is smoother and nonporous. But no material gets a free pass.

Design features – narrow necks, flip tops, straws, bite valves, and silicone gaskets – create nooks where residue and biofilms hide. Those tucked-away spots are often the last to be disassembled and scrubbed.

People who cleaned more often generally had better results, but intentions didn’t always match outcomes.

A quick rinse can leave sticky films intact. Dishwashers vary in heat and spray coverage; water may not reach beneath threads, seals, and straw assemblies.

Washing without separating the different parts of the bottle lets biofilms survive in hidden crevices.

“Do you wash your dishes after dinner?” Behnke asked. “Yes. But with water bottles, we often take them all over the place and don’t properly clean them.”

Human health concerns

Fukuta suggested that bacteria might include staphylococcus bacteria or strep. And there’s this, from the cohort study: “More than 20% of our samples had coliform bacteria, which is fecal matter,” says Behnke.

A bottle can act as a fomite – a surface that helps move microbes from place to place – when it isn’t kept clean. Hands touch the cap and rim.

Mouths contact the same areas. If the outside is grimy, it can become the pathway for microbes to reach the drinking surfaces.

Dr. Yuriko Fukuta agrees. “We’re constantly touching our water bottles with our mouths and hands, so it’s easy to transmit bacteria to them, and then it just grows,” she says. “In some cases this can make you sick, especially if you have a weaker immune system.”

Reusable water bottle tips

Treat a reusable bottle like a food-contact surface. Use hot water and dish soap. Scrub the walls with a bottle brush.

Use a small brush, skewer, or toothpick for threads and tiny crevices. Take lids, straws, bite valves, and silicone rings apart and scrub them individually.

Let everything dry completely with the cap off. Drying breaks the moisture part of the bacteria formula and prevents the musty smell that appears when water lingers.

If you use sweet or creamy drinks, wash the bottle the same day. If your current bottle has many small parts and you often skip steps, a simpler design with a wide mouth can help you stick with the routine.

Avoid sharing bottles. Saliva adds new microbes that can seed later growth, even if nothing seems wrong at the time.

If you often set your bottle on public surfaces, wipe the outside periodically, paying special attention to the rim and cap.

What it all means

Reusable bottles are still a smart, sustainable choice. They just need real cleaning.

The inside isn’t sterile, and the outside picks up residue fast. Pair frequent refilling with a consistent cleaning routine.

That set of small steps turns your bottle back into what you intended it to be: a safe hydration tool, not a traveling microbiology project.

The full study was published in the journal Food Protection.

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