Reusable water bottles are everywhere, from college campuses to gyms and office desks. They cut down on plastic waste, save money, and make staying hydrated easy. But a new study warns that they may also carry bacteria you wouldn’t want anywhere near your mouth, including strep and fecal organisms.

The research highlights how cleaning habits and design features influence what grows inside bottles and on their surfaces. Here, we’ll look at what the study found, why bacteria spread so easily, and how to keep your daily hydration safe without giving up the convenience of a reusable bottle.

What the study found about dirty water bottles

Researchers at Purdue University examined 90 student water bottles to see how daily routines affected cleanliness. They tested both the inside and outside using ATP hygiene swabs, a method commonly used in food safety, and then measured bacterial growth in the lab.

Every exterior surface failed the hygiene test, even new bottles used as controls. That result made sense once the team tracked how bottles are handled. Hands move from phones, keyboards, and doorknobs to bottle caps and rims throughout the day. Each contact leaves residue, which becomes a foothold for microbes. Frequent refilling was also linked to dirtier bottles, since each refill meant more touchpoints.

Inside the bottles, the findings were more concerning. Bacteria counts were measured in colony-forming units per milliliter, a standard way of estimating growth. Many bottles exceeded thresholds considered acceptable for drinking water. About 70 % had counts higher than 100 CFU/mL, and more than half surpassed 500 CFU/mL, levels that would be flagged in water quality testing.

Coliform bacteria, used as a marker for sanitation, appeared in about one in four bottles. Regulations for bottled water set the bar at zero coliforms in a sample of 100 milliliters. That means many bottles carried contamination levels that wouldn’t be allowed if they were sold commercially.

How to clean and handle bottles safely

The study makes clear that bacteria thrive in bottles because they provide exactly what microbes need: moisture, contact with hands and mouths, and time to multiply. Consistent cleaning habits can cut down those risks. Here are some practical steps to keep reusable bottles safer:

Wash daily with hot water and soap. Treat bottles like dishes. Use a bottle brush to scrub the walls and a smaller tool for threads and crevices.
Take everything apart. Disassemble lids, straws, bite valves, and gaskets. These tight areas trap films and biofilms that rinsing won’t remove.
Air-dry completely. Leave the bottle open after washing. Moisture is what bacteria need most, and drying breaks that cycle.
Avoid sugary or milky drinks. These leave residues that feed microbes. Stick to water when possible.
Don’t share bottles. It increases the risk of spreading strep, staph, and coliform bacteria.
Replace worn parts. Old silicone rings and valves can harbor bacteria even after washing. Swap them out regularly.
Wipe exteriors after public use. When bottles touch gym benches, desks, or sink counters, clean the outside to cut down contamination.

Reusable bottles remain a smart choice for the environment and for staying hydrated, but the same features that make them convenient also make them vulnerable to contamination. By treating them like food-contact items and cleaning them with care, you can reduce bacterial growth and keep each sip as safe as it should be.