When’s the last time you washed your water bottle? Might want to make that right now: A new study from Brazil suggests reusable shaker bottles (and water bottles) might be grosser than you thought.

In the study, researchers asked 30 gym members to hand over their shaker bottles for testing, and compared the results to that of 30, unused (contaminant-free) ones. They discovered bacteria contamination in 83 percent of the used plastic bottles.

Most prevalent were Staphylococcus aureus (found in 27 percent of the bottles) and E. coli (found in 17 percent).

“We tested in a real-world scenario, by surprise, asking for [bottles of] those who were arriving at the gym at those particular days,” study author Gilmar Weber Senna, Ph.D., a professor at the Federal University of State of Rio de Janeiro told Runner’s World. “We did this to avoid an intentional over-cleaning.”

Now, while the findings sound gross, they are not entirely unexpected, said Philip Tierno, Ph.D., professor of microbiology and pathology at the NYU School of Medicine. For one, staph bacteria is present in the noses of about 30 percent of people, and generally does not cause harm, according to the Centers for Disease Control and Prevention (CDC). Likewise, E. coli is present in healthy GI tracts, but certain strains can cause a diarrheal illness, the CDC says.

So how likely is it that your bottle bugs can make you sick? Well, according to Tierno, that depends on a few factors: the amount and type of bacteria present, and a person’s immune system.

The bacteria likely comes from contamination during handling, said Tierno. Since people are handling their bottles to make their recovery drink or just fill it up, bacteria can be transmitted through indirect contact. If you don’t wash your hands after going to the bathroom or touching your face, for instance, you can spread any bugs to the bottle.

It’s best to properly wash any bottle before each use to lower your risk of getting sick from present bacteria—or from transmitting it to others who may grab your water bottle. One way? Stick it in the dishwasher after each use.

To avoid spreading harmful bacteria to your shaker bottle or water bottle, Tierno suggests making sure that you properly wash your hands before fixing your gym drink to get rid of any residual organisms on your hands, remembering to pay extra-close attention to your nail bed, where germs can hide.

“Wash for 20 seconds. Get soap on the top and bottom of hands and in between digits and under the nail bed,” Teirno said. “Run your hands like a claw in the centre of the opposite palm to get suds into nail bed, and sing the song ‘Happy Birthday’ twice to wash hands adequately.”To help keep bacteria from growing, Tierno suggests using steel, metal, or glass bottles when possible, as bacteria can more easily adhere to plastics and other surfaces that are rougher. The smoother surface of steel, metal, glass-surfaced are more easily cleaned and prevent a biofilm (where bacteria can grow) from forming.

And keep your bottle as your bottle—don’t share water bottles unless they are dishwasher-washed first.