You may want to give your reusable water bottle or Stanley cup a good wash. A new study reveals that there are millions of harmful bacteria on the bottles. WaterFilterGuru.com sent samples from everyday reusable water bottles to a professional lab, revealing surprising levels of bacterial contamination. One big concern is the presence of antibiotic resistant bacteria. One gross stat from the study is that there’s more germs on the bottles than on a toilet. Experts say your water bottles and tumblers should be washed daily to avoid getting sick. The average person washes them only “a few times a month.”