Researchers tested four types of reusable water bottles and found that bottles that were used for a week without washing had more bacteria than a toilet seat. Specifically, the slide-top bottle had more bacteria than an average toilet seat, pet bowl, or kitchen sink. Over 60% of the bacteria found on the bottles could make people sick. The cleanest bottle type tested was the straw-top bottle, and researchers also found that stainless steel bottles were healthier options than plastic bottles.