A study showed that reused water bottles are worse than a toilet seat.....
Treadmillreviews.net, the site for treadmills, let twelve bottles in the laboratory study to answer this question. The bottles examined were one week without washing in use before being tested. The tested bottles didn't show any visible impurities.
More bacteria than a toilet seat
The smear test of the lids showed worrying amounts of pathogenic bacterias. Specifically, more than 300’000 colony forming units (CFU) per square centimetre have been found.
Worst were bottles with a slide shutter. They showed up to 933’340 CFU’s. Bottles where you can squeeze the water through a spout showed up with up to 161’971 CFU and those with screw off lids hat 159’060 CFU’s. Bottles with integrated straw did best, with 25.4 CFU but these are only 2 units of bacteria per square centimetre less than a toilet seat.