the worst part about reusing isn't the bacteria factor its the chemicals that offgas out of the plastic. your standard bottled water bottle should not be reused for this reason. the plastics used by say Nalgene are specifically designed to reduce this problem. standard water bottle plastic breaks down over time and offgases into the water. This process increases over time making reuse a bad idea. IF you doubt it, refill and leave for several days and then taste it!