Does bacteria content increase in a water bottle when the bottle is not properly washed every so often?

okay this is my science fair. i was supposed to start this along time ago. but i have only about 2 weeks left. i have all the info. but i need to experiment this. i don't know how to. im pretty sure that you cant see bacteria under a microscope. like im supposed to observe the bacteria that grows in the bottle. but how?? please help..