Urine is an excellent culture medium for bacteria to grow. At room temperature, one bacterium multiplies to become 1 lakh bacteria in two hours time in urine. Bacteria may be in the bladder due to urine infection, or may have come to urine form skin contact (contamination) while passing urine. If urine is kept for two hours at room temperature after collection, as usually happens if urine is collected in morning at 7 am and transported to lab at 9 am, it will contain 1 lakh bacteria in the reports even in absence of urine infection. If bacteria have come from the bladder causing urine infection inside, the urine will contain lot of pus cells too called pyuria, as well as you will have difficult urinary voiding symptoms including pain while passing urine. It is important to have the triad of urine voiding symptoms, pyuria and more than 1 lakh bacteria per ml of urine to diagnose urine infection worthy of treating. Most of positive urine cultures in the market are, in fact, caused by contamination instead.