Urine is sterile when in the bladder.
You have bacteria in the urethra (which goes from bladder to outside). When urinating, you usually wash out the patogens in the urethra, but some can still remain there. Then, if you don't urinate, bacteria can go up in the bladder and gives you patologies, like cystitis, or other type of infections.
Source: Bel Marra Health
Proteinuria: This is a condition in which protein is released in the
urine in high amounts. Protein is normally released in the urine but
in small amounts. When these levels become high, the urine can appear
foamy. This can be a sign of an impending kidney problem as the
kidneys are not filtering urine properly and thus ...