As a molecular geneticist by education (and therefore a microbiologist), all it takes is a cup of old sand. Most bacteria double in less than an hour - some as quickly as 10 or 15 minutes.

Remember the old chessboard equation. Put a grain of sand on the first square, 2 on the second, 4 on the 3rd, 8 on the 4ths, etc. By the time you get to the 64th square, you have a pile of sand a mile high with a mile base.