Here are some numbers for comparison, in microbes per cm3 volume:
Theoretical maximum limit: 1E15 cells per cm3
{this is based on the theoretical lower limit for microbial cells}
Human body: about 2E8 cells per cm3
Deep Ice (2.5 km deep in Greenland ice sheet): 1E5-1E4 cells per cm3
Instrument detection limit: 1E2 cells per cm3
Medically sterile: 0.1 cells per cm3