There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water.

There are approximately 5×1030 bacteria on Earth,[5] forming a biomass which exceeds that of all plants and animals.[6] Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere.

The nutrient cycle includes the decomposition of dead bodies and bacteria are responsible for the putrefaction stage in this process.