2. Single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered plants.
3. A single-celled, often parasitic microorganism without distinct nuclei or organized cell structures.
Various species are responsible for decay, fermentation, nitrogen fixation, and many plant and animal diseases.
Every square inch (6.4 square centimeters) of the human body has an average of some 32 million bacteria on it, with a grand total of 100 billion; over 22 times the estimated human population on the planet earth.
These 100 billion bacteria could fit inside a medium-sized pea.