Unicellular microorganisms with spherical (cocci), rod-like (rods) or corkscrew-like (spirals) shapes. Additionally, bacteria are distinguished by their physiological-biochemical characteristics, for example:
- their relation to oxygen,
- their dependency on temperature and pH value,
- their need for nutrients,
- and their staining (Gram-positive, Gram-negative)
As the do not have a cell nucleus, their DNA is located in the cytoplasm. Hence, bacteria belong to the group of prokaryotes (Greek "before nucleus").