👉 Ultramicrobe is a term used in microbiology to describe a bacterium with a diameter of less than 0.01 millimeters, which is typically considered too small for our usual definition of microorganisms. Ultramicrobes are often referred to as "microfossils" or "microscopic fossils". They can be found in various environments such as soil, water, and even in the human body. These organisms have been used as a source of information about the evolution