Etymologically, microorganisms are small organisms whose size is too small to be observed by the naked eye. The resolution of the human eye is 0.2 mm, and this establishes 1 mm as an artificial limit for the maximum size of microorganisms. Taxonomically, “microorganism” lacks a precise meaning, and the term includes prokaryots (bacteria and archaea) and eukaryots (protists, microscopic algae, and fungi), which are all unicellular or lacking true tissues. Exceptionally, virus should be included in the definition of “microorganisms,” even though they lack cellular arrangement. Microbiology is the area of biological sciences that studies microorganisms. Prokaryots are the main focus of study in Microbiology. All are unicellular and lack a nucleus (pro-karion); their response to oxygen is very diverse; they can be auto- or heterotrophic and they employ the entire range of energy-obtaining systems found in living beings: anaerobic or aerobic respiration, fermentation...