ABSTRACT
Microorganisms include unicellular organisms (archae, bacteria, some protista, some fungi, and some chlorophyta), few multicellular organisms (large fungi and some chlorophyta), and viruses. It is helpful to remember that classifying any organism as a microorganism had no biological criterion. The availability of the light microscope had revealed that at the cellular level, organisms can be called prokaryotes or eukaryotes. In 1977, Carl Woese proposed that archaebacteria are different from bacteria. Microbial life survives under extreme conditions of various kinds. These extremophiles survive under extreme temperatures and extreme pH (Thermophiles, Psychrophiles, Acidophiles, Alkaliphiles), dessicated conditions (Xerophiles), high weight or pressure (Barophiles, Piezophiles), radiation (Dienococcus radiodurans), and high salinity (Halophiles). Among the viruses infecting microorganisms, bacterial viruses, more commonly called bacteriophages or simply phages, are most important. Disordered structure in proteins is associated with the proneness to form abnormal protein deposits called amyloids (which are associated with several neurodegenerative diseases).
