Micro means small and organisms means living things. Micro-organisms are those organisms that are so small and they cannot be seen with naked eye. in order to observe them, we need a light microscope or an electron microscope.
Micro-organisms form a diverse group which includes different kinds of organisms, viruses, bacteria, unicellular plants, certain algae and some fungi.
Micro-organisms differ from one another with regard to structure and mode of nutrition. On the basis of structure viruses are not cellular while other micro-organisms have organized nucleus. According to mode of nutrition algae and a few bacteria are autotrophs while fungi and most unicellular animals are heterotrophs. Due to these characteristics micro-organisms form a diverse group which includes different kinds of living organisms.