Give a brief account of micro-organisms?

Those organisms that cannot be seen through unaided eyes are called micro-organisms. They can be observed only with the help of microscope or any other aid. Micro-organisms can be unicellular, example, Yeast, Amoeba etc. or  multicellular like Mucor, Rhizopus etc. They are broadly categorised as Bacteria, Algae, Fungi, Euglenoids, Slime moulds, Protozoa etc. They inhabit a particular habitat, except for Bacteria that are omnipresent. Based on their interaction (mode of nutrition), they can be termed as harmful or useful to other beings.