Microbiology is one of the most applied of all the biological sciences which did not exist as a true science before the later part of 19th century. Microbiology is the study of microorganisms that is the organism which are of microscopic dimensions. These organisms are too small to be clearly perceived by the unaided human eye. Microorganisms are living organisms that are usually too small to be seen clearly with the naked eye. An organism with a diameter of 1 mm or less are microorganisms and fall into the broad domain of microbiology. Because most of the microorganisms are only a few thousands of a mm in size, they can only be seen with the aid of microscope. Due to the invisibility of microbes tc the naked eye and the need for special techniques to study them, microbiology was the last of the three major divisions in biology (the other two are botany and zoology) to develop. At present there is general agreement to include five major groups as microorganisms.