ABSTRACT
Microorganisms isolated from various substrates or sources, need to be obtained in pure form as cultures contain millions or billions of individual cells. A culture that contains only one kind of microorganism is called a pure culture. A culture that contains more than one kind of microorganism is called a mixed culture; if it contains only two kinds of microorganisms that are deliberately maintained in association with one another, it is called a two-member culture. Pure cultures of microorganisms that form discrete colonies on solid media may be most simply obtained by one of the modifications of the plating method. This method involves the separation and immobilization of individual organisms on or in a nutrient agar medium. The spread plate technique is used for the separation of a diluted, mixed population of microorganisms so that individual colonies can be isolated. The streak plate method offers a most practical method of obtaining discrete colonies and pure cultures.
