1- In order to unsure that you get a pure sample.
2-pure culture: laboratory culture containing a single species of organism. A pure culture is usually derived from a mixed culture (containing many species) by methods that separate the individual cells so that, when they multiply, each will form an individually distinct colony, which may then be used to establish new cultures with the assurance that only one type of organism will be present. Pure cultures may be more easily isolated if the growth medium of the original mixed culture favors the growth of one organism to the exclusion of others. In mixed culture two or more microbial species formed colonies.
3-A bacterial colony is defined as a visible cluster of organisms growing on the surface of or within a solid medium, theoretically cultured from a single cell. Because all organisms within the colony descend from a single ancestor, they are genetically identical (except for mutations which occur at a low, unavoidable frequency, as well as the more likely possibility of contamination). Obtaining such genetically identical organisms (or pure strains) can be useful in many cases; this is done by spreading bacteria on a culture plate and starting a new stock of bacteria from a single colony. Fours characteristics by which colonies can be identified are shape, color, size and texture. 4-A Petri dish is used as a container for a medium used to grow cultures. Leaving the Petri dish open will increase the probability that your culture will be contaminated.
5- Between each set of streaks you sterilize the inoculating loop in the Bunsen flame. At the beginning of the next streak you overlap with the end of the one before. The effect of the technique is to "dilute" the bacteria by gradually spreading then over greater distances. Eventually, a point is reached where single bacteria are spaced sufficiently far apart for single colonies to...