[Microbiology] Atlas of Streak Plate Methods of Isolation

Streak Plate Methods of Isolation, Isolation Techniques and Selective Media, A Photographic Atlas for the Microbiology Laboratory,

Purpose

The identification process of an unknown microbe relies on obtaining a pure culture of that organism. The streak plate method produces individual colonies on an agar plate. A portion of an isolated colony then may be transferred to a sterile medium to start a pure culture.

Principle

A microbial culture consisting of two or more species is said to be a mixed culture, whereas a pure culture contains only a single species. Obtaining isolation of individual species from a mixed sample is generally the first step in identifying an organism. A commonly used isolation technique is the streak plate.

In the streak plate method of isolation, a bacterial sample (always assumed to be a mixed culture) is streaked over the surface of a plated agar medium (Figure 2-1). During streaking, the cell density decreases, eventually leading to individual cells being deposited separately on the agar surface. Cells that have been sufficiently isolated will grow into colonies consisting only of the original cell type. Because some colonies form from individual cells and others from pairs, chains, or clusters of cells, the term colony-forming unit (CFU) is a more correct description of the colony origin.

A common streaking technique is the quadrant method, which uses the four-streak pattern shown in Figures 2-2 and 2-3. Streaking for isolation is frequently performed on selective media

2-1 STREAKING A PLATE Hold the plate comfortably

and streak with the edge of the loop. Be careful not to cut

the agar.

2-2 THE QUADRANT METHOD As shown, the

quadrant method of streaking for isolation involves

four individual streaks. The best results are obtained

when the loop is flamed between streaks.

designed to encourage growth of certain types of organisms while inhibiting growth of others. The selective media considered in this section are used specifically to isolate pathogenic bacteria and yeast from human or environmental samples containing a mixture of organisms. Some selective media contain indicators that expose differences between organisms.

Such media are considered to be selective and differential. See Tables 2-1, 2-2, and 2-3 for summaries of terms related to organisms and media, and roles of common ingredients found in selective media.