Streak Plate Methods of Isolation

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 trans ferred 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.
During streak ing, 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. Streaking for isolation is frequently performed on selective media designed to encourage growth of certain types of organisms while inhibiting growth of others.
 Some selective media contain indicators that expose differences between organisms.
Such media are considered to be selective and differential. 

Bacterial Colony Isolation - quadrant method

Bacterial Colony Isolation – quadrant method

 

different-streak-methods-for-bacterial-plates

different-streak-methods-for-bacterial-plates

Reference :

Atlas of microbiology

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