What Is the Purpose of Streaking Bacteria for Isolation?

The purpose of streaking bacteria for isolation is to create a region in which the bacteria are so dilute that when each bacterium touches the surface of the agar, it is far enough away from other cells so that an isolated colony can develop. This allows for isolation of multiple organisms.

The two most common streak patterns are the three sector T streak and the four sector quadrant streak. Organisms are streaked through the first sector using either a disposable loop or needle, or a reusable loop, usually with nichrome or platinum wire, and then passed through a Bunsen burner for sterilization.

After cooling, the loop is dragged through the initial sector and carried over to the second sector to streak and dilute the bacteria. This process is continued through the third and a fourth quadrant on the four sector plate.

Bacteria that grow in moderate temperatures, called "mesophilic bacteria," are typically grown on media that is solidified with 1.5 percent agar or agarose in a sterile Petri dish. Bacteria that prefer higher temperatures, including thermophiles and hyperthermophiles, can also be streaked onto growth media solidified with agar substitutes, such as Gelrite and guar gum. Ensure the Petri dishes are labeled properly to avoid confusion.