Streaking (microbiology)

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A plate which has been streaked. Note the colonies thinning in progressive regions of the plate. The last region near the top of the picture contains pure colonies each grown from a single bacterium.

Streaking is a technique used in microbiology to isolate a pure strain from a single species of microorganism, often bacteria. A microbiological culture can be grown so that the organism can be identified, studied, or tested. A sterile cotton swab or inoculation loop is sterilized and dipped in a medium of a single species of bacteria. The loop is then spread across one quadrant of an agar plate containing a growth medium which has been sterilized in an autoclave. This introduces a solution of the bacteria to a substrate which provides them nutrients. Choice of which growth medium to use depends on which microorganism is being cultured, and which are being selected for, if any. Growth media are usually based on agar, a gelatinous substance.

The loop is resterilized and dragged across the inoculated quadrant of the streak plate. This is done to collect some bacteria on the loop. The loop is spread around another fourth of the plate much like the previous step. The loop is sterilized and the procedure is repeated. Each time the loop gathers fewer and fewer bacteria until it gathers just one single bacterial cell that can grow into a colony.

The streak plate is then incubated, usually for 24 to 36 hours, to allow the bacteria to reproduce. At the end of incubation there should be enough bacteria to form visible colonies in the areas touched by the inoculation loop.