Aseptic Techniques: Plate Streaking

Modern microbiology originated in Koch's laboratory with the development of agar-based media and the streak-plate technique for obtaining pure cultures. This method consists of spreading source material over an agar surface until one microorganism at a time falls off of the loop. The medium is then incubated until colonies arise. Theoretically, each colony represents a single type of microorganism that originated from a single cell.

Most source materials contain a large number of microorganisms. Pond water can be expected to contain 105 to 106 bacteria/ml , soil to contain more than 108 bacteria per ml, and a barely visibly turbid culture of Escherichia coli contains at least 107 bacteria per ml. This means that a loop of these materials spread over the surface of an agar plate would yield 103 to 105 colonies, producing a confluent lawn of growth on the plate. Thus, simply smearing a loop of source material over a plate will not yield isolated colonies. Additional steps are needed.

  • To streak a sample for isolation, draw a "T" on the back of the plate, dividing it into three sections.
  • Flame the loop, allow it to cool a few seconds, and then pick up a loop of sample. Turn the plate so that the top of the "T" is near the palm of your non-dominant hand.
  • Crack the lid of the plate by lifting it with your thumb and index finger.
  • Beginning at the edge of the plate above the top of the "T" and move the loop back and forth across the agar surface, pulling it towards the top of the "T" as you do so. Do this by flexing and extending the fingers holding the loop, not by moving your entire hand. Done properly, this process will result in a series of parallel lines at the top of the plate. (mouse over picture to see more details)
  • Remove the loop, flame-sterilize, and cool it. Do NOT insert the loop back into the sample.
  • Turn the plate 90 degrees counter-clockwise.
  • Repeat the streaking procedure, beginning at the edge of the plate. This time, cross the previous material three or four times to pick up cells from the part you first streaked. (mouse over picture to see more details)
  • Continue streaking until the loop reaches the right side of the upright of the "T."
  • Remove the loop, flame-sterilize, and cool it. Do NOT insert the loop back into the sample.
  • Turn the plate 90 degrees counter-clockwise.
  • Repeat the streaking procedure, beginning at the edge of the plate. Again, cross the previous material three or four times to pick up cells from the part you streaked second. (mouse over picture to see more details)
  • Continue streaking until the loop reaches the bottom of the top of the "T."
  • Label the plate with your name, date, experiment information, and course or lab section.
  • Incubate the plate agar side up as directed.

Plate Streaking Gallery

Take the Quiz

On to Broth Transfer