Streak Plate of Legionella Public Health Image Library 7925

Streak Plate Technique for Isolating Bacteria
Streaking to Obtain Pure Bacterial Cultures from Clinical Specimen
 
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The purpose of streak plating is to spread out  a bacterial sample on solid growth media, so that individual, isolated colonies will grow.

What Is Bacterial Growth Media?
Asolid  bacterial growth medium is a gel-like surface made from agar (a seaweed derivative), with an appearance similar to Jell-O. In addition to agar, growing media also contain water, nutrients and buffer to help the bacteria grow. Growth media are made to be sterile until inoculated with a bacterial sample.
Article Summary: Streaking a clinical sample onto a media plate is how specific bacteria are isolated so that the causative agent of a bacterial disease can be identified.
Streak Plate Technique for Isolating Bacteria
​Page last updated: 5/2014
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Prokaryotic Cell, Mariana Ruiz
Two different styles of streak plate techniques on Blood Agar
Two different styles of streak plate technique on Blood Agar.
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What Is a Bacterial Colony?
A colony is a visible spot on the surface of growing medium that containscontains millions of bacteria that, through cell division, arose from a single parent bacterium. This means that all bacteria in a colony are of the same species.

How to Isolate Bacteria Using Streak Plate Method
By using streak plate technique to spread a clinical sample out on the surface of a solid growth medium individual types of bacteria can be isolated. Here is the basic technique. See the instructional photo that appears with this article.

Supplies needed for streak plating:
  • Petri dish of sterile TSY medium (or a specialized growing medium)
  • Clinical sample (sample obtained from person)

Streak plate technique:
  • Streak swab of clinical sample over one quarter of the sterile Petri dish, quadrant #1.
  • Discard swab in biohazard bag.
  • Sterilize loop in flame of Bunsen burner or hub of microincinerator.
  • Allow loop to cool without waving it about.
  • Place loop on next quadrant of Petri dish, next to quadrant #1. Gently drag the loop into quadrant #1 a few times, to obtain just a bit of bacteria from that first sample, then spread that material over quadrant #2.
  • Again sterilize loop in flame of Bunsen burner or hub of microincinerator, and allow loop to cool without waving it about.
  •   Place loop on next quadrant of Petri dish, next to quadrant #1. Gently drag the loop into quadrant #1 a few times, to obtain just a bit of bacteria from that first sample, then spread that material over quadrant #2.
  •   Again sterilize loop in flame of Bunsen burner or hub of microincinerator, and allow loop to cool without waving it about.
  •   Place loop in next quadrant of Petri dish, adjacenty to quadrant #2. Gently drag the loop into quadrant #2 a few times, to obtain just a bit of bacteria from that sample, then spread that material over quadrant #3.
Isolation Streak Plate Technique
If using this technique to isolate a specific type of bacteria from a clinical sample, streak plate technique will need be repeated, possibly several times, in order to obtain a pure sample. Once this original pate is incubated, sample solitary colonies individually onto new pates. Streak quadrant #1 of the new plate with the isolated colony, and spread the bacteria out over quadrants #2 - #4 using technique described above.

Sources & Resources
  • Bauman, R. (2004) Microbiology. Pearson Benjamin Cummings.
  • Schauer, Cynthia (2009) Applied Microbiology HCR120 Laboratory Manual, Kalamazoo Valley Community College.

  • Again sterilize loop in flame of Bunsen burner or hub of microincinerator, and allow loop to cool without waving it about.
  • Place loop in next quadrant of Petri dish, adjacent to quadrant #3. Gently drag the loop into quadrant #3 a few times, to obtain just a bit of bacteria from that sample, then spread that material over quadrant #4.
  • Incubate plate at 37 degrees C for at least 24 hours.

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#4
#3
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How to Light a Bunsen Burner 
Using a Match