Streak Plate Technique

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What is the Streak Plate Method?

Streaking is a technique used in microbiology for the isolation of single colonies of microorganisms, either from a mixed species or from the same species. This technique is mostly applicable to bacteria but is also used for some yeasts. It is an old technique that has been in use since the time of Rober Koch. It was first demonstrated by Loeffler and Gaffky in Koch’s laboratory.

Streak Plate Method Principle

The streak plate method is based on the principle of dilution. It can be described as a rapid qualitative isolation technique. The main criterion of isolation is to obtain a reduced number of colonies. In this technique, a loopful of culture is spread on an agar plate to get individual cells far apart enough from each other. The streaking method gradually dilutes the inoculum such that the bacterial cells can be counted as colony forming units (CFUs).

Read: Inoculation and Plating Methods

Types of Streak Plate Method

1. Quadrant Streaking: This is the most common method of streaking, where the petri dish is divided into four quadrants and then inoculated. It is also known as a four-quadrant streak. A loopful of inoculum is taken and streaked such that the first quadrant contains the highest concentration of the inoculum, followed by the second quadrant, third quadrant, and fourth quadrant.

This is a discontinuous method where the loop is sterilised after streaking in every quadrant. By the time the fourth quadrant is streaked, the inoculum is diluted enough to give rise to individual colonies.

1. T-Streaking: In this method, the petri dish is divided into three quadrants by drawing the letter ‘T’. Similar to four-quadrant streaking, it is a discontinuous method where each quadrant is streaked after loop sterilisation, and the quadrant that is streaked last gives isolated colonies.
2. Continuous Streaking: In this type of streaking, the inoculum is spread from one edge to the centre of the plate. The plate is rotated 180°, and the remaining portion is streaked without sterilising the loop.

Another form of continuous streaking is used for diagnostic purposes. The petri dish is divided into sections, and different cultures, such as urine, sputum, pus, etc., are streaked in each section to get maximum output.

1. Radiant Streaking: In this streaking method, the inoculum is spread on one edge of the petri dish. From the edge, vertical lines are streaked in the upward direction. Next, the vertical lines are streaked horizontally to obtain pure isolated cultures.

Streak Plate Method Procedure

• Sterilise all the instruments, flasks and media that are required for the streaking procedure.
• Clean your work area using a disinfectant to minimise any contamination.
• Set up the bunsen burner in your work area carefully.
• Wash your hands with an antiseptic solution before handling any microbial solution.
• Label the petri dish with all important information, such as your name, date, media used and the culture being inoculated.
• To pick up the sample, you can use either a metal loop or disposable plastic loops.
• A loopful of sample is streaked on the first quadrant in a back-and-forth motion on the agar plate.
• Sterilise the loop by heating it in the bunsen burner if using a metal loop.
• Streak the other three quadrants by a similar method.
• Close the lid of the plate after streaking, and store the dish upside down in an incubator with optimal temperature.

Advantages of the Streak Plate Method

• It is one of the most popular techniques used to obtain isolated colonies of bacteria.
• It finds a great application in biotechnology as it can be used to identify transformed bacteria from non-transformed bacteria by adding an antibiotic to the growth medium.
• It also finds great application in diagnostic purposes.

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Also Read:

• Classification of Microorganisms – NEET Biology Notes
• Importance of Microbiology
• Microbiology MCQs with Answers Keys