How to Streak a Petri Dish
Streaking a Petri dish is a microbiological technique based on the principle that tiny microbial colonies can be pulled apart and individual cells can be isolated to form new colonies. The primary reason for using the streaking technique is to ensure the culture will grow for study purposes. A secondary purpose is to determine how many organisms are present in the culture. A culture with more than one type of organism present is considered contaminated while a pure culture consists of a single type of organism.
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Things You'll Need
- Sterile wire loops
- Petri dish
- Nutrient-rich agar well
- Culture broth medium
- Dry surface
- Bunsen burner
Instructions
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Streaking a Petri Dish
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1
Find an open and clean area of bench, making sure to disinfect it entirely before you begin. Ignite the Bunsen burner and flame the wire loop until it's heated red, then allow it to cool for a few minutes upright in a rack. Repeat this process for three or so more wire loops.
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2
Dip the sterilized loop in the culture broth. Delicately spread the liquid across the agar surface in a straight line along the edge of the Petri dish. Spread three more lines with the same loop to make four parallel lines in the agar and quickly cover the Petri dish with a lid.
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3
Flame the loop with the Bunsen burner and allow a couple minutes for it to cool. Rotate the Petri dish slightly to start a new set of lines. This new set should intersect with the end half of the first set of lines along the edge of the Petri dish. Grab another sterilized loop and make the new set of parallel lines, then cover the dish quickly with the lid.
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4
Repeat the process for two more sets of parallel lines around the remaining edge space of the Petri dish, promptly covering the dish with its lid after each set. For the final line in the fourth set, make a zigzag pattern in the dish.
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5
Close and seal the Petri dish. Write a label including the culture type, your name, and the date and store in an incubator upside down.
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1
Tips & Warnings
Using nitrile gloves for your hands will help ensure that no new possible contaminants are introduced to the agar culture.
Exercise caution around open flames or cooling wire loops to help avoid injury.
References
Resources
- Photo Credit hands in gloves tuning microscope image by Antonevich Anastasia from Fotolia.com
