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A Good Way to Grow Bacteria in Petri Dishes

Growing pure cultures of bacteria is an important technique for molecular biologists and microbiologists to master. Microbiologists need to isolate specific strains of bacteria to characterize and study them, while molecular biologists often use E. coli bacteria in culture to produce desired proteins or to make copies of a gene. When you are working with bacterial cultures in a lab class or other lab setting, the most important thing to realize is that you and everything you handle is potentially contaminated with other species of bacteria. To avoid introducing these other species into your culture, you must practice sterile technique at all times.

Things You'll Need

Gloves and eye protection
Broth culture of bacteria you want to culture
Prepared nutritive agar plate (Petri dish)
Bunsen burner and flint lighter
Glass dish containing 95 percent ethanol
Inoculation loop
Parafilm
Incubator set to 37 degrees C

Instructions

- 1
  Put on your gloves and eye protection.
- 2
  Take the broth culture of the bacteria you want to cultivate to your lab bench, together with a prepared Petri dish containing nutrient agar. Make sure the broth culture remains covered at all times.
- 3
  Connect the Bunsen burner to the gas outlet at your lab bench. Turn on the gas and carefully light the flame. Make sure that no flammable materials are nearby, especially the glass dish containing the ethanol. Keep the glass dish covered to ensure that flammable vapors do not accumulate.
- 4
  Dip the tip of the inoculation loop into the ethanol, then hold it in the Bunsen burner flame until the metal turns red hot. Be very careful not to touch the metal yourself so you do not get burned. Make sure you re-cover the dish with the ethanol in it immediately.
- 5
  Remove the lid from the test tube containing the bacterial broth culture. Place the tip of the tube in the flame for just a second or so to kill any bacteria on the tip of the tube. Do not place the whole tube in the flame, just the tip only, and do not allow it to remain in the flame for too long.
- 6
  Hold the tip of the inoculation loop near the tube containing the broth, then hold it inside the tube for fifteen seconds or so but without touching the broth -- the hot metal will kill your bacteria.
- 7
  Dip the tip of the loop into the broth for a second.
- 8
  Remove the tip of the loop and hold it in the air. Meanwhile, stick the tip of the tube in the flame for a second again, then recap it.
- 9
  Hold the lid of the plate just barely off the plate itself. Gently streak the inoculation loop back and forth across the agar in a zig-zag pattern.
- 10
  Replace the lid of the plate and sterilize the inoculation loop again just as you did before.
- 11
  Turn off the Bunsen burner.
- 12
  Turn the plate and lid upside-down so the agar side is on top. Use the Parafilm to seal the seam between plate and lid, going all the way around and stretching each piece of film so the seam is completely covered. Parafilm is a kind of plastic film often used in labs to seal test tubes and other containers.
- 13
  Transfer the plate to the incubator and store it there for 48 hours or so. The incubator should be set at physiological temperature, which is at 37 degrees Celsius.

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