Sterile Technique

Microbes are everywhere. When you culture microbes, you need to be very careful not to let any contaminating microbes from the environment land in your samples. If they do, they will rapidly grow and multiply in your nutritious culture media. You also need to be careful not to let any microbes from your cultures escape from your work area or be accidentally ingested.

If you follow proper sterile technique, you will grow only the microbes you intend to grow, and you will keep them in their proper place. Mostly, you want to avoid dust and fingers, the most common carriers of contaminating bacteria and molds, from entering your cultures.

1. Choose a clean, quiet work area

  • When you are streaking or pouring plates choose a time (usually after classes end for the day) and place where you will not be interrupted by students, colleagues or janitors. Lock the door to your work area.
  • If possible, choose a work area that is far from plants, animals, and other biological materials that are sources of mold.
  • Choose a work area with as little air turbulence as possible. Do not talk, sing, whistle, cough or sneeze in the direction of sterile items or your work area. Avoid leaning over your work area as dust can fall off of you and contaminate it.

2. Clean your work area

  • Before you begin, clean your work area with a household cleaner, 10% bleach solution, or soap and water. Then wipe it down with 70% ethyl or isopropyl (rubbing) alcohol.
  • Wash your hands with soap and water or wear gloves, then wipe them with alcohol or hand sanitizer.

3. Keep sterile things sterile

  • Keep sterile media, plates, toothpicks, etc. covered as much as possible.
  • Avoid setting lids down; it's better to hold them near by in your hand as you work.
  • Only touch the samples and sterile things with other sterile things.
  • Work at arm's length. This will keep contaminants from falling off you and landing on or in your sterile materials.
  • Breathe, cough, and sneeze away from sterile materials.

4. Treat contaminated materials properly

  • Keep a plastic bag nearby and use it for any used disposable items such as toothpicks, pipets, and disposable spreaders.
  • Non-disposable items such as spreaders and loops should either be heated in a flame or placed into a container of 70% ethyl or isopropyl (rubbing) alcohol after each use to kill any microbes they may be carrying.
  • Contaminated liquids should be disinfected with bleach. Add 10% of the total volume of bleach and swirl to mix. Leave for 20 minutes, then wash down the drain with plenty of water.

5. Clean your work area again after you finish

  • This will keep anything that has escaped from your plates or cultures from spreading.
  • Use the same cleaning methods you used in Step 2.