Sterilizing Solid Objects

All tools and supplies that contact your microbial cultures and samples should be sterile. Non–sterile objects may introduce bacteria and mold spores into your cultures. You can effictively sterilize tools and supplies in an autoclave, a pressure cooker, or an oven. Some tools can also be sterilized in alcohol or a flame. Read on for details.

Sterilize with Heat

This method works well for supplies such as paperclip spreaders, wooden coffee stirrers, and non–sterile toothpicks.

  1. Wrap objects in aluminum foil, or place them in glass test tubes with metal or aluminum foil lids.
  2. Sterilize
    • In an autoclave on dry cycle
    • In a 320 °F oven for 2 hr
    • In a pressure cooker at 15 psi for 15 minutes. Put an inch or two of water in the bottom, then add your wrapped objects in a beaker or other heat-proof container.

Sterilize by Flame

Metal objects such as inoculating loops and paperclip spreaders can be sterilized just before use by heating them in a flame until they are red hot. Use a hot, clean flame such as a Bunsen burner; other sources may leave a black residue. Cool before touching to living material by dipping in sterile water or by touching to the edge of an unused petri dish.

Sterilize with Alcohol

Glass and metal spreaders and inoculating loops can be sterilized in a beaker of 70% ethyl or isopropyl alcohol. Just before use, burn off the alcohol by passing the tool through a flame (even candle and lighter flames are sufficient). Cool before touching to living material.

If you plan for more time, you can skip the flame step and simply let the alcohol evaporate.

Pre-Sterilized Items

Pre-sterilized items are great time savers. The following items come sterile in the package:

  • Toothpicks in unopened boxes are sterile and can be used directly from the box. Use scissors to cut a small hole (about 1/4 in. across) in one corner of the box to serve as a dispenser. Cover the hole with tape when not in use. You can also use individually wrapped toothpicks.
  • Facial tissues: Discard the top one or two tissues that have been exposed to air and dust. The ones underneath will be sterile.
  • Paper towels (the kind in rolls): The side that contacts the roll is sterile.
  • Items specifically made and packaged for microbiology, including Petri dishes, disposable inoculating loops and tubes, and individually wrapped transfer pipets.