Preparing, Sterilizing and Dispensing Media

Preparing and Dispensing Media

Basic types of media used in bacteriology:

1. LIQUID MEDIA or BROTH MEDIA are dispensed into test tubes or flasks.

2. SOLID MEDIA are usually prepared by adding AGAR to a broth of some kind. A solid medium is then dispensed into test tubes to make SLANTS or DEEPS or it may be dispensed into PETRI DISHES or PETRI PLATES.

AGAR is an undigestible POLYSACCHARIDE derived from seaweed (Algae). It melts at 100^oC and it solidifies at 40^oC. Why is this a desirable characteristic for bacteriological media?

There are nutrients in the broths used to make media and these include:

1. Sugars and polysaccharides (Name a few)

2. Proteins and amino acids (Name a few)

3. Fats or lipids (Name a few)

4. Vitamins and minerals which are often called growth factors (Name a few)

Media are usually heated to dissolve all the ingredients and then they are AUTOCLAVED to sterilize.

Cultures are manipulated with LOOPS and NEEDLES. These tools are easily STERILIZED between use by FLAMING them in a BUNSEN BURNER flame.

Types of DISINFECTION and STERILIZATION commonly used in the bacteriology lab.

1. Chemical agents used for disinfection

Zephirin (QUATERNARY AMMONIUM COMPOUND, QUAT)

Chlorox (10% aqueous solution)

2. Dry Heat (oven)

350^oF / 180^oC for 2 hours is a sterilizing procedure

3. Steam Heat - AUTOCLAVE or pressure cooker

15 pounds per square inch

250^oF / 121^oC, for 15 minutes is a sterilizing procedure

BOILING is disinfecting but not sterilizing

4. Bacteriological filtration

Used for liquids that are heat sensitive. This is a sterilizing procedure as far as bacteria and fungi are concerned. Do you think that viruses would be removed by this method?

5. Gas - Ethylene Oxide

Used for certain plastics and instruments which are sensitive to heat. Treatment is in a chamber for about 4- 6 hours and treated materials must be aired out for a long period of time to make sure all the gas is gone.

6. Radiation

nonionizing (ultraviolet) - used to disinfect air and surfaces. It is very effective but it has poor penetrating ability.

ionizing (gamma rays) - very powerful sterilizing agent widely used in industry to sterilize disposable medical/surgical supplies. Especially useful since items can be packaged before exposure to the radiation.