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• Anaerobic Systems
• Antibiotic Sensitivity Testing
• Culture Media, Supplements and Raw Materials
• • Are there any guidelines for preparation and reconstitution of culture media?
  • Are there any guidelines for sterilising culture media?
  • Can I prepare culture media in a microwave?
  • Can I store selective supplements after reconstitution?
  • How can I re-melt solid media?
  • How long can I store culture media after preparation?
  • How should I dispose of used culture media?
  • How should I test the pH of culture media?
  • My culture medium does not look right - what has gone wrong?
  • My culture medium is not performing correctly - what has gone wrong?
  • Should I boil media before autoclaving?
  • What are the main components of culture media?
  • What exactly is agar?
  • What grade of water should I use to prepare culture media?
  • What is the shelf life of dehydrated culture media after the pot has been opened?
  • What quality control testing should I carry out on culture media?
  • Which blood product should I choose to make blood agar?
• Diagnostic Reagents
• Dipslides
• General
• Toxin Detection

PREPARATION OF DEHYDRATED MEDIA
Dehydrated media are hygroscopic and are sensitive to moisture, heat and light. They are adversely affected by drastic changes in temperature e.g. hot/cold cycling temperatures which may occur between day and night laboratory temperatures in winter.

Storage conditions are usually indicated on the product label and should be followed.

1 Write on the label the date of receipt in the laboratory.

2 Store as directed on the label; usually below 25°C in a dry area, away from direct sunlight, autoclaves, drying ovens or other heat sources. Where indicated store at 2-8°C.

3 Check expiry date on the label, some media have significantly shorter shelf-lives than others.

4 Use stock in lot/batch number order. Do not open a new bottle until the previous bottle has been emptied. Note on the label the date the container is first opened. After use, make sure the container is tightly closed and return it to the designated storage area.

5 Order the medium in an appropriate size of container and in a quantity which accords to normal use requirements. A medium in a large container which has been opened many times will deteriorate on storage. Discard the medium if the powder is not free flowing, if the colour has changed or if it appears abnormal in any way.

RECONSTITUTION OF DEHYDRATED MEDIA
Complete instructions for the preparation of culture media are given on the label of each bottle. As a general rule it is wise to prepare one week's requirement only.

1 Use water prepared by distillation, deionisation or reverse osmosis. Toxic metal ions such as copper must be absent. Check the pH of the water, if below 5.5, heat to drive off CO2 and re-check. The conductivity of the water should ideally be below 15 micro siemens. Rinse glassware before use.

2 Prepare the medium in a vessel about twice the final volume of the medium to allow adequate mixing. Follow the instructions given on the label of each product.

3 Open the culture medium container away from draughts and moisture. Avoid inhaling the powder and prolonged skin contact. Weigh the powder quickly, accurately and without creating `clouds of dust'. Reclose the container as soon as possible.

4 Pour half the required volume of water in the vessel, then the weighed quantity of medium and agitate briskly for a few minutes. Pour the rest of the water down the sides of the vessel to wash any adherent medium back into solution. This is an important step because dry culture media powder above the level of the water may not be sterilised in the autoclave and may be a source of contamination.

Agar-free media will usually dissolve with gentle agitation.

Media containing agar should be heated to dissolve the agar before autoclaving. Bring the medium to the boil without scorching or burning. Those media which should not be autoclaved will be ready to pour into dishes or other containers after this amount of heating. Most culture media will require final sterilisation in an autoclave at 121°C for 15 minutes.

The pH of the dehydrated medium has been adjusted so that the final pH of the prepared medium conforms with the label specification when the medium has been cooled to 25°C. Do not adjust the pH before sterilisation.