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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

WATER QUALITY

The quality of water used for reconstitution of dehydrated culture media can affect the performance of the medium being prepared. The quality of the water must be regularly monitored and maintained by the Microbiology Department.

Fresh, high quality water prepared by distillation, de-ionisation, or reverse osmosis is recommended for the satisfactory reconstitution of culture media. Tap water should not be used as it contains impurities such as calcium and magnesium and their metal ion traces. In addition chlorine and fluorine will alter the characteristics of selective media. In areas of hard water, it is advisable to soften the water prior to distilling. Condensed boiler distillate should not be used owing to the carry-over of chemicals used to treat the water (Gifford 1960). Boilers have also been known to contaminate media during autoclaving (Human 1979). It may be necessary to add trace elements to media in order to allow fungi to exhibit characteristic morphologies (King et al 1991). However, tap water is too variable in its ion content to be considered.

Equipment for preparing de-ionised or distilled water should be maintained in accordance with the manufacturer's recommendations. The conductivity of the water should be within the manufacturer's range, typically under 10 micro Siemens/cm. It is advisable not to exceed a maximum limit of 15 micro Siemens. The conductivity and pH should be checked daily. A high conductivity reading indicates a malfunctioning still or de-ioniser cartridge. Portable conductivity meters are widely available and are suitable for this purpose.

The pH of water is difficult to measure accurately because of its low ion content (Hunt 1985). The pH of the water should, however, be checked if there is a problem with the medium. The pH can be measured by adding 0.3ml of saturated KCl solution to 100ml of the de-ionised water before using a calibrated pH meter. A pH of 6 is considered typical and satisfactory for the majority of culture media.

Water should be stored for as short a period as possible owing to potential build up of algal and bacterial colonies, especially Bacillus spp., in stills and de-ionisers. Store media in clear glass sealed containers. Water stored in polyethylene can become acid on storage, whereas soda glass can give an alkali drift in pH. The use of metal containers or contact with pipework may risk leaching of metallic ions into the water.