• Home |
• Contacts |
• Quality Standards |
• Instructions for Use |
• Careers

• 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

PH TESTING GUIDELINES

1. The water used in the preparation of culture media should be distilled or de-ionised. This water should have a conductivity measurement of <15 micro-Siemens and a pH of not less than 5.5 and not more than 7.7. If the de-ionisation process is inadequate, residual acid used during the process may cause the final pH of the medium to drop.

2. The pH of the prepared medium should be tested when the medium is in its final form at ambient temperature (25°C). Agar products will, therefore, be in solid form and ideally require the use of a flat-bottomed pH probe in order to facilitate measurement. The use of temperature compensation is not recommended. Our laboratory suggests an electrode such as the BDH double-junction flat-tipped combination electrode.

3. The pH should lie within the range given on the product label. When the medium is supplemented with a selective agent, blood, serum or egg yolk, the pH limits apply to the complete medium.

4. If the stated pH is not achieved, the following should be considered:

• Check that the pH of the water is not excessively low or high;
• Adjust the water, if necessary, with dilute alkali (NaOH) or acid (HCl). The water used should be as fresh as possible, to prevent absorption of CO2 from the atmosphere. To check the pH of pure water a small quantity of KCl can be added in order to provide ions and enable a pH reading to be taken;
• Ensure that the glassware is of suitable and appropriate quality for the preparation of culture media. Avoid soda glass where possible and restore acid washed glassware to neutrality by rinsing;
• The adjustment of media prior to autoclaving should, where possible, be avoided. This measure may be used as a short-term expedient to avoid 'non-compliance' to set standards;
• Ensure that the heat processing time and temperature are not exceeded, as this can affect the final pH of the product.

5. Validation, to demonstrate that a medium has performed satisfactorily when slightly out of the stated pH specification, should be sufficient to comply with accreditation requirements. In addition, the laboratory should inform the manufacturer to obtain appropriate advice in this respect.