The preparation of culture media is one of the basic skills for microbiology laboratory personnel. Appropriate preparation procedures, correct sterilization methods, proper storage conditions and quality control tests guarantee high quality culture media.
I. Cause analysis of common quality problems
1.Failure of medium solidification
Excessive heating during preparation
Acid hydrolysis of medium caused by low pH value
Incorrect weighing; incomplete dissolution of agar
Insufficient mixing of medium components
2.Incorrect pH value
Excessive heating during preparation
Poor quality water
Contamination by exogenous chemical substances
Improper temperature during pH measurement
Uncalibrated pH meter
Poor quality of dehydrated culture medium
3.Abnormal colour
Excessive heating during preparation
Poor quality water
Incorrect pH value
Extraneous contamination
Poor quality of dehydrated culture medium
4.Precipitate formation
Excessive heating during preparation
Poor quality water
Incorrect pH value
Impurities in raw materials
Poor quality of dehydrated culture medium
5.Growth inhibition / low growth rate
Excessive heating during preparation
Poor quality water
Poor quality of dehydrated culture medium
Incorrect components or incorrect dosage of supplements
Toxic residues in containers or water
6.Poor selectivity
Excessive heating during preparation
Poor quality of dehydrated culture medium
Incorrect formula applied
Improper supplemented components, e.g. supplements added to over heated medium, wrong supplement concentration, etc.
7.Contamination
Deficiencies in aseptic operation
Contaminated supplements
Inappropriate sterilization
II. Precautions
1.Sub packaged culture media shall be sterilized on the same day (sterilization within 2 hours is optimal). The filling volume shall not exceed two thirds of the container volume to prevent overflow during sterilization.
2.Moist heat sterilization is adopted for culture media in laboratories. Sterilize at appropriate temperature and duration in accordance with the instructions for the culture medium. For media with high sugar content in particular, excessive temperature shall be avoided, as high temperature will cause sugar caramelization and compromise medium quality.
3.Preparation records shall be kept for every batch of medium. A duplicate copy shall be made: the original record shall be filed for future reference, and the duplicate shall be stored together with the finished medium to avoid confusion.
4.Culture media shall be stored in a cool dark place, preferably in a conventional refrigerator. The storage period shall not exceed one week; poured plate media shall be used within 3 days. Each batch of medium must be accompanied by a duplicate preparation record or an obvious label.
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