Stability of Ready-to-Use and Laboratory-Prepared Culture Media

Abstract

The culture media quality, which depends to a large extent on growth promotion properties, determines the reliability and accuracy of test results obtained by microbiological methods. The procedure for culture media preparation is quite labourconsuming and includes several stages: successive dissolution of components exactly as specified in the recipe (qualitative and quantitative composition), sterilisation, adjusting the pH of the medium, and testing of growth promotion properties—therefore it is important to demonstrate the stability of each particular culture medium.The aim of the study was to evaluate growth promotion properties of the culture media prepared in the laboratory from a dry mixture, and to assess their stability during long-term storage.Materials and methods: stability testing was performed for fluid thioglycollate medium (FTM) and soybean-casein digest broth (SCD) prepared in the laboratory. FTM growth promotion properties were tested  using the following test microorganisms: Bacillus subtilis ATCC 6633,  Clostridium sporogenes ATCC 19404, Pseudomonas aeruginosa ATCC 9027,  Staphylococcus aureus ATCC 6538. SCD growth promotion properties were  tested using: Aspergillus brasiliensis ATCC 16404, Candida albicans ATCC  10231, Bacillus subtilis ATCC 6633. The stability study was carried out for 6  months, assessing changes in appearance and growth promotion properties. The test media were stored at room temperature and in a refrigerator at 2–8 °C.Results: no growth of the anaerobic microorganism Clostridium sporogenes was observed in FTM after 3 months, regardless of storage conditions. Later on, there was no growth of the gram-positive microorganisms Bacillus subtilis and Staphylococcus aureus, which are more sensitive to the storage conditions than Pseudomonas aeruginosa. The growth promotion properties of the SCD prepared in the laboratory did not change during 6 months of storage.Conclusions: FTM prepared from a dry mixture remains stable and retains its growth promotion properties for no more than two months when stored in a refrigerator at 2–8 °C. SCD can remain stable for 6 months, both at room temperature and when stored in the refrigerator.