In order to be able to determine the quality of decontamination tests, SKAN has developed a method for counting viable, isolated spores. The method is based on the scientific principle that spores are capable of germination and growth on solid culture media and that each microbial colony-forming unit (CFU) represents a direct progeny of a single spore in the original inoculum.
This is done by serial dilution of the spore in a suitable medium after resuspension from the carrier. The final dilution is then plated out on a suitable medium. After the spores have grown into colonies on this medium, they are counted. The Viable Spore Count results correspond to the amount of Geobacillus stearothermophilus spores that were initially inoculated on the biological indicator.
- How many biological indicators are used for a Viable Spore Count?
To generate reliable results, we recommend using at least four biological indicators for a Viable Spore Count. In this way, test errors can be reduced and valid results can be achieved.
- What diluent do you use for serial dilutions ?
Distilled, sterile water is used for serial dilutions. Ideally, a narrow, flat-bottomed tube is filled with approximately 9mL of distilled water for the process. This provides the best operating base for serial dilution of spores.
- What culture medium do you use for final plating and growth?
As part of the Viable Spore Count procedure, the spores are plated onto the Trypcase Soy Agar (TSA) culture medium. This is the standard medium used in the pour plating method. This nutrient medium offers the best conditions for healthy spore growth and allows easy counting of the colonies generated.