Hi, I'm new here. I hope somebody can help me with this problem. If I want to prepare a 2X108 and 2X109 cfu/ml contamination fluid, how do I do it? I mean how to know how much bacteria to put in? What does 'standard quantitative culture technique in accordance with prEN 12054' means? Thank you very much for your solutions!!!!

Suspend some bacteria in the vol you need....then you will need some sort of counting chamber (e.g. a hemocytometer) to find out how many bacteria you have.

then you can dilute your solution to the desired density or add more bacteria.

EN = European norm. I think 12045 is currently in (re)draft form. I'd definitely go on the net and track down the document as it exists - registration of disinfectants etc. under BPD requires compliance.