This test is also known as: - Microbial Challenge, Microbial Kill testing, Die off testing or antimicrobial activity testing)
Suspension of organisms are used to challenge an antimicrobial solution/product under evaluation. Samples are inoculated with a pre-defined high concentration number of organisms and numbers of survivors remaining assessed over time.
The result is quoted as a log reduction the product has achieved, at each time point, compared with a control count at the start of the test.
Used to assess the activity of antimicrobial products using one of three variations.
- Standard Die-off tests
- Modified Die-Off tests -evaluation in the presence of a nutrient rich medium e.g. serum
- Can be used for repeat challenge testing. Adaptable method.
Principles of Log Reduction Test
Antimicrobial dressing/sample prepared, (for example, 2cm x 2cm or 2.5cm x 2.5cm pieces for dressings or 1ml volumes for liquid samples)
107 cfu/ml culture added to dressing/sample
Incubated at desired temperature (e.g. 32oC, 37oC) for required time (e.g. 0, 30 minutes, 4 hours 24 hours)
Dressing sample is placed into tube containing a neutraliser(s) and then further decimal 1 into 9ml dilutions carried out using standard diluents
From each dilution 1ml placed into each of duplicate petri dishes and mixed with molten agar ("pour plates") or plated onto aerobic petri film.
These are incubated at 32oC for 24-72hrs before recording the results from the dilutions which are countable and the reduction in numbers worked out
The count in the original solution is determined, the numbers converted to log10 counts. The control Time 0 log count us used as baseline from which to subtract the sample log counts at the different time points and reported as "log reduction". The higher the log reduction in the shortest time, the better the antimicrobial activity
There are many variations in the detail of how this test is carried out, including the inoculum level used, but the principle of using this test to determine the number of microbes killed by an antimicrobial product remains the same.