Preservative Challenge Test

USP 51: Procedure at a Glance

The Antimicrobial Effectiveness test method or USP 51 utilizes the following microorganisms Candida albicans (ATCC No. 10231), Aspergillus niger (ATCC No. 16404), Escherichia coli (ATCC No. 8739), Pseudomonas aeruginosa (ATCC No. 9027), and Staphylococcus aureus (ATCC No. 6538). The test product is inoculated with a concentration between 10⁵ to 10⁶ CFU/ml (0.5%-1.0% the volume of the test product) and incubated for a pre-determined amount of time which is dependent on the product category.  The inoculated product is then periodically evaluated for surviving microorganisms. 

The test product, can range from cleansers to cosmetics and other personal care products. The criteria of antimicrobial effectiveness for the test products is dependent on the route of application.  Generally a 1.0-3.0 log10 reduction of microorganisms is required at 14 day interval and no increase from the starting inocula from day 14 to 28 days.

Test controls are implemented into the study to account for any variables that could affect the test outcomes.  Test controls performed in the study include assay of the concentration of the microorganisms used to inoculate the product,  assay of the starting concentration of the bacteria as inoculated into the product, viability control of test medium, neutralization control, and media sterility.  The measurement of the starting concentration of the microorganisms, or time zero, is used to compare against the concentration during each interval, the viability control confirms media isn’t toxic to the test germs, neutralizing controls validates that the neutralizing recovery medium stops the bactericidal activity of the test product.

USP 51: Microchem’s expertise

The staff at Microchem laboratory have carried out the USP 51 challenge test for several years on products ranging from cleaners to personal care products.  Scientists are present to explain test results and answer questions about how results from preservative challenge testing relates to shelf life and microbiological stability.