Preservative Challenge Test - USP 51

This method is used to assess the efficacy of a preservative used in cosmetics, personal care products and drug products. All useful preservatives are toxic at certain levels and during product development a balance between efficacy and human toxicity must be established. The test utilizes a total of 5 challenge microorganisms, three bacteria and one mold and yeast. These microorganisms are inoculated into the tested product and evaluated at specific time points (0, 7, 14, 28 days). The antimicrobial preservative must be neutralized to recover viable cells in the microbial enumeration test at each time point.  

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Test Procedure

Before testing can begin a validation study must be performed in order to determine the proper method of preservative neutralization. This step is important because without proper neutralization the efficacy of the preservative will be overestimated.

 

Once a suitable neutralization step has been validated the product to be tested is separated into 5 separate sub samples. Each sub sample is challenged with one of the five microorganisms specified in the method at a concentration of 1 x 10^6 cfu/mL or g.

 

  • Staphylococcus aureus (ATCC 6538)

  • Escherichia coli (ATCC 8739)

  • Pseudomonas aeruginosa (ATCC 9027)

  • Candida albicans (ATCC 10231)

  • Aspergillus brasiliensis (ATCC 16404)

 

The inoculum is plated to determine the initial concentration of each microorganism. After inoculation the product is then held at storage conditions or room temperature for a minimum of 28 days. The product is then evaluated at intervals depending on the product category specified in the method. During each evaluation, the inoculated product is neutralized, plated, and incubated for 48-72 hrs to determine viable cell count of each microorganism. The resulting log reduction of each microorganism is then reported for each microorganism and the effectiveness of the preservative is determined. Established passing criteria are defined by USP 51 for each category.​​​