Once it has been determined that products are free from harmful microorganisms, it is important to ensure that they remain free from contamination during manufacturing consumer use. That is accomplished through challenge testing the formula with known microorganisms to verify that if the products become contaminated, the preservative system is sufficient to inhibit the growth of the microorganisms and spoilage. The CTFA M-3 (much like the USP 51) is a challenge method that is designed for water-based products.
A summary of the CTFA M-3 method is described below:
- The product is separated out into 5 containers, each being challenged with one of the 5 method-specified microorganisms (S. aureus, E. coli, P. aeruginosa, C. albicans, and A. brasiliensis) at fairly high concentrations.
- The initial concentration of each microorganism is determined by standard dilution and plating techniques.
- A separate aliquot of the product is diluted along with a control substance, to be used in the neutralization and recovery validation to verify that the methods and media used are sufficient to recover the microoganisms.
- The product is held at room temperature for a period of no less than 28 days.
- The product is evaluated at specific intervals within the 28 day period that predetermined by the study sponsor.
- Test sample colonies are counted at each specified interval to determine the amount of microorganisms remaining.
- The log reduction of each microorganism at each interval is reported.
- OPTIONAL: The product is separated into fewer containers and mixed culture inoculations are used.
- OPTIONAL: The product is re-inoculated with even more microorganisms and allowed a longer incubation period (specified by study sponsor).
1. Cosmetic, Toiletry, and Fragrance Association 2007. “CTFA Microbiology Guidelines"