It is important in preservative challenge testing to consider the typical use of the finished product and to test them in their finished state. For example, nonwoven substrates such as presaturated wipes or towelettes, should be tested in the wipe form and not on just the add-on solution. The CTFA M-5 guidelines are designed for personal care products such as wipes or towelettes that contain aqueous-based add-on solutions.
A summary of the CTFA M-5 method is described below:
- A pre-determined number of wipes or towelettes are aseptically transferred to sterile containers for inoculation. Enough wipe samples are prepared such that there is a separate sample for each sampling time point for each microorganism in the test.
- Each set of wipes are inoculated with one of the 5 method-specified microorganisms (S. aureus, E. coli, P. aeruginosa, C. albicans, and A. brasiliensis) at fairly high concentrations using a pipette to distribute the inoculum in a uniform pattern throughout the sample.
- Initial concentrations are determined by placing inoculated wipe samples into neutralizing broth, mixing appropriately to remove all microoganisms from sample and plating by standard dilution and plating techniques.
- A separate wipe sample is prepared 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 inoculated sample containers are sealed to avoid dryout and samples are held at room temperature for a period of no less than 28 days.
- The product samples are evaluated at specific intervals within the 28 day period. Sampling is recommended immediately after inoculation and at 14 and 28 days.
- 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.
1. Cosmetic, Toiletry, and Fragrance Association 2007. “CTFA Microbiology Guidelines”Cosmetics