It is every cosmetic manufacturers' responsibility to ensure the microbiological safety of their products. The first step in microbiological safety testing is to verify that each cosmetic product is free from contamination that could affect the quality of the product or consumer health. The CTFA M-1 method (much like the USP 61 method) will help cosmetic and personal care product producers assure that their products are free from contamination. See suggested microbial limits for more information.
A brief summary of the M-1 test method is described below:
- An aliquot of the well-mixed product sample is diluted 1:10 in a suitable neutralizing broth such as Dey / Engley Broth.
- The sample is mixed by vortex or other appropriate means to ensure homogeneity.
- The 1:10 and the 1:100 dilutions are plated in duplicate (2 plates per dilution per agar type) in sterile petri plates.
- Approximately 10 - 20 milliliters of bacterial and fungal growth agar (typically trypticase soy agar for bacteria and potato dextrose agar for fungi) tempered to approximately 45-50°C is added to the plates and the plates are gently swirled to mix.
- Bacterial plates are incubated at 30-35°C for a minimum of 48 hours.
- Fungal plates are incubated at 20-25°C for a minimum of 5 days.
- Sterility and growth controls are included for the neutralizing broth and agar media.
- After the incubation period is complete colonies on the plates are counted. The number of colony forming units (CFU) per mL or g is determined by multiplying the colony count by the appropriate dilution factor (10 or 100).
- Neutralization of the preservative is validated for each product tested. This is accomplished either by diluting the product in neutralizer as per the test and then inoculating with a 1:10,000 dilution of an appropriate organism and then plating to the appropriate agar and incubating alongside the test plates or by streaking a 1:10,000 dilution of an appropriate organism onto the surface of test plates at the end of the incubation period. Failure of the inoculum to grow invalidates the test and the test is repeated using a greater dilution of the test material or a more suitable neutralizer.
1. Cosmetic, Toiletry, and Fragrance Association 2007. "CTFA Microbiology Guidelines"