The aim of antimicrobial efficacy testing (AET) performed during cosmetic product development is to predict microbial stability and consumer safety during use. The AET design includes referenced microbial strains and acceptance criteria. However, such tests do not include specific situations resulting from consumer use; for example, microbial flora encountered in normal environments; repeated insults; environmental condition variability; the impact of accessories and packaging; and the possibility of localized inoculation, e.g., via caps.
Therefore, it was deemed necessary to develop an additional test to strengthen the investigation. Here, the authors propose an approach to assess the microbial stability of a product during use, referred to as the Microbiological Use Test (MUT), and apply this analysis in a few case studies to predict the microbiological risk of commercial products. The described test has been used successfully in the development process of cosmetic products.
The MUT test assesses, during the product development phase, the ability of a product to prevent its own microbial contamination during “standard” conditions of use. The aim is to perform a quantitative and qualitative assessment of potential contamination after a specified period of use. To ensure results, some parameters must be fixed; for example, blind testing, to ensure the product is used under conditions close to reality. Panelists should not be informed of the aim of the study—especially that the product will be microbiologically tested upon return. Also, packaging must be as close as possible to the final form, including being comprised of the same materials and utilizing the same closure system. This is a key point, as packaging plays an important role during product use and, therefore, in product contamination.
Timing and conditions also are important. Samples should be returned directly to the microbiological laboratory without extra manipulation prior to testing. The time between the last use and the first test must be fixed, e.g., 72 hr, to limit the recovery of transient microbes but allow for the detection of the more critical persistent contaminants. Further, it is necessary to have at least 20 samples involved to ensure a relevant assessment.