A controlled forearm washing ex vivo method for assessing the impact of personal cleansing products on skin's acid mantle properties and antimicrobial defence against transient bacteria.

Abstract

An important trend in the personal care industry involves the development of advanced personal cleaning products that not only provide skin mildness but support skin's acid mantle properties and skin's natural antimicrobial defence function. The objective of this study was to develop a controlled forearm washing ex vivo method for assessing the impact of personal cleansing products on skin's acid mantle properties and antimicrobial defence against transient bacteria. We developed a controlled forearm washing ex vivo method (ex vivo NET method) to compare the impact of two representative personal cleansing products on skin's acid mantle properties and antimicrobial defence against transient bacteria: one was a low-pH skin cleanser, and the other was high-pH soap cleanser. Skin pH was measured at baseline and 4 h after the product application. Concurrently, D-squame tape stripping procedure was followed to sample the stratum corneum surface layers. Then, two selected transient bacteria, Staphylococcus aureus and Escherichia coli, were inoculated onto the D-squame tapes and incubated under controlled conditions, respectively. The residual bacteria counts can provide an objective measure of skin's acid mantle properties against transient bacteria. Results from the ex vivo NET method were compared with the traditional in vivo cup-scrub RET method. The skin pH was significantly lower 4 h after washing the forearm with the low-pH cleanser versus the high-pH soap, consistent with literatures. Interestingly, the skin surface washed by the low-pH cleanser showed significantly higher hostility against representative transient bacteria as demonstrated by the lower counts of S. aureus by 1.09 log and E. coli by 0.6 log versus the high-pH soap based on the ex vivo NET method. Results from the ex vivo NET method were further supported by the traditional in vivo RET method which also showed the skin washed by the low-pH cleanser had significantly lower counts of S. aureus and E. coli versus the high-pH soap. The skin's acid mantle properties and antimicrobial defence can be directly impacted by the personal cleansing products. The low-pH skin cleanser works better than the high-pH soap for supporting skin's acid mantle properties and antimicrobial defence against transient bacteria. Results from the ex vivo NET method are consistent with the in vivo RET method. It is important that the ex vivo NET method offers many advantages since it is quicker to run with higher throughput and has better safety without the constraint of inoculating harmful microorganisms onto the human subjects.