A leather-based in vitro model to determine the effect of cosmetics on skin microbes

Human skin is home to a wide range of microorganisms, including fungi, bacteria and viruses. In fact, recent estimates indicate that more than 1011 microbial cells could be present on the surface of the skin.

As a rule, the abundance and composition of microorganisms vary depending on the specific conditions of different areas of the skin. Some of the bacterial genera found primarily on the skin include Cutibacterium, Staphylococcusand Corynebacterium.

Study: A new model to study the effects of cosmetics on skin microorganisms in vitro. Image Credit: Kateryna Kon/Shutterstock.com

Background

Many prebiotic supplements have been used in cosmetics to promote the growth of beneficial bacteria, such as coagulase negative bacteria. Staphylococci, while deleting Cutibacteria growth.

Prebiotics play an important role in rebalancing skin microbiota by promoting the growth of beneficial microbes and inhibiting pathogenic growth. Putative prebiotics can be obtained from various sugars, natural extracts and fermented products of bacteria.

A recent Beauty products Newspaper study discusses leather product development in vitro model due to its resemblance to human skin. The model was used to study the role of cosmetic ingredients on bacterial growth under realistic conditions.

About the study

Seven ubiquitous Gram-positive skin bacteria were selected to analyze how they were affected by the prebiotic actives tested. The actives used in this study included herbal glycerol, diglycerol, glyceryl caprylate and a multifunctional ingredient.

The pH of all active ingredients has been adjusted and thickened by the addition of acacia senegal gum and xanthan gum. The serum without the active ingredients was used as a control.

The growth curve of each bacterial strain was recorded in the absence or in the presence of the active principles. To assess the effect of cosmetic serum on skin microbes under more realistic conditions, a leather model imitating human skin was developed.

Study results

The possibility of microbial growth or growth inhibition was investigated using liquid culture media containing prebiotic actives at different concentrations. Culture media with greater optical density (OD) at any given time indicated promotion of bacterial growth. Likewise, cultures with lower ODs indicated inhibition of bacterial growth.

Promotion Corynebacterium jeikeium growth and partial inhibition of Microshells were observed. Even at a higher concentration of prebiotic actives, most bacterial strains remained unchanged.

Varied growth results were observed for some Staphylococci species. For example, maximum growth promotion was observed in S. aureus and S. hominis, while S. epidermidis remains unchanged.

A dissimilarity in microbial growth promotion and inhibitory effects can be observed between live and in vitro experiments using microbial strains and similar active ingredients. Indeed, the complexities of the interaction between bacteria and skin cells cannot be analyzed in a cell culture-based model.

In contrast, the leather skin model mimicked important characteristics of human skin, including bacterial adhesion and absorption of the cosmetic formula.

The newly developed leather skin model helped scientists examine how cosmetic formulations containing prebiotic actives affect natural bacterial colonizers in the skin. After 30 minutes of treatment with the serum, the serum without any added active showed a considerable effect on bacterial colonization. More specifically, the growth of S. epidermidis, S. hominisand M. flavus was marginally promoted, while S. aureus growth was inhibited.

Although general active ingredients containing serums showed a similar pattern of bacterial colonization, some minor changes and exceptions were observed. Compared to the serum colonization model alone, the addition of actives resulted in minor alterations, including less pronounced inhibition of S. aureus and a higher rate of C.jeikeum inhibition.

Consequences

The leather skin model data indicated that the cosmetic treatment had a discriminatory impact on the skin microbiota. Comparisons between skin models treated only with serum and actives containing serum did not show statistically significant different effects.

In human skin, bacterial strains do not occur individually but in a complex microbial community; thus, the evaluation of the effect of actives containing serum on a single bacterial strain is not justified. In particular, the growth effects of cosmetic active agents on the microorganisms of the skin are generally weak.

A manufacturer should consider leather skin model data to reconfirm whether their cosmetic formula promotes the growth of pathogens such as S. aureus. The current model would provide manufacturers with a complete view of the growth-inhibiting or growth-promoting effects of bacterial species in the skin microbiota.

This model could also be used to study the effect of skin-derived media, such as sweat or sebum, as well as cosmetic formulations, on skin bacteria.

Journal reference:

  • Rademacher, M., Zinn, M., Beinio, R. and Bockmühl, DP (2022) A new model to study the effects of cosmetics on skin microorganisms in vitro. Beauty products. 9(4), 88. doi:10.3390/cosmetics9040088.