577 COSMETIC INGREDIENTS THAT RESPECT SKIN MICROBIOTA
The maximum LR that can be obtained for a species in the present model is –8.5 (maximal
concentration =8.5 log CFU/mL). A score of approximately –42.5 is therefore the maximum
score that can be obtained. Between –5 and –42.5, ingredients have deleterious effects on
the microbiota. Indeed, scores for phenoxyethanol (1%) and parabens (1%) were –28.4 and
–41.9, respectively, which is congruent with their antimicrobial properties. It is interesting
to note that a concentration of 0.5% of phenoxyethanol has no inhibitory effect on the
consortium after 8 hours of contact. A concentration inferior to 1% of phenoxyethanol is
recommended in finished formulas,42 yet inferior concentrations still exhibit antimicrobial
activity. More specifically, Wang et al. showed that S epidermidis and M luteus monocultures
were inhibited by phenoxyethanol at 0.5% in an in vitro minimum inhibitory concentration
(MIC) assay.14 On the contrary, no effect was observed on these bacteria in the present
assay (data not shown). The reduced contact time and the presence of several species in
the consortium, combined with specific growth conditions (lower pH, low temperature)
could explain the low efficacy of phenoxyethanol at a concentration of 0.5%. In general,
antimicrobial substances lead to expected reductions of bacteria in the model, which
indicates that the designed protocol is adapted to test ingredients with unknown effect.
Finally, among the common substances used in cosmetics tested here, most of them
had little effect on the consortium. Pentylene glycol (2.5%), a solvent not listed as an
antimicrobial, reached the score of –3.6. While no significant impact of pentylene glycol
was observed at this concentration, pentylene glycol is known to enhance the efficacy of
preservatives in cosmetic formulas by lowering the water activity.42 Puschman et al. (2018)
also demonstrated that the antimicrobial activity of a preservative (phenoxyethanol) in
emulsion gels is dependent on the lipophilicity of the oil phase and the concentration of
emulsifiers.43 Future studies should focus on testing several ingredients simultaneously, and
possibly complete cosmetic formulas, to have a more precise understanding of the effect of
substance combinations on the skin microbiota, and identify synergistic or antagonistic
effects.
Evolutions of the protocol should also be considered. For instance, increasing the incubation
time could be useful to analyze potential beneficial ingredients. The introduction of
pathogen species or the disruption of the proportions of the consortium would also bring
interesting data on known dysbiosis such as atopic dermatitis or acne vulgaris.
CONCLUSION
As proposed by van Belkum et al., cosmetic products should have the least possible impact
on the skin’s microorganisms.13 To do so, the authors evoke the need of first intention
in vitro prescreening methodologies, and in vivo testing of cosmetic products to assess the
friendliness toward the microbiota. Since in vivo assays are more complex to implement,44
i.e., require extensive toxicity assessment, ethic validation, specific sampling strategies
and complex analysis, one would recommend reserving in vivo assays for finished products
or formulas. Instead, in vitro assays such as the one presented here, are useful to analyze
the precise impact of ingredients composing formulas. To date, there is no consensus
or standard methodology for such evaluations, nor regulation to claim the “microbiota
friendliness” of cosmetic products. The present article is a proposition of an in vitro protocol
to measure the impact of ingredients and, potentially, combinations of ingredients on the
skin’s main bacterial commensals, which brings additional data, complementary to the
safety and efficacy data.
The maximum LR that can be obtained for a species in the present model is –8.5 (maximal
concentration =8.5 log CFU/mL). A score of approximately –42.5 is therefore the maximum
score that can be obtained. Between –5 and –42.5, ingredients have deleterious effects on
the microbiota. Indeed, scores for phenoxyethanol (1%) and parabens (1%) were –28.4 and
–41.9, respectively, which is congruent with their antimicrobial properties. It is interesting
to note that a concentration of 0.5% of phenoxyethanol has no inhibitory effect on the
consortium after 8 hours of contact. A concentration inferior to 1% of phenoxyethanol is
recommended in finished formulas,42 yet inferior concentrations still exhibit antimicrobial
activity. More specifically, Wang et al. showed that S epidermidis and M luteus monocultures
were inhibited by phenoxyethanol at 0.5% in an in vitro minimum inhibitory concentration
(MIC) assay.14 On the contrary, no effect was observed on these bacteria in the present
assay (data not shown). The reduced contact time and the presence of several species in
the consortium, combined with specific growth conditions (lower pH, low temperature)
could explain the low efficacy of phenoxyethanol at a concentration of 0.5%. In general,
antimicrobial substances lead to expected reductions of bacteria in the model, which
indicates that the designed protocol is adapted to test ingredients with unknown effect.
Finally, among the common substances used in cosmetics tested here, most of them
had little effect on the consortium. Pentylene glycol (2.5%), a solvent not listed as an
antimicrobial, reached the score of –3.6. While no significant impact of pentylene glycol
was observed at this concentration, pentylene glycol is known to enhance the efficacy of
preservatives in cosmetic formulas by lowering the water activity.42 Puschman et al. (2018)
also demonstrated that the antimicrobial activity of a preservative (phenoxyethanol) in
emulsion gels is dependent on the lipophilicity of the oil phase and the concentration of
emulsifiers.43 Future studies should focus on testing several ingredients simultaneously, and
possibly complete cosmetic formulas, to have a more precise understanding of the effect of
substance combinations on the skin microbiota, and identify synergistic or antagonistic
effects.
Evolutions of the protocol should also be considered. For instance, increasing the incubation
time could be useful to analyze potential beneficial ingredients. The introduction of
pathogen species or the disruption of the proportions of the consortium would also bring
interesting data on known dysbiosis such as atopic dermatitis or acne vulgaris.
CONCLUSION
As proposed by van Belkum et al., cosmetic products should have the least possible impact
on the skin’s microorganisms.13 To do so, the authors evoke the need of first intention
in vitro prescreening methodologies, and in vivo testing of cosmetic products to assess the
friendliness toward the microbiota. Since in vivo assays are more complex to implement,44
i.e., require extensive toxicity assessment, ethic validation, specific sampling strategies
and complex analysis, one would recommend reserving in vivo assays for finished products
or formulas. Instead, in vitro assays such as the one presented here, are useful to analyze
the precise impact of ingredients composing formulas. To date, there is no consensus
or standard methodology for such evaluations, nor regulation to claim the “microbiota
friendliness” of cosmetic products. The present article is a proposition of an in vitro protocol
to measure the impact of ingredients and, potentially, combinations of ingredients on the
skin’s main bacterial commensals, which brings additional data, complementary to the
safety and efficacy data.
