573 COSMETIC INGREDIENTS THAT RESPECT SKIN MICROBIOTA
Phenoxyethanol (1%) was used as a negative control in each test series to validate the
efficacy of the assay.
EVALUATION OF NEUTRAL SUBSTANCES
Substances without bactericidal effect were evaluated: Two carbohydrates (glucose and
inulin), two amino acids (glycine and proline) and glycerol. After 8 hours, the bacterial
concentrations were measured and the score of each tested ingredient (sum of LR, Table II)
was calculated according to the method described above (Figure 6).
Glucose and inulin were tested at 0.1% and 1%, as well as 2% for glucose. Even at the
highest concentrations, the LRs were negligible for all bacteria and consequently the scores
were close to zero, indicating an absence of effect of the materials.
Amino acids were tested at 0.1% and 1%. The maximum LR was observed for proline
at 1% on S epidermidis (LR =–2.7). The other LR were close to –1 and overall, scores
were comprised between 0 and –5. The maximum LR for glycerol (1%) was observed for
S epidermidis (–2.1). The maximum score on the consortium was –4.2. The scores obtained
by summing up all the LR indicate that the products had a negligible effect on the
consortium. As expected, glucose, inulin, proline, glycine, and glycerol can be considered
friendly toward the microbiota.
Table II
Logarithmic Reductions (LR) of Each Five Bacterial Species After Contact With Neutral Ingredients
Concentration (%)C acnes S epidermidis S mitis M luteus C xerosis
Glucose 0.1 –0.24 –0.82 –0.13 –0.27 –0.23
1.0 –0.44 –0.82 –0.20 0.03 –0.25
2.0 –0.23 –0.16 –0.27 0.18 0.02
Inulin 0.1 –0.86 –0.59 0.13 –0.05 –0.08
1.0 –1.20 –0.31 0.14 0.37 –0.16
Glycine 0.1 –0.70 –1.23 0.00 0.64 –0.55
1.0 –2.53 –0.75 –2.60 1.19 0.02
Proline 0.1 –1.70 –1.80 –0.30 0.10 –0.10
1.0 –0.80 –2.70 –0.80 0.20 –0.50
Glycerol 0.1 –1.20 –1.10 –0.30 0.10 0.00
1.0 –1.50 –2.10 –0.40 0.20 –0.40
Figure 6. Evaluation of neutral substances on the consortium. The gray hatched histogram represents the
maximum score (maximum LR).

574 JOURNAL OF COSMETIC SCIENCE
EVALUATION OF COSMETIC INGREDIENTS
Finally, three common ingredients used in cosmetic formulas (butylene glycol, pentylene
glycol, 1,3-propanediol) were tested, at a maximum concentration of 5%. In addition, a
water-soluble composite polymer based on acacia gum and xanthan gum was tested at a
concentration of 1%. This dose was chosen because it has the most stabilizing effect in most
formulations. Results are presented in Table III and Figure 7.
Among all tested products, most of them presented a score between 0 and –5, indicating that
they are neutral toward the skin microbiota. Pentylene glycol at 2.5% (max concentration
in finished products) reached a score of –3.6.
DISCUSSION
How do cosmetic products affect the skin microbiota? To date, there is a general assumption
that cosmetic products have a negative impact. However, the multiplicity of protocols to
test the impact of cosmetics on the skin microbiota, combined with technical difficulties
in manipulating finished formulations, due to their physical characteristics (viscous, greasy,
opaque etc.) and complex chemical nature, make it difficult to address such a simple question.
The present work aims at designing a relevant in vitro methodology to test cosmetic
ingredients, and to analyze their effect on the main bacterial components, in a coculture
assay.
Table III
Logarithmic Reductions (LR) of Each Five Bacterial Species After Contact With Cosmetic Ingredients
Concentration (%)C acnes S epidermidis S mitis M luteus C xerosis
Pentylene 2.5 –0.24 –0.82 –0.13 –0.27 –0.23
Butylene 2.5 0.63 0.37 –0.83 –0.30 0.12
5.0 0.30 0.12 –1.30 0.12 –1.38
1,3-propanediol 2.5 0.43 0.21 –0.21 –0.16 –0.39
5.0 0.64 0.37 –1.38 –0.10 –1.11
Composite polymere 1 1.0 –1.50 –2.10 –0.40 0.20 –0.40
Figure 7. Evaluation of cosmetic ingredients on the consortium. The gray hatched histogram represents the
maximum score (maximum LR).