572 JOURNAL OF COSMETIC SCIENCE
are also subjected to ratio variations: CI 6.9–10.4% and 7.6–12.6%, respectively. However,
the coefficients of variation on the concentrations were comprised between 2.9 and 4.2%.
Overall, the variability is low with regard to the bacterial concentrations (Figure 4B),
indicating that the consortium preparation protocol is very reproducible and can be used
to analyze the effect of cosmetic products.
EVALUATION OF PRESERVATIVE SUBSTANCES
First, two common preservatives used in cosmetic products were tested on the model, at
two concentrations: phenoxyethanol and parabens (mix of ethyl- and methyl parabens).
Scores calculated from the LR (Table I) are presented in Figure 5.
The score observed for phenoxyethanol at 0.5% was near zero, indicating an absence of
effect on the skin microbiota. At the use concentration in cosmetic products (1%), C acnes
and C xerosis were completely inhibited (maximum LR), while the effect on S epidermidis,
M luteus and S mitis was more moderate (Table I). The score was –28.4, indicating a strong
deleterious effect on the skin microbiota.
At low dose (0.1%), the score observed for parabens was –5.2. However, at the use
concentration of 1%, maximal LRs were observed for all bacteria, leading to a score of
–41.9. Even at a low concentration, parabens have a negative effect on the consortium. In
accordance with what was expected, these preservative substances had deleterious effects on
the consortium and thus cannot be considered friendly to the microbiota.
Figure 5. Evaluation of preservative substances on the consortium. The gray hatched histogram represents
the maximum score (maximum LRs).
Table I
Logarithmic Reductions (LR) of Each Five Bacterial Species After Contact With Preservative Ingredients
Concentration(%) C acnes S epidermidis S mitis M luteus C xerosis
Phenoxyethanol 0.5 –0.78 –0.92 –0.49 0.16 –0.81
1.0 –8.71 –2.09 –8.32 –1.47 –7.83
Parabens 0.1 –1.40 –1.00 –1.08 –1.12 –0.66
1.0 –8.68 –8.65 –8.54 –7.93 –8.06

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).