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).

575 COSMETIC INGREDIENTS THAT RESPECT SKIN MICROBIOTA
MODEL DESIGNING, STRAINS SELECTION
Skin microbiota is composed of more or less 1,000 species per person,17 according to data
from 16S rRNA amplicon sequencing, or Shotgun sequencing. These communities do not
consist in a random combination, but rather in the specific association of microorganisms
particularly fit to their environment, as illustrated by the different consortia colonizing the
different zones of the skin (sebaceous, dry, moist). Considering that microorganisms adhere
to the skin surface, some authors describe these communities as biofilms. Indeed, Jahns
et al. (2014) were able to observe biofilms of C acnes in the pilosebaceous units.18 In complex
consortia, microorganisms have complex relationships.19 Today, there is growing evidence
showing that bacteria in microbial communities behave differently than in axenic culture,20
and produce a wider variety of metabolites.21 Recently, Benitez et al.22 also demonstrated
that some bacteria were more resistant to antibiotics when incubated in a coculture with
resistant strains, highlighting the beneficial impact of interactions on the bacterial behavior.
Therefore, a culture model using multiple wild-type species, in proportions relevant to the
skin microbiota, was designed to study the effect of exogenous substances. This model
could be the first step of a more complex design to study multispecies biofilms.
The methodology used here to collect wild type bacteria on the skin involved an enrichment
and a growth step on laboratory media. Consequently, only cultivable bacteria were collected
and integrated into the model. It also means that the isolated species is the one that grew
faster on artificial media, but not necessarily the most representative of a bacterial genus on
the skin (i.e. for Corynebacteria).
While the proportions in the microorganisms vary in each individual’s microbiota, it is
clear that C acnes is the most prevalent representative of the face microbiota. Cutibacterium
acnes is adapted to thrive in low-oxygen environments and able to metabolize lipids.
Consequently, it is mostly found on sebaceous areas (back, shoulder, face), and more
specifically in the pilosebaceous units. While C acnes is notoriously known to be involved in
the acne pathology, it is now admitted that the loss of balance between the different strains
(phylotypes) is partly responsible for this skin condition, rather than the simple presence
of the bacteria.23 Besides, positive effects have been identified (secretion of antioxidant
molecules,24 maintenance of a protective acidic pH in the pilosebaceous unit against
pathogens25), and C acnes is now recognized as an important part of the microbiota, with
proportions reaching more than 90% of the face microbiota.26,27
S epidermidis is a coagulase negative Staphylococcus, and one of the most studied skin
commensal.28 Its beneficial roles for the skin are now better understood. For instance, the
secretion of bacteriocins like phenol soluble modulins and lantibiotics allow S epidermidis to
control the proliferation of potential pathogens. Other competing commensals like C acnes
can also be the target of S epidermidis, through various mechanisms like the secretion of
deleterious fermentation products or type VII secretion systems.29 The positive roles of
S epidermidis for the skin now extend to the participation in the regulation of inflammation30,31
or, as described recently, in the maintenance of the skin barrier, through the secretion of a
sphingomyelinase triggering ceramide synthesis in the skin.32 The literature indicates that
it is present on all body sites, with a predominance on moist areas.33
Corynebacteria are a diverse phylogenetic group, as illustrated by 16S rRNA amplicon
sequencing studies of the skin microbiota.34 They are composed of lipophilic bacteria4 that
lack essential enzyme activities (proteinase, phospholipase, saccharolytic enzymes) and are
sensitive to low pH, which is unexpected for skin residents. As a result, Kwaszewska et al.35