582 JOURNAL OF COSMETIC SCIENCE
Furthermore, lifestyle choices, including the use of cosmetics5 and dietary habits,6 have also
been shown to influence the skin’s microbial community. Numerous studies indicate that
the composition and balance of the skin microbiota can vary based on both external and
internal factors, such as skin integrity and physiological conditions.7 These conditions can
lead to various skin disorders, including atopic dermatitis,8 acne,9 psoriasis,10 and vitiligo.11
However, the origins of many skin diseases are complex and multifactorial. In particular
cases, such as SS syndrome, the precise role of microorganisms is still not fully understood,
although changes in the skin microbiota were already reported and suspected to contribute
to this condition.
For instance, we can document the findings of a first bacterial analysis by mass spectrometry
using MALDI-Biotyper of bacterial isolates collected and cultivated under aerobic
conditions. Isolates were collected on the forehead, cheek or scapula from 2 groups of
panelists of 3 men and 3 women (20–35 years old) having normal skin (NS) or SS evaluated
by lactic acid sting test (LAST). This well-recognized test allows diagnosis of the degree
of skin sensitivity and consists of the application of an aqueous lactic acid solution wiped
on the test site, while an inert control substance is used on the contralateral test site. After
application, a moderate to severe tingling sensation occurs and participants were asked to
grade the intensity of stinging using a 4-point scale (0 =no stinging 1 =slight stinging
3 moderate stinging and ≥3 strong stinging). The results of the study showed that,
for both genders of the SS group, the relative abundance of Staphylococci decreased, and
Acinetobacter or Proteus were absent. Other bacterial variations were influenced by gender as
Pseudomonas and Bacillus that were absent in female SS. Acinetobacter and Proteus decreased
only in female SS, whereas Micrococcus increased only in male SS. Kocuria evolved in the
opposite sense between male and female SS, with an increase for female SS. However, in
this small cohort, no significant skin microbiome profile associated with the SS phenotype
was identified.12
In 2020, a larger study was performed on 42 Korean women (22–52 years old, cheek
area, sensitive selection by sodium lauryl sulfate patch test and LAST). The uncultivated
microbiome samples were analyzed for the first time using a more relevant but longer
method of microbiome identification at the species level based on analyses of hypervariable
bacterial 16S rRNA gene sequences or Internal Transcribed Spacer 1 (ITS1) fungal regions
(phylotype-based approaches using the closest type of strain assignment). The results
showed an equivalent diversity of skin bacteria between the 2 groups (NS and SS), but a
significant increase of Lactobacillus and Mucor racemosus, and a decrease in Malassezia restricta
for SS.13 Another study was conducted using bacterial 16S rRNA gene sequencing on cheek
skin samples from 77 female panelists ranked in 3 groups according to LAST grading (31
NS samples with stinging test score 0 24 lightly SS with stinging test score 3, and 22 SS
with stinging test score ≥3). If skin bacterial diversity and richness were equivalent among
the 3 groups, on the genus level, there was no significant difference in the abundance
of Cutibacterium, while the abundance of Staphylococci in SS group was significantly lower
than in NS group. Interestingly, this study provided clear evidence that the abundance
of Staphylococcus epidermidis gradually decreased as skin sensitivity increased, whereas the
slight rise in Staphylococcus aureus observed was not statistically significant.14
In 2021, the first microbiome study associated with a clinical evaluation was also performed
in China on 24 subjects with SS (face or chest, LAST-based recruitment) and 10 subjects
with NS (face).15 The cuticle moisture content, transepidermal water loss, facial skin sebum

583 Specificities of Microbiota From Sensitive Skin
secretion, and facial skin surface morphology were evaluated in parallel to 16S rRNA
sequencing. No change in diversity was observed for SS and NS face skin. In the three
most common microbiota genera, including Cutibacterium, Paracoccus, and Corynebacterium,
there was no significant difference in the relative frequency among the SS and NS groups.
Nevertheless, at the genus level the authors found that in the SS group a significantly
higher frequency of rare bacteria such as Acidaminococcus, Alloiococcus, Fimbriimonas,
Pseudoclavibacter, Reyranella, and Roseococcus, but a lower frequency of Bradyrhizobium.
Spearman correlation analysis showed correlation between the microbiota genera and skin
physiological parameters. However, they found no bacterial genus with strong association
with skin physiological parameters.
A study on Caucasian women (cheek, SS evaluation using capsaicin test (61 NS 23–50
years old, 23 NS 51–69 years old, and 44 SS 20–50 years old) likewise did not show
a difference in diversity. Nevertheless, SS compared to NS with the same age class was
reported here to display a specific microbiota profile with a significant decrease in SS in
Cutibacterium, Lawsonella, and Bacillus (nonsignificant decrease for Staphylococcus), and an
increase in Corynebacerium, Snodgrassela, Kocuria, and Micrococcus.16
Finally, in 2024, Lu et al. performed a study on 10 panelists (cheek, 22–35 years old,
LAST-based recruitment) using a 2bRAD-M sequencing method enabling accurate
characterization of the low-biomass microbiome at species level. They showed in the
SS group a decrease of alpha diversity and an increase of abundance in the 3 top genera
Cutibacterium, Corynebacterium, and Staphylococcus. Among Staphylococci, S. epidermidis was
the more abundant and increased by 1.34-fold, whereas less abundant S. aureus was
the most increased (2.98-fold). They also observed that the relative abundance of rare
bacteria such as Dermabacter hominis and Chryseobacterium was significantly increased,
whereas those of Neisseria and Streptococcus decreased. Advantageously, Staphylococci
(S. epidermidis, S. capitis, and S. aureus) and Micrococcus luteus from a panelist with SS
were extracted and compared to standard S. epidermidis and aureus for their ability to
induce Interleukin-8 (IL-8) in keratinocytes. Interestingly, S. capitis and M. luteus from
the panelist with SS significantly promoted more IL-8 expression than S. epidermidis and
S. aureus.17
In this study, our goal was to first compare the composition of the skin microbiota of
individuals with NS and SS using the entire 16S rRNA gene sequencing from PacBio
technology to obtain unprecedented taxonomic resolution of the microorganisms present.
However, at the same time, we evaluated for the first time droplet-based microfluidic
technology (DBMT) to create a collection of specific bacterial species isolated from
individuals with SS and NS. DBMT was chosen as this microbiome-on-a-chip high-
throughput technology can generate more than 100 million parallel picoliter-sized droplets,
allowing the isolation of live bacterial species that grow to clonal populations at a much
higher diversity compared to standard microbiology methods when using specific culture
media. In addition, the picoliter-sized droplets containing bacteria can be isolated drop
by drop for subsequent bacterial clones’ characterization and further cosmetic ingredient
testing. The bacterial collection was used here to study the impact on bacterial growth
of cosmetic ingredients. Isolates of C. acnes from both the NS and SS collections were
also compared for their potential to stimulate the proinflammatory cytokine IL-8 in
keratinocytes with the final aim to build a screening tool using a set of bacteria previously
shown to be modulated in SS and to have impact on SS features.