534 JOURNAL OF COSMETIC SCIENCE
skin microbial community. In their novel approach, the authors developed a co-culture
system, where five bacterial species normally associated with the microbiota of the face were
collectively grown up from natural isolates, including: Cutibacterium acnes, Corynebacterium
xerosis, Micrococcus luteus, Streptococcus mitis, and Staphylococcus epidermidis. This was a two-
part endeavor: first, to establish a procedure allowing for the co-culture of all five bacteria
without interference, and second, to investigate the utility of the approach to evaluating
the relative impact of certain cosmetic care ingredients on the growth patterns of the five
bacterial species commonly detected on the face. The co-culture model was effective at
distinguishing between materials with established antagonistic and neutral effects on skin
bacteria. This represents a new tool in the ongoing process of developing cosmetic care
ingredients that are not harmful to the beneficial microbes on the skin
Gault et al. delve into a new technological approach to studying the skin microbiome:
a droplet-based microfluidic methodology. In their report, they have capitalized on the
advantages of this technology platform to investigate sensitive skin syndrome, a condition
that has been often overlooked in the study of the microbial residents of the skin. In a
comprehensive research study, the authors discovered unique changes in the populations
of several species and strains of skin bacteria between healthy and sensitive skin. The
technology was also utilized to determine the relative impact of certain skin care ingredients
on the balance of the skin microbiota, further establishing the potential applications of
their approach.
Drohan et al. tackle the important issue of how best to communicate the many advances
being made in our understanding of the skin microbiome to the average consumer.
They argue that much of the cutting-edge knowledge that has been amassed is largely
inaccessible to the public due to brands opting to push non-substantive claims and that
there needs to be a greater consensus around the language used to convey the importance
of the scientific findings from microbiome investigations. The authors adeptly touch upon
many of the topics being researched in the field. Special emphasis is given to the skin-gut
axis and its bidirectional nature as well as the participation of the host immune system in
the form of dendritic cells.
Criton et al. present interesting results of an investigation into the capacity of a pentapeptide
to counter the effects of C. acnes on blemished skin. This research study provides an
interesting new approach to ameliorating blemished skin by targeting the bacterial species
implicated in its development. Their approach to utilizing a unique pentapeptide to limit
the negative impacts of C. acnes was most notably seen with its disruption and diminishment
of C. acnes biofilms, which are often observed with the most acneic strains of the bacteria.
Further testing produced a remarkable series of beneficial effects including a reduction
in inflammation, induction of hyaluronic acid synthesis, and increased production of key
fibers in the assembly of the extracellular matrix. These findings represent another step
forward in the development of countermeasures to acne vulgaris.
Garlet provides an eloquent update on the continually growing connections between the
skin microbiome and the aging process. The author describes the history of investigational
studies into the various microbes that inhabit the human body’s largest organ: from the
early days of culture-dependent methods that limited the scope of our understanding given
that most microbes cannot be cultivated in a laboratory setting to contemporary culture-
independent research utilizing the ever-advancing genomic sequencing technologies
that no longer confine our understanding of the skin microbiota to a small subset of

535 Foreword
microorganisms. As the research marches forward, Garlet details the exciting correlations
that have been identified between the signs of aging and fluctuations in specific populations
of microbes on the skin.
Schaefer et al. conclude this special skin microbiome issue with an elegant in-depth
elaboration of the current state of knowledge involving the interconnectivity of the gut
and the skin microbiomes. The so-called “gut-skin axis” has been described in some depth
elsewhere and more rarely the reverse “skin-gut axis” has been speculated upon. Here,
the authors comment on the bi-directionality of this important axis of communication
between members of the human microbiome. They also highlight how metagenomic
sequencing technology has been a major paradigm change from the early days where our
understanding of the skin microbiome was limited to what could be cultured from the
skin, and how these new research efforts are rapidly expanding our understanding of these
complex interactions and how they provide greater resolution of the microbes that are part
of these pathways.
Collectively, these articles contribute to the substantive advancement in our understanding
of not just the components of the skin microbiome, but the intricate interconnectedness of
this diverse community and its impacts on skin health.
Many unanswered questions and nebulous aspects remain, assuring exciting future
revelations to be made in this area of research. This frontier of skin microbiome topics
includes the unheralded members of the skin microbiota that are less often mentioned:
the fungi, protists, archaea, viruses, and others yet to be fully characterized. Indeed, the
discovery of a new group of acellular biological entities composed entirely of RNA called
“obelisks” was reported in early 2024, and they appear to be associated with the gut and
oral microbiomes, which potentially could extend to the skin. Also, pathogenic varieties
of prions, another acellular biological entity, have been found to be “seeded” into human
skin under certain conditions. The archaea certainly present an intriguing mystery given
that they are representative of the third cellular domain of life on Earth and are frequently
characterized as environmental extremophiles. Their precise role in skin health has yet to
be defined, but there appears to be an age component as they are detected at their highest
levels at early ages up to adolescence when there is a pronounced drop in their abundance
only to resurge again around the age of 60. Similarly, metagenomic sequencing efforts
have shown the viral component of the skin microbiota seems to be dominated by DNA-
based viruses, which could have deeper meaning or be an artifact of genomic sequencing
techniques that favor DNA genomes. These are just a few of the tantalizing questions yet to
be answered, which will certainly provoke discussions of how and whether the personal care
industry can capitalize on the resulting findings to design, develop, and launch new classes
of active ingredients that can improve skin health by modulating these microbial targets.
As we celebrate the 75th anniversary of the Journal of Cosmetic Science, it is our sincerest hope
that you will appreciate these intriguing reports relevant to the ongoing effort to expand
our knowledge and understanding of the skin microbiome. This is a subject area that
continues to demonstrate a profound relevance to the maintenance of healthy, youthful skin.