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While richness refers to the total number of species present in an ecosystem, diversity also accounts for the evenness of the relative abundances of such species. For example, a system of two species


where the relative abundance of each is 50% is more diverse than a system where one of the two species is dominating (e.g., 90% of species A and 10% of species B). Interestingly, reduced skin microbial diversity has been reported in neonates born by cesarean delivery compared to those born from natural childbirth and this perhaps in part explains the greater susceptibility of babies born by cesarean section to certain pathogens and allergies. The development of next-generation sequencing technique has facilitated the study of the human microbiome, first in the gut and later also on the skin.13


Like the skin, also the


gastrointestinal (GI) tract is one of the largest interfaces (30 m2


) between the host and its


environment. It is estimated that the skin has about 1012 for 1014


microbial cells while the gut accounts microbial cells and both are characterized


by a low microbial diversity at the phylum level but high diversity at the species level.14


The gut-skin axis While the skin microbes participate in epidermal development and differentiation, the gut microbiome is involved in nutrient uptake and metabolism. The intimate relationship between these organs is referred to as the ‘skin-gut axis’ and the role of the gut in the development of many inflammatory disorders of the skin, and vice versa, are currently being investigated. It seems that several skin pathologies pose


as gut comorbidities. Indeed, an impaired intestinal barrier can facilitate the entrance of intestinal bacteria as well as their metabolites in the bloodstream, which can reach and disrupt the skin microbiome.15,16


Also diet can influence


the gut microbiome, which can directly or indirectly affect skin health. For instance, a Western diet has been


associated with the development of numerous immune-mediated inflammatory diseases (IMIDs), such as rheumatoid arthritis, psoriasis, and AD. Instead, high-fibre diet (in particular inulin) and nutrients such as short chain fatty acids (SCFAs), resulting from fibre fermentation of components of the gut microbiome, are known to promote the growth of beneficial bacteria such as Bifidobacteria in the colon, stabilize gut homeostasis and regulate skin inflammation through direct or indirect mechanisms.17 However, the gut–skin axis also acts


bidirectionally: skin exposure to ultraviolet B (UVB) and therefore indirectly to serum vitamin D levels, increase the diversity of the gut microbiome.18


In addition to diet, we are


exposed to other numerous extrinsic factors that can shape the skin microbiome, including the ecosystem where humans live in (which comprises biodiversity, climate, pollution and urbanization), UV exposure, antimicrobial agents (antibiotics or anti-inflammatories), pets and cosmetics. The massive growth of urbanization is


PERSONAL CARE November 2022


reducing human interaction with nature and this create differences in cutaneous microbiota of people living in rural areas compared with urban adults. In particular, it has been shown that the bacterial diversity of the skin decreases in individuals living in urban environments and this could reasonably explain why they are more prone to suffer from skin sensitivity. Instead, early-life exposure to


rural environments and animals seems to be protective against the development of asthma and allergies.19


Even cosmetics and


the excessive hygiene through the daily use of detergents and hygienic products might affect the skin microbiome and have an important effect on its development. Conventional beauty and skincare products


contain synthetic chemicals and antimicrobial preservatives which might favour or inhibit the growth of certain cutaneous microorganisms. However, despite the widespread use of this products, their effects on the structure and functionality of the skin microbiota needs to be further investigated.


Microbiome-friendly claims As today’s consumers have realised the importance of taking care for their body by seeking for cosmetic products that improve their overall skin health and the following the exploding interest on the skin microbiome topic, an increasing number of companies have focused on this market opportunity and started to develop ‘microbiome-friendly’ skincare products. A ‘microbiome-friendly’ claim requires


scientific evidence to ensure that the product preserves microbiome diversity, does not compromise the balance of the skin and is free from contamination. Currently, the cosmetic market offers products added with prebiotics or probiotics which are generally recognized to have beneficial effects on health. Several studies, performed in animal


models, have shown the potential health benefits by the ingestion of probiotics and the improvement of both mucosal and skin health, supporting the theory of the gut-skin axis. Nevertheless, the use of live bacteria in a cosmetic formulation is not easy, and there is no regulatory definition or a clear nomenclature of probiotics in cosmetics. Indeed, the term ‘probiotic’ often includes


ingredients that are not living bacteria, but which have been obtained by means of probiotic bacteria including products of bacterial fermentation, cell lysates, non-viable bacteria.20


Next-generation sequencing In recent years, the development of next- generation sequencing has revolutionised research on the human cutaneous microbiome. This innovative approach is largely afforded by two main techniques: amplicon sequencing and metagenomics. The first is based on the sequencing of small subunit 16S ribosomal RNA (rRNA) gene as it is ubiquitous and highly conserved between


different species of bacteria and it enable us to identify the bacterial community composition settling on the skin at genus and, sometimes, at species level. Metagenomics, for example, uses the


whole-genome shotgun approach to fragment and sequence the entire DNA of a microbiome sample and should allow the identification of microbes down to the strain level, but with rather high costs and significantly more data.21 However, the accuracy of sequencing-


based analysis depends on how well the sample handling and processing have been made. Furthermore, the choice of the subjects recruited for a microbiome study needs special attention, considering the great individuality and variability of the skin microbiota and both exogenous and endogenous factors that can impact its function and composition, such as age, sex, antibiotic usage, diet, season etc. Once the pool of volunteers has been


selected, sampling of skin microbiota can be carried out. Skin swabbing is one of the most common currently used sampling methods to harvest the skin microbiota as it is quick, simple, non-invasive and suitable for all body sites. Other methodologies are tape stripping and biopsy. However, it is important to maintain a consistent, standardised approach for sample collection throughout a study.22


An integrated approach At Eurofins Cosmetics and Personal Care Italy, we proposed a multidisciplinary, integrated approach by combining clinical investigation, analytical testing and genome sequencing to confirm, in a panel of healthy human subject (male or female), the skin acceptability, the efficacy and the effect on skin microbiome of cosmetics or personal care products under the normal condition of use. This is for us the best way to correlate


microbiome structure data with the skin barrier status and to provide a better picture of any changes. As mentioned before, we pay special attention to the planning of clinical studies, including clinical protocol information, pool of volunteers (number, gender, age etc.), inclusion and exclusion criteria, medical and medication history and skin preparation. To give an example, we conducted a


microbiome study that involved the application of a moisturising cream on face twice a day for 28 consecutive days to a panel of 22 female


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