82 SKIN MICROBIOME
Phylum Firmicutes Actinobacteria Proteobacteria Bacteroidetes
Others
Relative abundance Csr
0% Newborn
No skin site specificities
Its heterogeneous topography creates distinct niches where temperature and humidity vary by site: higher in flexural areas such as the groin and axillae, close to 37°C, and lower at the extremities, such as fingers and toes, around 30°C.5
The density of sebaceous glands influences
lipid secretion, including fatty acids, which contributes to acidifying the skin surface. As a result, surface pH fluctuates by site within an approximate range of 4.2 to 7.9.5 These gradients in temperature, humidity, sebum, and pH generate conditions that shape the density and composition of resident communities of bacteria, fungi including Malassezia, viruses, archaea, and mites.5 Within this ecosystem, the microbiome
contributes to barrier function and supports cutaneous homeostasis through continuous interaction with its environment.5
Study techniques Advances in molecular biology and next generation sequencing (NGS) have enabled precise characterization of the skin microbiome, providing access to diversity and structural profiles that were previously inaccessible.5 Culture methods remain essential to
characterize microbial properties, including phenotypes, functional interactions, and sensitivity to actives, as a complement to molecular approaches.5 Key functions: protection, hydration, and immunomodulation Cutaneous microorganisms play a protective barrier role against exogenous and opportunistic microorganisms, notably
PERSONAL CARE November 2025
Infant Children Adolescent Skin site specificities
Figure 1: Bacterial diversity increases with age, whereas the relative abundance of bacterial families decreases11 Adult Senior Vgl
through competition and antagonism.6
This
protection relies, among other mechanisms, on the production of antimicrobial peptides by commensal bacteria, notably Staphylococci and Corynebacteria.7
Malassezia synthesizes
indoles that can inhibit other yeasts and moulds, thereby limiting certain colonisations.5 Staphylococcus epidermidis produces
a sphingomyelinase that hydrolyses sphingomyelin into ceramides, which strengthen the barrier and limit water loss, and into phosphocholine, which nourishes and protects the bacterium. This activity also promotes the
colonization of S. epidermidis, contributing to the stability of the local community.8,9 In addition, certain acidic metabolites
produced by S. epidermidis can activate receptors involved in recruiting T lymphocytes (T cells) to the skin, support keratinocyte proliferation, and increase the number of tight junctions between cells, mechanisms that strengthen barrier integrity.9
Activation of TLR2
by commensal S. epidermidis also increases the tight junction barrier in keratinocyte cultures in vitro, which contributes to maintaining epithelial barrier homeostasis.9
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