507 Evolution and Challenges of Sustainability
The crosstalk between commensals and the skin may also depend on the effects of host-derived
molecules on the skin microbiome. Kang et al. demonstrated that oral supplementation
of human subjects with vitamin B12 (i.e., cobalamin), which is an essential co-factor for
humans, resulted in a transcriptional and metabolic shift in C. acnes that involved down-
regulation of genes involved in vitamin B12 biosynthesis and differential expression of
genes with both known and unknown association to vitamin B12 metabolism.128 Vitamin
B12 supplementation to test subjects also promoted the production of porphyrin by C.
acnes. Prophyrins are proinflammatory and are implicated in acne pathogenesis.129,130 This
demonstrated that host-acquired micronutrients are accessible to the cutaneous microflora
and that back-and-forth crosstalk between the host and the skin microbiome may play
important roles in disease development. Although vitamin B12 is essential to human
health, this work suggests that avoiding excessive amounts in the diet may reduce porphyrin
production by C. acnes, which in turn should decrease inflammation and the severity of acne.
THE SKIN ENVIRONMENT FOR MICROORGANISMS
The skin provides a living environment for resident microorganisms. The structure of skin
and the nutrients on skin for microorganisms depend on factors including an individual’s
age, sex, health status, hygiene practices, lifestyle, and environmental exposure. The outer
layer of the epidermis continually sheds keratinized skin cells with complete turnover
about every 28 days, and it has been estimated that about 10% of exfoliated cells contain
bacteria.69,106 A large proportion of the skin microbiome consists of resident microorganisms
that generally are stable, but there is a smaller percentage of transient microorganisms
that can opportunistically colonize niches when the skin is compromised.131 The skin
microflora at different sites on the body are those that are best able to take advantage of
the physical and chemical environments in various niches using substrates in sweat, sebum,
and exfoliated cells along with varying levels of exposure to ultraviolet radiation, moisture,
oxygen availability, and pH.106
The skin surface pH is reported to be around pH 5.6 66 however, the skin surface pH
of healthy individuals may range from around pH 3.5 to over pH 7, depending on the
individual and the anatomical site tested.132 Eccrine glands are primarily responsible for
excretion of water and electrolytes, but may also include minor amounts of glucose, pyruvate,
lactate, cytokines, immunoglobulins, and AMPs.133,134 The low pH and dry environment
(i.e., low water availability) at relatively dry skin sites (e.g., volar forearm) restrict growth
of many microorganisms, such as the Proteobacteria that are not acid and low a
w tolerant.
More moist/higher pH regions such as the navel, axillary, umbilical, and inguinal regions
allow growth of a greater diversity of microorganisms that are able to utilize the available
substrates for growth.
CHANGES IN THE SKIN MICROBIOME WITH AGE
Until recently, it was believed that microorganisms colonize the infant’s skin at the time
of birth. However, studies now show that colonization occurs in the mother’s body and
that the uterine cavity, placenta, and amniotic fluid may not be sterile.135,136 The placental
microbiome consists mainly of commensal microorganisms from Firmicutes, Tenericutes,
Proteobacteria, Bacteroidetes, and Fusobacteria phyla.66 Tenericutes (now renamed to

508 JOURNAL OF COSMETIC SCIENCE
Mycoplasmatota) is a phylum of bacteria that lack a cell wall around their cell membrane
and which includes Mycoplasma spp. The mother-child relationship in the first few months
after birth allows transmission of microorganisms, and a study of one-year old children
revealed predominance of bacteria belonging to Firmicutes (about 50%), Actinobacteria
(about 20%), and Bacteroidetes (about 20%) phyla. Microbial diversity increases until about
age 8, with the number of staphylococcal and streptococcal species decreasing and the
amount of Actinobacteria and Proteobacteria species increasing.137,138 The skin microbiome
experiences a substantial shift at puberty when sex hormones drive maturation of sebaceous
glands that start sebum production.86 In adolescents, numbers of C. acnes increase due to
the increased availability of sebum.68 Adult skin has higher levels of Cutibacterium spp. and
Corynebacterium spp., in contrast to young children who have Gammaproteobacteria (which
includes Pseudomonadales and Enterobacterales) and Streptococcaceae at multiple sites.72 The
microbiome of adult skin remains fairly stable, but the composition at different sites is
dictated by different conditions (e.g., dry, moist, lipid-rich).66
Unlike other organs, skin aging is affected by both intrinsic and extrinsic factors. Sun
protected sites, such as the buttocks, largely undergo intrinsic aging processes influenced
by genetic, metabolic, and hormonal changes.98 Intrinsically aged skin is characterized by
reduced sebaceous gland functioning and lipid content, reduced activity of AMPs, decreased
blood flow, and degradation of collagenous and fibrous extracellular matrixes which leads
to atrophy, xerosis with fine lines and wrinkles.139,140 Lower sebum levels may result in
decreased numbers of Firmicutes (including S. aureus and C. acnes) and more Corynebacterium
spp. in the microbiome.68,80,138 Extrinsic aging is influenced by environmental factors,
primarily due to exposure to ultraviolet radiation.141 Both intrinsically and extrinsically
aged skin have a higher pH, less hydration, and reduced expression of tight junction
proteins compared to young skin however, photoaged skin exhibits increased rates of
proliferation and increased sebum.139,142 Changes in skin lipids with age leads to changes in
substrates and pH in the skin microenvironment, which results in alterations in the skin
microflora. The overall abundance of bacteria increases with age, but there are changes in
the bacterial populations within the skin microbiome so that Corynebacterium spp. increases
while Cutibacterium spp. and Lactobacillus spp. decrease in number.143-146
Dimitriu et al. studied the contributions of extrinsic (i.e., lifestyle) and intrinsic (i.e.,
age) factors to skin microbiome variation by profiling bacterial microbiomes of 495
North American subjects (ages, 9 to 78 years) at four skin sites using 16S rRNA gene
amplicon sequencing.68 These workers found the strongest associations involved aging,
demographics, and lifestyle factors, with bacteria in the genera Anaerococcus, Peptoniphilus,
Prevotella, and Corynebacterium dominating these associations. Corynebacterium spp. were
associated with chronological age and skin aging (e.g., hyperpigmented spots and wrinkles)
on the forehead, and a Propionibacterium (Cutibacterium) taxon was associated with forehead
porphyrins. Interestingly, they found that age was associated with two corynebacterial
taxa that mutually co-excluded each other and that the relationship between age and the
two corynebacteria taxa was similar at different skin sites. These workers also observed
that bacterial interactions at skin sites exposed to the environment showed higher
connectedness than those at occluded sites, which led them to suggest that exposed skin
sites have diverse microflora that are potentially more resilient to community disruption by
changes in the environment. Readers are directed to reviews on the skin microbiome for
additional information on gender, ethnicity and living/working environment on the skin
microbiome.66,74,77,82,86,98