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Page 4 of 10 Abadías-Granado et al. Plast Aesthet Res 2021;8:27 https://dx.doi.org/10.20517/2347-9264.2020.199

All these communities of bacteria, viruses, fungi, and mites present in different skin ecosystems can
influence the health of the host in both senses, either as a protective mechanism disease or by contributing
to the initiation or development of different dermatoses and cutaneous infections [11,15] .

Regarding the biological mechanisms that could explain the relationship between the alteration in the skin
microbiota and the development of disease, its role inducing inflammation and modulation of the immune
response is considered very important [16-18] . All these microorganisms can produce beneficial or pathogenic
substances, and the interaction among them can also participate in the pathophysiology of some
dermatoses. Examples of dysbiosis related to skin diseases include: increase density of pathogenic bacteria,
such as in atopic dermatitis; reduced bacterial diversity, such as in psoriasis; increase of commensal
organisms, such as in acne; and alterations of microenvironments and colonization by unique species, such
[11]
as in chronic wounds [Table 1].

THE MICROBIOME AND AGING
The microbiota changes as the host ages, but also it seems that the relationship between the host and the
[19]
microbiota impacts host aging and life expectancy . The changes in the microbiota with age have been
[20]
extensively studied in the human gut . In this sense, there is a proliferation of opportunistic Proteobacteria
at the cost of symbionts Firmicutes and Bacteroidetes with age, as well as less abundance of Bifidobacterium
[21]
(Actinobacteria) compared to younger adults [Table 2]. These changes associated with age have been
related with different factors implicated in dysbiosis and disease: dietary changes, especially those related
with a scarce consumption of fibrous foods, and increased antibiotic administration, among others .
[22]
Furthermore, aging and dysbiosis have in common the inflammation, which is a known risk factor for the
[23]
progression of several diseases related with age. Smith et al. conducted an interesting study in African
turquoise killifish by recolonizing middle-age fish (after being treated with antibiotic) with the gut
microbiota from young fish. Surprisingly, they found that this change was associated with a significant
increase in life expectancy. In addition, they also performed the opposite experiment: they recolonized
young fish with the microbiota from middle-aged fish, finding that the metabolism of hyaluronic acid, a
fundamental component of the extracellular matrix associated with skin aging, was increased in this
model .
[23]
THE MICROBIOME AND SKIN AGING
The skin structure and function change with age, and this could be due not only to intrinsic factors such as
cellular metabolisms, the immune system, or hormone changes, but also to extrinsic factors such as
ultraviolet irradiation . In this sense, the microbiota also changes over the lifetime , not only due to age,
[5]
[24]
but also due to geography, age, diet, lifestyle, and pollution, among others [8,25-27] [Figure 1].
Skin aging is characterized by a decrease in sebum and hydration levels as well as immune dysfunction,
[28]
which results in significant alterations in skin physiology . These physiological changes also imply changes
in the cutaneous ecology, inducing a disbalance of cutaneous microbiota .
[29]
The composition of the microbiome is different in old and young skin [7,30,31] . In puberty, the density of
lipophilic bacteria proportionally increases with the increase of sebum levels, whereas it is much lower in
elderly skin [5,32] . Moreover, metagenomic studies have shown a decrease of Actinobacteria in older skin [32,33] .
However, the number of total bacteria increases in older people; specifically, more Corynebacterium species
are found on the aged skin . Shibagaki et al. found that the diversification of skin microbiome in older
[34]
[33]
skin is related to chronological and physiological skin aging, but it is related to the oral bacteria
composition. Another study suggests that gut, oral, and skin microbiomes predict chronological age, being