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RNA) fingerprint sequences that each organism contains, which allows identifying, characterizing, and
[10]
measuring the true relative abundance of each bacterial operational taxonomic units .
Finally, the essential portion of accurate microbiome analysis is the bioinformatics processing. Generally,
large-scale computing clusters and specific bioinformatic pipelines must be established to understand and
analyze these diverse bacterial communities from the millions of sequencing reads .
[11]
THE MICROBIOME OF THE SKIN
The majority of the “regular” bacterial inhabitants of the skin are included in four phyla: Actinobacteria,
[3]
Proteobacteria, Bacteroidetes, and Firmicutes . The three most common genera are Propionibacteria,
Corynebacteria, and Staphylococci .
[6]
The commensal microbes of the skin have also been classified as resident or transient depending on if they
belong to the fixed microbiota or not [3,10] . The fixed microbiota tends to reestablish after disturbance. It is
considered as commensal, which means that these microorganisms are normally harmless and most likely
provide some benefit to the host. Transient microorganisms are temporarily found in the skin. They come
from the environment and persist for hours or days and then disappear . Under normal circumstances,
[10]
[10]
both groups are nonpathogenic . Recent research has shown that, even though the skin is constantly
exposed to the environment, the healthy human skin microbiome is stable [12,13] .

[3]
The body site is one of the most influential factors in the types of microbes inhabiting the skin . The three
main types of environments on the human skin are sebaceous, dry, and moist. Moist areas mostly include
the body folds: the navel, axilla, antecubital and popliteal fossa, or groin. Sebaceous areas include the
forehead, nasolabial folds, retroauricular crease, middle chest, and back, whereas the upper buttock area,
forearm, and hypothenar palm are drier sites [3,6,8-10] . Other microenvironments include the hair follicles,
sweat glands, and dermal layers .
[10]

The microbial communities found in these cutaneous environments are different. Corynebacterium and
Staphylococcus genera, of the phyla Actinobacteria and Firmicutes, respectively, are the most abundant
microbes colonizing moist regions. The diversity of the microbes present in sebaceous sites is lower. In this
anaerobic lipid-rich environment, there is a higher density of Propionibacterium, a lipophilic genus. The dry
areas of the skin show the highest diversity in microbial inhabitants, predominantly Staphylococcus,
Propionibacterium, Micrococcus, Corynebacterium, Enhydrobacter, and Streptococcus species [3,10] . Addition-
ally, even microenvironments such as sebaceous, apocrine, and eccrine glands and hair follicles are
associated with their own singular microbiota. In this sense, whereas Propionibacterium is especially
adapted to the anaerobic environment rich in lipids of the sebaceous follicles, Gram-positive bacteria of the
genera Corynebacterium, Micrococcus, Staphylococcus, and Propionibacterium are the main microbiota of
the axillar area, rich in sebaceous glands .
[10]

Although microbiota research has focused primarily on identifying bacteria, we have to keep in mind that
other types of microorganisms also live on the skin [3,10] . The fungal community is similar all over the body
regardless of physiology. The genus Malassezia predominates at the head, trunk, and upper extremities,
whereas feet are colonized by a combination of Malassezia, Aspergillus, Epicoccum, Rhodotorula,
Cryptococcus, and other genera . Demodex is a tiny mite that is also present in normal skin, especially
[13]
[14]
inside the follicles, although its role as a commensal organism remains uncertain . To our knowledge,
there is little information about the viral composition of the cutaneous microbiota.