Micro Skims

[Geri Cutcher]

TheSkims Plush Pointelle Henley is reminiscent of the snug warmth of holiday mornings. Tailored from soft, stretchy micro modal, it gently hugs your figure and has a half-length button closure for a customisable fit.

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The diffusion of microplastics in the food supply chain is prompting public concern as their impact on human health is still largely unknown. The aim of this study was to qualitatively and quantitatively characterize microplastics in skim-milk powder samples (n = 16) from different European countries (n = 8) through Fourier-transform infrared microspectroscopy in attenuated total reflectance mode analysis. The present study highlights that the use of hot alkaline digestion has enabled the efficacious identification of microplastics in skim-milk powders used for cheesemaking across European countries. The adopted protocol allowed detection of 29 different types of polymeric matrices for a total of 536 plastic particles. The most abundant microplastics were polypropylene, polyethylene, polystyrene, and polyethylene terephthalate. Microplastics were found in skim-milk powders in 3 different shapes (fiber, sphere, and irregular fragments) and 6 different colors (black, blue, brown, fuchsia, green, and gray). Results demonstrate the presence of microplastics in all skim-milk powder samples, suggesting a general contamination. Results of the present study will help to evaluate the impact of microplastics intake on human health.

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Functioning as the exterior interface of the human body with the environment, skin acts as a physical barrier to prevent the invasion of foreign pathogens while providing a home to the commensal microbiota. The harsh physical landscape of skin, particularly the desiccated, nutrient-poor, acidic environment, also contributes to the adversity that pathogens face when colonizing human skin. Despite this, the skin is colonized by a diverse microbiota. In this Review, we describe amplicon and shotgun metagenomic DNA sequencing studies that have been used to assess the taxonomic diversity of microorganisms that are associated with skin from the kingdom to the strain level. We discuss recent insights into skin microbial communities, including their composition in health and disease, the dynamics between species and interactions with the immune system, with a focus on Propionibacterium acnes, Staphylococcus epidermidis and Staphylococcus aureus.

Our skin is home to millions of bacteria, fungi and viruses that compose the skin microbiota. Similar to those in our gut, skin microorganisms have essential roles in the protection against invading pathogens, the education of our immune system and the breakdown of natural products1,2,3. As the largest organ of the human body, skin is colonized by beneficial microorganisms and serves as a physical barrier to prevent the invasion of pathogens. In circumstances where the barrier is broken or when the balance between commensals and pathogens is disturbed, skin disease or even systemic disease can result. Human skin sites can be categorized by their physiological characteristics, that is, whether they are sebaceous (oily), moist or dry (Box 1). Studying the composition of the microbiota at different sites is valuable for elucidating the aetiology of common skin disorders, which often have a preference for specific skin sites, such as eczema inside the elbow4 and psoriasis on the outside of the elbow5.

Traditionally, skin microbial communities were explored by use of culture-based methods. As this approach selects for microorganisms that thrive in artificial growth conditions, it underestimates the total diversity of the community. For example, the skin genus Staphylococcus is cultivated more easily than Propionibacterium spp. or Corynebacterium spp., which were frequently underestimated in culture-based surveys6. Thus, to circumvent the bias imposed by culture and to capture the complete diversity of the microbiome, investigators began applying sequencing methods. These original sequencing approaches utilized sequence variation in conserved taxonomic markers as molecular fingerprints to identify members of microbial communities7. For bacteria, the 16S ribosomal RNA (rRNA) gene is used, whereas for fungi, the internal transcribed spacer 1 (ITS1) region of the eukaryotic ribosomal gene is preferred8.

Most skin microbiome surveys have used amplicon sequencing. Over the past few years, however, major technical and analytical breakthroughs have enabled shotgun metagenomic sequencing studies. Figure 1 highlights the technical and procedural differences between amplicon and shotgun metagenomics and the different types of analyses that are possible with the data sets. As shotgun metagenomics does not sequence specific target regions, it simultaneously captures all genetic material in a sample, including human, bacterial, fungal, archaeal and viral, thus allowing relative kingdom abundances to be inferred, with the limitation that the DNA of different microorganisms may be differentially extracted depending on the sample preparation method14,15,16. Another advantage of shotgun metagenomic sequencing is that these data sets provide sufficient resolution to differentiate species and even strains within a species. This is crucial for identifying members of the Staphylococcus genus, which are difficult to classify to the species level with most amplicon sequencing approaches9. The ability to differentiate strains is important as more studies reveal the functional differences that exist between strains within a species17,18,19.

To study the members of a microbial community, two sequencing strategies can be utilized. For amplicon sequencing (left), primers are utilized that amplify conserved regions within a kingdom. For bacteria, the 16S ribosomal RNA (rRNA) region of the ribosomal gene is amplified, whereas for fungi, the internal transcribed spacer 1 (ITS1) region is amplified. By contrast, whole genome sequencing (right) captures the entire complement of genetic material in a sample without a targeted amplification step. Analyses of sequenced amplicons can identify the genus-level and the species-level community composition, but only shotgun metagenomics can reveal kingdom relative abundances and resolution to the strain level. Colours not defined may be grouped as 'Other'.

In this Review, we discuss recent insights into skin microbial communities, including their composition in health and disease, assembly and ecology, and interactions with the immune system. We end by considering important unanswered questions in the field and future research priorities. A greater understanding of these topics is important as interest in targeting the skin microbiome for therapeutic approaches increases.

Structurally, the skin is composed of two distinct layers: the epidermis and dermis (Figure). The outermost layer (the epidermis) is composed of layers of differentiated keratinocytes. The top layer, or stratum corneum, is composed of terminally differentiated, enucleated keratinocytes (also known as squames) that are chemically crosslinked to fortify the barrier of the skin113.

In addition to this conserved layered structure, body sites provide diverse microenvironments that vary in ultraviolet light exposure, pH, temperature, moisture, sebum content and topography22. On the basis of these characteristics, sites can be grouped into broad categories: sebaceous or oily (face, chest and back); moist (bend of elbow, back of knee and groin) and dry (volar forearm and palm). The environment of these sites is influenced by appendages, such as sweat glands, hair follicles and sebaceous glands. More abundant in moist sites, sweat glands are important for thermoregulation through the evaporation of water, which also acidifies the skin, making conditions unfavourable for the growth and colonization of certain microorganisms22. In addition, sweat contains antimicrobial molecules, such as free fatty acids and antimicrobial peptides, that inhibit microbial colonization114. Connected to the hair follicle and denser in oily sites, sebaceous glands secrete lipid-rich sebum, a hydrophobic coating that lubricates and provides an antibacterial shield to hair and skin.

Depending on the method used to sample the skin microbiota (swab, biopsy, surface scrape, cup scrub or tape strip), microorganisms residing at different depths or subcompartments of the skin are captured92,115,116,117. Although most major bacterial taxa are similarly identified regardless of sampling method92, some microorganisms are variably present at the surface compared with deeper skin layers118,119,120; this emphasizes the importance of maintaining consistent sampling techniques throughout a study. In general, the studies highlighted throughout this Review utilize methods that capture microorganisms on and within the stratum corneum; additional studies with more invasive sampling techniques are necessary to fully understand the spatial distribution of microorganisms in the skin.

Composition of the skin microbiota. In sequencing surveys of healthy adults20,21,22,23, the composition of microbial communities was found to be primarily dependent on the physiology of the skin site, with changes in the relative abundance of bacterial taxa associated with moist, dry and sebaceous microenvironments. Sebaceous sites were dominated by lipophilic Propionibacterium species, whereas bacteria that thrive in humid environments, such as Staphylococcus and Corynebacterium species, were preferentially abundant in moist areas, including the bends of the elbows and the feet (Fig. 2; Table 1). In contrast to bacterial communities, fungal community composition was similar across core body sites regardless of physiology23,24. Fungi of the genus Malassezia predominated at core body and arm sites, whereas foot sites were colonized by a more diverse combination of Malassezia spp., Aspergillus spp., Cryptococcus spp., Rhodotorula spp., Epicoccum spp. and others24 (Fig. 2). Bacteria were the most abundant kingdom across sites, and fungi were the least abundant25 (Fig. 2); however, there are many more bacterial reference genomes than fungal reference genomes available, which may partially contribute to this observed difference. Interestingly, the overall fungal abundance was low, even on the feet where fungal diversity was high.

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