Tremendous strides have been made in the identification of microbes (animalcules) since their first discovery on epithelial surfaces by Antonie van Leeuwenhoek in 1677 [1].
For example, Staphylococcus aureus, a bacterium associated with atopic dermatitis, was identified by Friedrich Rosenbach in 1884 [2] and Propionibacterium acnes, now called Cutibacterium acnes – a major commensal bacterium found primarily on the face – by Raymond Sabouraud in 1897 [3]. Thus, we know that there is plenty of food derived from eccrine, sebaceous and apocrine glands together with the stratum corneum itself to constitute us being a good symbiont.
However, our understanding around how, and what, microbes bind to in healthy skin is still in its infancy. To comprehend this better we need to focus on the fragile interface of the facial stratum corneum (SC).
The stratum corneum, initially likened to a brick wall, is now considered to be a continuous poly-proteinaceous membrane structure of varying thickness. The bricks (corneocytes) which compose the majority of the SC, are tightly interconnected by corneodesmosomes in all layers of the SC, and by tight junction molecules in the lower layers. Rather than rigid like bricks, corneocytes are more reminiscent of sponges formed of keratin and containing absorbent natural moisturising factor (NMF) molecules; they hydrate extensively and are interspersed between a continuous, mostly highly ordered lamellar and largely hexagonal phase and an orthorhombically-packed lipid phase.
This thermodynamically unstable (desquamating), yet kinetically stable (as constantly rebuilt), tissue undergoes both catabolic and anabolic biochemical reactions as it matures from its lower to its upper layers to facilitate its intrinsic hygroscopicity properties and the process of desquamation. While this physical and chemical barrier acts as a substrate for the attachment of bacteria, the process of cell shedding counteracts this in order to prevent infection. In a similar fashion, a variety of exquisite lipid and protein biochemicals present in the skin aim to inhibit and control microbial growth.
Our understanding around how, and what, microbes bind to in healthy skin is still in its infancy.
Still, the question remains: what ligands in the skin are these microbes binding to? The first clues came from the work of Gary Cole and Nancy Silverberg in 1986 who examined the adherence of S. aureus to human corneocytes [4]. They found that these potentially pathogenic bacteria bound more extensively to corneocytes from subjects with atopic dermatitis (AD) compared to those with psoriasis, xerosis or healthy skin – but what caused the difference remained unknown.
To start to understand the propensity for bacteria to bind to the SC, we need to consider the architecture and biochemistry of the SC, especially that of the face. Facial SC is very different to other non-palmoplantar body sites; it is thinner and enriched in sebum but depleted of intercellular barrier lipids, such as ceramides, cholesterol and fatty acids. The facial SC also contains less NMF than other body sites and has higher proteolytic activitie, including that of plasmin, a serine protease which is associated with the outer surface of corneocytes [5][6].
Importantly, facial skin is considerably more fragile than other non-palmoplantar sites [7].
The normal maturation of the corneocytes, from the lower layers of the SC to its upper layers, is impeded due to reduced activities of transglutaminase and 12R lipoxygenase; two enzymes that strengthen and hydrophobically coat the corneocyte envelope with isodipeptide bonds and ceramides and fatty acids [8][9][10].
As a result, the corneocytes of the face are fragile and have a very specific morphology, compromising the impaired integrity of this skin [11]. Due to the differences in the desquamatory process on the face versus other areas of the body, corneodesmosomes are here found extensively over all the corneocyte surfaces rather than just at their peripheries [12].
Using scanning electron microscopy (SEM) and / or atomic force microscopy (AFM), villus-like projections can be observed protruding from the typically flattened upper surface of corneocytes – pointing to the existence of additional structures / ligands that might be accessible to microbes and that would not be normally present [13]. Notably, this architecture is also found from corneocytes derived from subjects with AD, perhaps providing an explanation to Cole and Silverberg’s findings. Fragile cell morphology is especially apparent in facial samples taken from photodamaged cheeks, infants and subjects with sensitive skin. This fragile morphology is mostly associated with reduced NMF levels [14][15][16].
Facial skin is rich in sebum, which contains particular lipids that are inhibitory to bacterial growth. However, in addition to increased fragility, it has also been noted that the antimicrobial defences of the outer layers of facial cheek skin are reduced compared to other body sites. Antimicrobial proteins dermcidin and lysozyme have also been shown to be lower in facial skin. Furthermore, protease inhibitors cystatin-A, alpha-2-macroglobulin and serpin A12, which prevent activity of proteases produced by microbes to invade corneocytes, are diminished on the face [8]. Thus, facial skin shows frailty not only in its morphology but also in its antimicrobial defences and is thus ‘prime real estate’ for microbes.
It has been found that fibronectin-binding proteins present on S. aureus, such as clumping factor B, can bind to fibronectin in AD subjects (which is not normally present in healthy skin) – indicating fibronectin as a ligand candidate contributing to increased colonisation. Further, facial keratinocytes, which are affected during the pathogenesis of AD, may help to reduce binding by clumping factor B in healthy skin by decreasing corneocyte levels of structural proteins loricrin and keratin 10 [17][18][19][20].
In contrast, plasmin is heavily expressed on facial SC with greater quantities associated with a disrupted skin barrier, as in diseased states like AD [21][22][23]. It has been suggested that interactions of moonlighting glycolytic enzymes secreted by microbes, such as triosephosphate isomerase and enolase, may aid in microbial binding to extracellular corneocyte-associated plasminogen (inactive plasmin). Indeed, research by DSM showed that inhibition of plasmin (and urokinase (uPA), using benzylsulfonyl-D-Ser-homoPhe-(4-amidino-benzylamide) (BSFAB)) to improve the SC barrier [24] caused changes to the microbiome, including an increase in the levels of S. Epidermidis – a commensal skin bacterial species associated with healthy skin [25].
It has been suggested that interactions of moonlighting glycolytic enzymes secreted by microbes, such as triosephosphate isomerase and enolase, may aid in microbial binding to extracellular corneocyte-associated plasminogen (inactive plasmin).
In addition to identifying docking sites for attachment to the corneocyte surface, some microbes colonise by compromising activity of skin surface enzymes which would otherwise inhibit their activity, for example Candida albicans fungi express substrates for transglutaminase.
In summary, we know a lot about the architecture and biochemistry of corneocytes – particularly those in facial skin – and we can gain valuable insights of corneocyte ligands used by microbes by examining skin samples in various disease states.
In this respect, there are many similarities between the SC, and its associated fragile corneocyte envelopes, in AD and that of cheek skin, especially in subjects with photodamaged or sensitive skin or facial skin in infants.
There is, however, still so much more to be understood regarding specific facial corneocyte ligands and their interaction with microbes, especially Cutibacterium acnes; a better understanding could help us develop new skincare solutions targeting the microbiome – with plasmin already being identified as a promising target.
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