Adhesion of Staphylococcus aureus to Corneocytes from Atopic Dermatitis Patients Is Controlled by Natural Moisturizing Factor Levels

Abstract

The bacterial pathogen Staphylococcus aureus plays an important role in atopic dermatitis (AD), a chronic disorder that mostly affects children. Colonization of the skin of AD patients by S. aureus exacerbates the disease, but the molecular determinants of the bacterium-skin adhesive interactions are poorly understood. Specifically, reduced levels of natural moisturizing factor (NMF) in the stratum corneum have been shown to be associated with more severe AD symptoms, but whether this is directly related to S. aureus adhesion is still an open question. Here, we demonstrate a novel relationship between NMF expression in AD skin and strength of bacterial adhesion. Low-NMF corneocytes, unlike high-NMF ones, are covered by a dense layer of nanoscale villus protrusions. S. aureus bacteria isolated from AD skin bind much more strongly to corneocytes when the NMF level is reduced. Strong binding forces originate from a specific interaction between the bacterial adhesion clumping factor B (ClfB) and skin ligands. Remarkably, mechanical tension dramatically strengthens ClfB-mediated adhesion, as observed with catch bonds, demonstrating that physical stress plays a role in promoting colonization of AD skin by S. aureus Collectively, our findings demonstrate that patient NMF levels regulate the strength of S. aureus-corneocyte adhesion, the first step in skin colonization, and suggest that the ClfB binding mechanism could represent a potential target for new therapeutic treatments.IMPORTANCE Bacterium-skin interactions play important roles in skin disorders, yet their molecular details are poorly understood. In this study, we decipher the molecular forces at play during adhesion of Staphylococcus aureus to skin corneocytes in the clinically important context of atopic dermatitis (AD), also known as eczema. We identify a unique relationship between the level of natural moisturizing factor (NMF) in the skin and the strength of bacterium-corneocyte adhesion. Bacterial adhesion is primarily mediated by the surface protein clumping factor B (ClfB) and is enhanced by physical stress, highlighting the role of protein mechanobiology in skin colonization. Similar to a catch bond behavior, this mechanism represents a promising target for the development of novel antistaphylococcal agents.

Keywords: Staphylococcus aureus; adhesion; atopic dermatitis; biofilms; skin.

FIG 1

Exploring the forces between Staphylococcus aureus and AD corneocytes. (A) We used AFM-based single-cell force spectroscopy and multiparametric imaging to study the forces between bacterial cells and skin samples from AD patients immobilized on tape strips. Single bacterial probes were prepared by attaching S. aureus bacteria onto AFM cantilevers. (B) Optical microscopy image showing the bacterial AFM probe scanning across the surfaces of corneocytes. (Inset) Fluorescence image of the probe showing that the bacterial cell is alive (BacLight viability kit) (bar, 20 µm).

FIG 2

Topographic imaging reveals major structural differences between corneocytes with high and low NFM levels. Height images (z-range = 2 µm) (A, D, G, and J) and corresponding deflection images (B, E, H, and K) recorded in PBS for corneocytes from patients with high NMF levels (high NMF₁ and high NMF₂) or low NMF levels (low NMF₁ and low NMF₂). (C, F, I, and L) High-resolution height images recorded in the square areas shown in panels A, D, G, and J, together with vertical cross sections taken along the dashed lines.

FIG 3

S. aureus AD08 bacteria bind weakly to high-NMF corneocytes. (A and B) Adhesion force and rupture distance histograms with representative force profiles obtained in PBS between S. aureus AD08 bacteria and corneocytes from two patients with high NMF levels. For each patient, three representative bacterium-corneocyte pairs out of a total of seven pairs are shown. See Fig. S1 in the supplemental material for additional bacterium-corneocyte pairs, and see Fig. 5 for a statistical analysis of all samples analyzed.

FIG 4

Bacterial adhesion to low-NMF corneocytes is strong and mediated by ClfB. (A and B) Adhesion force and rupture distance histograms with representative force profiles obtained in PBS between S. aureus AD08 bacteria and corneocytes from two patients with low NMF levels. For each patient, three representative bacterium-corneocyte pairs out of a total of seven pairs are shown. (C and D) Data obtained under the same conditions for bacterial cells from the AD08 ΔclfB strain. Results obtained for three representative bacterium-corneocyte pairs out of a total of nine pairs are pooled. See Fig. S1 and S2 for additional bacterium-corneocyte pairs, and see Fig. 5 for a statistical analysis of all samples analyzed.

FIG 5

Statistical analysis of the interaction strength between S. aureus and AD corneocytes. (A and B) Adhesion frequencies (A) and adhesion forces (B) recorded between S. aureus AD08 bacteria and corneocytes from AD patients with high NMF levels (two patients; n = 1,431 adhesive curves from 14 different cells) or low NMF levels (two patients; n = 3,577 curves from 14 cells). Data obtained between low-NMF corneocytes and the AD08 ΔclfB strain are also shown (two patients; n = 1423 adhesive curves from 18 different cells). The horizontal lines in panel A represent the mean adhesion frequency. Box-charts in panel B show the mean adhesion (full square), median, first and third quartiles (boxes), and range of data without outliers (whiskers, 5 to 95 percentiles). Statistical analysis was performed by one-way ANOVA and Bonferroni post hoc tests with a P value of <0.001 indicated by three asterisks.

FIG 6

Mechanical tension potentiates bacterium-corneocyte adhesion in low-NMF conditions. (A) Adhesion forces measured at increasing loading rates (LRs) between S. aureus AD08 bacteria and low-NMF₁ corneocytes (data pooled from 4,009 adhesive peaks on three cell pairs). (B) The force distribution switches with the rate at which force is applied, i.e., while weak bonds dominate at low LR (<1,000 pN ⋅ s⁻¹), strong bonds are favored at s high LR (>10,000 pN ⋅ s⁻¹). These results support a catch bond model where ClfB-mediated adhesion is enhanced through force-induced conformational changes.

FIG 7

Nanoscale adhesion imaging shows that ClfB ligands are largely exposed on low-NMF corneocytes. (A and B) Simultaneous height (left) and adhesion (right) images of corneocytes recorded in PBS between different S. aureus AD08 cell probes and different high-NMF₂ (A) or low-NMF₁ (B) corneocytes. (C) Images obtained on different low-NMF₁ corneocytes with different AD08 ΔclfB cell probes. The insets in the top right images show representative force curves. More images are presented in Fig. S5.