Key Role of Staphylococcal Fibronectin-Binding Proteins During the Initial Stage of Staphylococcus aureus Keratitis in Humans.

Corantin Maurin,Frédéric Laurent,Zhiguo He,Paul O Verhoeven,Jérôme Josse,Philippe Gain,Gilles Thuret,Emilie Courrier,Josselin Rigaill

doi:10.3389/fcimb.2021.745659

Abstract

Objectives Staphylococcus aureus is one of the main causes of bacterial keratitis in humans. This study was aimed at investigating the mechanisms of S. aureus adhesion to the human corneal epithelium involved during the initial stage of infectious keratitis.MethodsHuman corneas stored in a specific active storage machine that restores a normal pluristratified epithelium were used to assess S. aureus adhesion level to intact and injured tissues using immunostaining. S. aureus adhesion to immobilized fibronectin was measured in microtiter plate. Internalization of S. aureus clinical isolates recovered from keratitis was assessed on human corneal epithelial HCE-2 cells.ResultsSuperficial corneal injury unmasked fibronectin molecules expressed within the human corneal epithelium. S. aureus adhesion level was increased by 117-fold in the area of injured epithelium (p < 0.0001). The deletion of staphylococcal fnbA/B genes decreased by 71% the adhesion level to immobilized fibronectin (p < 0.001). The deletion of fnbA/B genes and the incubation of the corneas with anti-fibronectin blocking antibodies prior to the infection significantly reduced the S. aureus adhesion level to injured corneal epithelium (p < 0.001). Finally, S. aureus clinical isolates triggered its internalization in human corneal epithelial cells as efficiently as the 8325-4 wt.Conclusion S. aureus was almost unable to bind the intact corneal epithelium, whereas a superficial epithelial injury of the corneal epithelium strongly increased S. aureus adhesion, which is mainly driven by the interaction between staphylococcal fibronectin-binding proteins and unmasked fibronectin molecules located underneath the most superficial layer of the corneal epithelium.

Highlights

Bacterial keratitis (BK) is a potentially eye-threatening infection characterized by a loss of integrity of the corneal epithelium and an inflammation of the stroma
S. aureus strains 8325-4 and 8325-4 DfnbA/B were used to assess fibronectin binding to injured corneal epithelium
The deletion of fnbA/B genes decreased by 71% the adhesion level to immobilized fibronectin

Summary

Introduction

Bacterial keratitis (BK) is a potentially eye-threatening infection characterized by a loss of integrity of the corneal epithelium and an inflammation of the stroma. Like many other bacterial species found in humans, S. aureus harbors a number of virulence factors, including adhesins capable of binding to receptors expressed at the surface of eukaryotic cells and extracellular matrix (ECM) molecules (Josse et al, 2017). Bacterial adhesins are important at the early stage of colonization and infection to let the bacteria attach to the host tissues. The microbial surface component recognizing adhesive matrix molecules (MSCRAMMs) [e.g., fibronectin-binding protein A and B (FnBPA/B), clumping factor A and B (ClfA/B), collagen adhesin (CNA)] are major cell wall-anchored proteins that mediate the attachment to ECM such as collagen, fibrinogen, or fibronectin (Foster et al, 2013). The FnBP-Fn-a5b1 integrin pathway is widely acknowledged to be the main internalization process, but other S. aureus factors [e.g., autolysin, extracellular adherence protein (Eap), lipoprotein-like lipoproteins] can trigger internalization but in a lesser extent

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