Staphylococcus aureuspenetrate the interkeratinocyte spaces created by skin‐infiltrating neutrophils in a mouse model of impetigo

Abstract

Background-Impetigo is a bacterial skin disease characterized by intraepidermal neutrophilic pustules. Previous studies have demonstrated that exfoliative toxin producing staphylococci are isolated in the cutaneous lesions of human and canine impetigo. However, the mechanisms of intraepidermal splitting in impetigo remain poorly understood. Objective-To determine how staphylococci penetrate the living epidermis and create intraepidermal pustules in vivo using a mouse model of impetigo. Methods-Three Staphylococcus aureus strains harbouring the etb gene and three et gene negative strains were epicutaneously inoculated onto tape-stripped mouse skin. The skin samples were subjected to time course histopathological and immunofluorescence analyses to detect intraepidermal neutrophils and infiltrating staphylococci. To determine the role of neutrophils on intraepidermal bacterial invasion, cyclophosphamide (CPA) was injected intraperitoneally into the mice to cause leucopenia before the inoculation of etb gene positive strains. Results-In mice inoculated with etb gene positive S. aureus, intraepidermal pustules resembling impetigo were detected as early as 4 h post-inoculation (hpi). Neutrophils in the epidermis were detected from 4 hpi, whereas intraepidermal staphylococci was detected from 6 hpi. The dimensions of the intraepidermal clefts created in mice inoculated with etb gene positive strains at 6 hpi were significantly larger than those in mice inoculated with et gene negative strains. In CPA treated mice, staphylococci or neutrophils were not detected in the deep epidermis until 6 hpi. Conclusion-Our findings indicate that intraepidermal neutrophils play an important role in S. aureus invasion into the living epidermis in a mouse model of impetigo.

Exfoliative Toxins of Staphylococcus aureus

“…To address this issue, we recently established a mouse model of bullous impetigo [72]. S. aureus harboring etb gene was inoculated epicutaneously to murine inner pinnae after the stratum corneum was partially removed by tape stripping.…”

Impetigo Animal Models: A Review of Their Feasibility and Clinical Utility for Therapeutic Appraisal of Investigational Drug Candidates

“…In this model [ 49 ], there was no intraepidermal or sub-corneal pustules nor the migration of neutrophils, which is inconsistent with the picture for human impetigo. Similar studies [ 50 , 51 ] that used this model and followed the same procedure have reported contrasting results. One study [ 50 ] claimed the formation of intraepidermal pustules resembling human impetigo through epicutaneous inoculation of S. aureus on the inner pinna of the mice.…”

“…Human impetigo initially develops as a small vesicle after minor injuries or cuts on the skin, such as abrasions and skin scratches resulting from eczematous lesions or insect bites, rapidly progressing into pustular lesion, and subsequently evolving into an erosion covered by a thick crust [ 8 , 60 , 61 , 62 ]. Other authors [ 45 , 46 , 49 , 50 , 63 ] also suggest that the application of topical inoculation of organisms to the surface of slightly damaged or traumatized skin is likely to more closely resemble the natural route of transmission for GAS and S. aureus . Minor skin traumas perforate the stratum corneum, causing a temporary breach in the integrity of the skin barrier system that enables the inoculated bacteria to permeate or penetrate the epidermis with little hindrance [ 62 , 64 ].…”

“…In human impetigo involving S. aureus , the first step for the bacterial skin invasion involves adhering to the outer layer of the epidermis (stratum corneum), followed by disruption of the epithelial barriers of the inner layers of the epidermis comprising cell-adhesion structures such as desmosomes and adherence junctions [ 73 ]. This process separates the epidermis just below the stratum granulosum and large intraepidermal pustules, which may contain bacteria, and these are described as typical histologic characteristics of impetigo [ 45 , 50 , 74 ]. Besides, in the case of impetigo, the upper dermis also contains an epidermal inflammatory infiltrate of neutrophils and lymphocytes which are reported to be crucial for the staphylococcal percutaneous invasion [ 50 ].…”

“…This process separates the epidermis just below the stratum granulosum and large intraepidermal pustules, which may contain bacteria, and these are described as typical histologic characteristics of impetigo [ 45 , 50 , 74 ]. Besides, in the case of impetigo, the upper dermis also contains an epidermal inflammatory infiltrate of neutrophils and lymphocytes which are reported to be crucial for the staphylococcal percutaneous invasion [ 50 ].…”