A scoring system for histological evaluation of skin during Staphylococcus aureus infections

The histological evaluation of skin changes during Staphylococcus aureus (S. aureus) infection requires an exact approach with an accurate definition of individual changes and their parameters, including a classification score facilitating comparison among experiments. To propose methods and procedures for the evaluation of histological changes in skin infected by S. aureus strains in experimental pig models. Four pigs were included in the experiment. In the dorsal region of each pig, 10 wounds per pig (5 × 5 cm, 2 cm spacing) were created. Wounds in the individual pigs were inoculated with three different strains of methicillin- (oxacillin) and amoxicillin-resistant S. aureus at concentrations of 2 × 109—5 × 109 CFU/ml. One pig was not inoculated and served as control. Subsequently, samples were collected from each wound on Days 3, 7, 10, ad 14 after inoculation processed by frozen section technique and the samples were stained with hematoxylin-eosin. Classification scores of selected parameters were determined and an average classification index was established at the end of the experiment. Histologically, granulomatous arrangement and superficial re-epithelialization were observed in the wounds of all animals. Compared to control, S. aureus infection documents slow healing of skin wounds with impaired epidermal formation (0.88 infected vs 1.45 control), re-epithelialization (1.25 vs 2.08), edema in the dermis (1.50 vs 2.67), inflammation in the dermis (0.50 vs 1.69), fibrosis in the dermis (4 vs 2.56), fibrosis in the hypodermis (4 vs 3.17) and, finally, necrosis in the dermis (1 vs 2.2). The results in the scores for scabs (1.25 vs 1.42), dermis (2.63 vs 2.67), and, finally, necrosis in the hypodermis (0.25 vs 0.75) did not differ among groups. The proposed methodological approach allowed quantification of histological changes and we recommend employing this scoring system in other studies evaluating the healing of skin wounds.