Background: Streptococcus pyogenes and Staphylococcus aureus are often simultaneously detected from many cases of non-bullous impetigo with atopic dermatitis. Objectives: Using confocal laser scanning microscopy (CLSM), to investigate formation of S. pyogenes microcolonies in skin lesions. Methods: The S. pyogenes cells in the stationary growth phase alone were strongly stained with fluorescein isothiocyanate-concanavalin A (FITC-ConA), and this staining was reduced by pretreatment with amylase. Although the components of sugars in glycocalyx produced by S. pyogenes cells are unknown, we suggested that the materials stained by FITC-ConA were consistent with the presence of ConA-reactive sugars in glycocalyx produced by S. pyogenes cells. Results: S. pyogenes cells associated with streptococcal impetigo skin and croton-oil inflamed mouse skin formed microcolonies encircled by materials (glycocalyx) that stained strongly with FITC-ConA, and these findings were consistent with those in biofilms. In croton-oil inflamed mouse skin, polymorphonuclear leukocytes (PMNs) infiltrated to just below the epidermis in the cefdinir-treated group but only to the middle dermis in the cefdinir-non-treated group. In this case S. pyogenes and S. aureus cells formed separate microcolonies and existed independently in the outer walls of pustule lesions of streptococcal impetigo. Conclusion: In skin infections, S. pyogenes and S. aureus formed aggregates of microcolonies (similar to that in biofilms) encircled by glycocalyx, which can make the infection hard to eradicate using an antimicrobial agent alone. The effect of conventional antimicrobial agents against biofilm is mainly due to the increase of the invasion of PMNs into the biofilm. (C) 2003 Japanese Society for Investigative Dermatology. Published by Elsevier Science Ireland Ltd. All rights reserved.