The skin barrier, the other major interface with the microbial world, commensalism and pathogenicity

The fourth lecture, entitled "The skin barrier, the other great interface with the microbial world, commensalism and pathogenicity", dealt with a relatively neglected topic in the field of infection pathogenicity. The lecture focused on the cells and highly sophisticated integrative processes that ensure the skin's barrier effect, which combines several protective requirements in a single entity: against physical and chemical aggressions and UV rays, against dehydration, and of course against infectious agents. The subtle interplay between keratinocytes and Langerhans cells, and the transition between epidermis and stratum corneum, which is accompanied by the secretion of powerful anti-infectious agents (antimicrobial peptides, lipids), were examined, as were the nature of the commensal flora, its composition and its possible role in the homeostasis of the whole "system". on this basis, the major mechanisms of subversion of this powerful barrier were considered, with particular emphasis on the model represented by Streptococcus pyogenes infection, which can be responsible for dramatic cellulitis. The central role of stem cells in homeostasis and repair processes was also considered.