An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria.¹²³ The S-layers of both archaea and bacteria consists of a monomolecular layer composed of only one (or, in a few cases, two) identical proteins or glycoproteins, the latter case making it a type of glycocalyx.²³⁴ This structure is built via self-assembly and encloses the whole cell surface. Thus, the S-layer protein can represent up to 15% of the whole protein content of a cell.⁵ S-layer proteins are poorly conserved or not conserved at all, and can differ markedly even between related species. Depending on species, the S-layers have a thickness between 5 and 25 nm and possess identical pores 2–8 nm in diameter.²³⁶⁷

The terminology "S-layer" was used the first time in 1976. The general use was accepted at the "First International Workshop on Crystalline Bacterial Cell Surface Layers, Vienna (Austria)" in 1984, and in the year 1987 S-layers were defined at the Workshop on "Crystalline Bacterial Cell Surface Layers", Vienna as "Two-dimensional arrays of proteinaceous subunits forming surface layers on prokaryotic cells" (see "Preface", page VI in Sleytr "et al. 1988"). For a brief summary on the history of S-layer research see "References" A comprehensive historical account of the development of fundamental and applied S-layer research is given in the following current review.