Capsule and slime layer are two structures that are found in the outside cell wall of many bacteria. Capsule allows bacteria to invade the immune system of the host. Hence, it is common in parasitic bacterial forms. Both capsule and slim layer are composed of a sugar shell called glycocalyx. Glycocalyx layer is considered as an additional layer to the cell wall. The main difference between capsule and slime layer is that capsule is a thick glycocalyx layer that is tightly bound to the cell, defining boundaries of the cell whereas slime layer is a thin glycocalyx layer that is loosely bound to the cell.
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. It consists of a monomolecular layer composed of identical proteins or glycoproteins. 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. 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 with 2–8 nm in diameter
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