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An S-layer is a part of cell envelope commonly found in gram-positive and gram-negative bacteria, as well as among Archaea. It is composed of protein and glycoprotein. In gram-negative bacteria, the S-layer is directly attached to the outer membrane. In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer (the outer membrane is absent). The S-layer protects the cell against ion and pH changes as well as osmotic stress and foreign enzymes. The S-layer may be useful in helping the cell adhere to surfaces and protecting pathogens from phagocytosis. It also helps to maintain the shape and envelope rigidity of some bacterial cells. It is not a cell membrane
