2.3.2 Capsules and slime layers Many prokaryotic cells are covered with polysaccharides. In some cases the polymers are tightly integrated with the cell while in others they are loosely associated. The former is called a capsule, and the latter a slime layer (Figure 2.2). Slime layer materials can diffuse into the medium with their structure and composition being dependent on growth conditions. An impoitant role for these structures is adhesion to host cells for invasion or to a solid surface to initiate and stabilize biofilm formation. These structures are also responsible for resistance to phagocytosis, thereby increasing virulence. In some bacteria the capsule functions as a receptor for phage. Since the polysaccharides are hydrophilic, they can also protect cells from desiccation. The term glycocalyx can be used to describe extracellular structures including the capsule and S-layer, the latter being described below. 2.3.3 S-layer, outer membrane and cell wall Unicellular prokaryotes have elaborate surface structures These include the S-layer, outer membrane and cell wall. The cell wall