Outer Layers of the Cell Envelope

Many prokaryotes are covered with sticky and slimy materials outside their cell wall, which exists in two forms- capsules and slime layers.

Capsules are highly organized layers of polysaccharides that are firmly attached to the cell wall.

In contrast, less organized layers of polysaccharides form the slime layer. A bacterial species can produce both the layers, but they are typically not found on the same cell at the same time.

Capsules protect cells from dehydration and prevent phagocytosis, enhancing virulence. For example, Bacillus anthracis uses its capsule to evade the host’s immune defenses.

The outer layers also aid cell attachment to diverse surfaces and biofilm formation, as seen in Streptococcus mutans, which can adhere to teeth and cause dental caries.

Most archaea and many bacteria have highly organized crystalline S-layers made of proteins or glycoproteins.

In archaea, the S-layer is the primary protective structure, providing structural support and protection against osmotic lysis.

In bacteria, the S layer helps adhesion to host cells and protects against environmental stress.