Capsules & Slime layers

Capsule

Capsules are the well organized, not easily washed off layer lying outside the bacterial cell. They are tightly bound to the cell wall and they are found in both Gram positive and Gram negative bacteria. They are generally made up of polysaccharide but they may contain other materials like Bacillus anthracis has D-glutamic acid (a polypeptide); Escherichia coli contains peptidoglycan and muramic acid.

A capsulated bacteria is resistant to phagocytosis by host phagocytic cell. They help cells to adhere to solid surfaces and host tissues. Capsules contain water and hence protect bacteria against desiccation. They also protect bacteria against viruses and hydrophobic toxic materials like detergents.

Slime layer

A slime layer is a less compact, unorganized, easily removable film that surrounds the bacterial cell wall. They are usually made up of exopolysaccharides, glycoproteins and glycolipids. They help bacteria to adhere to smooth surface such as catheters. They provide resistance to phagocytosis as well. They act as a protective layer against antibiotics, desiccation and chemicals like chlorine, iodine and many others. Flushing with boiling water or autoclaving are the only means to remove slime layer containing bacteria.

Difference between capsule and slime layer

Characteristics	Capsule	slime layer
Composition	polysaccharide	exopolysaccharide, glycolipids and glycoproteins
Thickness	thick	thin
Binding	tightly bound to the cell wall	loosely bound to the cell wall
Organization	well organized; not easily washed off	unorganized layer; can be easily washed off
Function	mainly evades phagocytosis	mainly helps in adherence