What is the protective outer layer of some bacteria called?

The protective outer layer of some bacteria is called a capsule. A capsule is a well-organized coating outside the bacterial cell envelope, usually made of polysaccharides. Some capsules instead contain materials such as poly-D-glutamic acid, as in Bacillus anthracis.

Capsules help bacteria retain water and resist drying, attach to surfaces, and avoid being engulfed by immune cells such as macrophages. For this reason, capsule formation can increase a bacterium’s virulence. Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis are important examples of encapsulated pathogens.

A capsule is not the same as a cell wall, pilus, or flagellum. The cell wall provides structural strength, pili commonly assist attachment or DNA transfer, and flagella enable movement. A loose, diffuse coating is usually called a slime layer; capsule and slime layer are often grouped under the broader term glycocalyx.

Because capsules are difficult for ordinary stains to penetrate, they may appear as clear halos around cells in India ink preparations. Capsular polysaccharides are also used in several conjugate vaccines.