Which layer surrounds the cells of certain bacteria for additional protection?

The correct answer is the capsule, which is a thick, gelatinous layer that surrounds the cell walls of certain bacteria. The capsule serves several important functions, providing an additional layer of protection against environmental threats, such as dehydration and phagocytosis by immune cells. Additionally, it can play a role in the bacterium's ability to adhere to surfaces and form biofilms, which can contribute to its virulence and survival in various environments. This structure makes it particularly advantageous for pathogenic bacteria, enhancing their ability to evade the host's immune system.

In contrast, the peptidoglycan layer is a crucial component of the bacterial cell wall that offers structural support and shape but does not provide the same level of external protective function as the capsule. The cell membrane is a fundamental barrier that regulates the entry and exit of substances but is not an external protective layer. The cell wall, while providing structural integrity, does not offer the additional protective benefits that the capsule does in specific bacteria.