Ans.
Bacterial cells are surrounded by a very large structure, known as capsule or slime layer. It is made up of polysaccharides and protect the bacteria from harsh, external conditions, such as antibiotics, acidic conditions, and desiccation. The capsule also enhances the virulence (disease causing ability) of bacteria. In given diagram, capsule is represented by label (A).
Thus, the labeled structure that protects bacteria and allows it to cause infection in harsh, acidic conditions is (A).