Answer & Explanation:
Bacteria are really complex and developed organisms. As for flagella, this appendage allows them to move and some other features. This particular structure has an extremely structured anchorage in different parts of the cell's wall. It's assembled in the plasma membrane and later in the external membrane, which contains a thick coat of peptidoglycan. This molecule is extremely flexible but also strong, due to its numerous connections between the monomers. Bacteria don't have tissues so their protection restricted to their membrane. Therefore, this membrane must have enough strength and complexity so it can be protective and utterly functional.