What protects most bacteria from swelling up and bursting in hypotonic solutions?

Exoskeleton (cell wall) which provides the bacteria with several benefits. This cell wall protects the bacterium from damage by encircling it with a tough, rigid structure. The primary function of the cell wall is to maintain the cell shape and prevent bursting from osmotic pressure, so this bursting process only occurs in animal and protozoa cells which do not have cell walls.