Why does Salt work as a preservative?

Bacteria have a cell membrane and a cell wall. Bacteria must live in an aqueous (watery) environment. Most often this is a hypo-tonic environment, in other words, the concentration of water outside the cell is greater than the concentration of water inside the cell. 

This causes the net movement of more water into the

cell than outside. If the bacterium did not have a cell wall, this could cause the cell to burst. (In fact, many antibiotics work by causing an ineffective cell wall to be made, which allows the bacterial cell to burst underwater pressure). 

So why does salt work as a preservative?

Because when the outside environment around a cell is salty, then the concentration of water in the solution is less than inside the cell and water tends to leave the cell. This causes the cell to dehydrate, which eventually kills the cell. By subjecting bacteria to a salty environment, it keeps them from growing. Some bacteria, however, have adapted to living in salty environments, such as Staph bacteria a common skin inhabitant. Your skin tends to be salty-this is one way your body protects you against bacteria on your skin.