It seems that bacteria and molds should grow well in jam. It's extremely high in sugar, a substance many cells can use for energy, and contains enough other nutrients to support cell growth. Yet jam can be left unrefrigerated for a very long time without contamination by microorganisms. How can this be?

Jam has a very high sugar content. Although sugar can be used as energy source by microorganisms but too much of sugar can actually harm the microorganism. Microorganisms need a favorable surrounding environment for growth and high sugar content hinders their growth.

When the outside environment has very high sugar content, it becomes hypertonic compared to the internal cellular environment of the microbes. Water moves from low solute to high solute concentration so it moves out of the cell into the jam. Cells shrivel and die by plasmolysis which is when the protoplasm shrivels away from cell wall due to water loss. So, the growth of microbes is hindered.

Bacteria and mold do not grow well in the jam because jam acts as a hypertonic environment for the bacterial cell which causes water loss from the bacterial cell.

Jam has a very high concentration of solute in the form of sugar so when any bacteria or mold attacks on jam then due to its higher concentration of solute outside the cell the bacterial cell loses water from inside to outside which causes shrinkage of cell and ultimately resist the bacteria to grow.

Therefore the high concentration of sugar in jam do not allow bacteria to grow so it can be left unrefrigerated for a long time without contamination by microorganisms.