Does glycerin promote bacterial growth the same as water?
If I understand correctly if the concentration of water is greater on the outside of a bacterial cell the water will go inside the bacteria allowing it to multiply or grow faster. Does glycerin have the same effect?


Usually the point of adding salt to water is to lower its activity, in order to make the water concentration outside the cell lower than on the inside. This leads to water wanting to escape the cell, causing osmotic pressure to swell the cell.  With strong enough salt, the osmotic pressure causes water to flow out of the cell, so much that the cells dry up and aren't able to function.

Water is miscible with glycerol, which makes glycerol an osmolyte.  That means that glycerol would have the same effect as salt.  Water mixed with glycerol is lower in concentration than pure water.  Putting a happy aqueous bacterium into 100% glycerol will cause immense osmotic shock, probably resulting in death for most bacteria, due to too much of their intracellular water trying to escape to the extracellular, water-free (but glycerol rich) environment.

The downside of using glycerol is that at low concentrations (say 10 g/L or even 100 g/L) it is a great food for many types of microorganisms.  Sure at &gt;1000 g/L it is a deadly osmolyte, but at low concentrations, its a yummy food.  Salt would merely lose its effectiveness as it gets diluted, but glycerol on dilution becomes a potent agent that would enhance microbial growth.  For this reasons, and others (costs, viscosity) it is not often used (outside of laboratory settings)