Re-evaluation of osmotic effects as a general adaptative strategy for bacteria in sub-freezing conditions.

We studied the molecular mechanisms of adaptation of a Siberian psychrophilic bacterial strain. Upon adaptation to 4 degreesC from 24 degreesC, the major changes observed were in the membrane and cell surface chemistry. There was no evidence for the formation of so-called 'compatible metabolites' that are thought to be responsible for the survival at very low to sub-freezing temperatures. The membrane fatty acids were shorter with an increased amount of unsaturation in the 4 degreesC cells compared to the 24 degreesC cells. The absence of a significant amount of phosphorylation in the membrane lipids at 4 degreesC compared to the levels at 24 degreesC was another significant difference. At 4 degreesC, the cell size was reduced in volume by a factor of approximately 14 compared to its size at 24 degreesC. The polar polysaccharide capsular layer was also significantly reduced. There were no significant changes in the protein profiles indicating that antifreeze proteins were not being produced. The results obtained here are consistent with observations and established theories and principles on and about the behavior of water in confined spaces. These indicate that ordering effects, surface charge and polarity are the key determinants of the freezing point and the type of ice structure that will be formed when water is confined to an area of the size of a bacterial cell.