Influence of water activity on the ice-nucleating activity of Pseudomonas syringae

Pseudomonas syringae is known as a biological ice-nucleating agent. The bacterium has the unusual property of increasing the temperature at which water freezes by a few degrees, However, the ice-nucleating activity (INA) always remains lower for in vitro cultivated cells, than for cells grown in planta. We examined the effects of the hydrophobic environment and of water availability, on the in vitro growth and INA of P. syringae. The hydrophobic environment was modified by addition of fatty acids, vegetable oils or silicone oil to the culture medium. Addition of olive oil (1%), or traces of silicone oil in the culture medium had a positive effect upon the expression of INA. Variations In water activity from 0.990 to 0.988 by addition of sugar beet fibres or sodium chloride in the culture medium were followed by an increase in INA. This study suggested that control of the medium's water activity must be considered as an important parameter for optimization of INA in P, syringae.