Sparing effect of lithium ion on the specific requirement for sodium ion for growth of Vibrio parahaemolyticus.

The role of NaCl in the growth of Vibrio parahaemolyticus strain A-55 was investigated. Maximal growth was obtained at 0.5 M NaCl in a synthetic medium. Na+ could not be replaced for growth by any other cations or substances. When the medium was kept isotonic with 0.5 M NaCl by the addition of sucrose, good growth was obtained with 0.1 M NaCl. By reducing the osmotic pressure and decreasing the NaCl concentration from 0.5 M to 0.1 M, the same growth yield was obtained as when using a medium containing 0.02 M NaCl and 0.08 M LiCl. This was not the case with sucrose. Therefore, it is concluded that ionic strength and osmotic pressure are important environmental factors affecting growth. The minimal essential requirement for Na+ for growth of V. parahaemolyticus was 0.003 M, because this was never replaced with any other cations and agents. Hence, requirement for Na+ for growth involves a specific requirement for Na+, ionic strength, and osmotic pressure, respective concentrations being 0.003 M, 0.047 M, and 0.45 M. Osmotic support was required for growth when the concentration of NaCl was decreased to 0.05 M. The effect of ionic strength of monovalent cations other than Na+ on growth was examined at 0.003 M NaCl. Among LiCl, NH4Cl, KCl, and RbCl, Li+ was the most accelerative for growth in the synthetic medium.