The role of osmotic effects in haloadaptation of Vibrio costicola

Growth rates of Vibrio costicola showed a broad optimum between 0.8 and 1.5 M-NaCl, and there was no growth above 3.3 M-NaCl in a peptone-based medium. The minimum requirement of 0.5 M-NaCl for growth in NaCl alone was reduced to 0.3 M-NaCl when the total solute concentration was raised to 0.5 to 1.0 M equivalent with sucrose or glycerol. Compared with equivalent NaCl concentrations, higher concentrations of sucrose were more inhibitory to growth, whereas glycerol had less effect. Increasing the medium NaCl concentration suddenly by 2- or 3-fold with either a constant starting, or final, salt concentration showed that, after the shift-up, the lag in growth, the rate of growth, and the inhibition of phospholipid synthesis depended both on the final NaCl concentration and the magnitude of the shift in salinity. The time-courses of phospholipid synthesis following a 2- or 3-fold shift-up in NaCl or sucrose media were very similar and exhibited a relative increase in phosphatidylglycerol synthesis over that of phosphatidylethanolamine. This 'switch-over' was not seen following shift-up in glycerol media when there was also a stimulation, rather than inhibition, of phospholipid synthesis. It is concluded that during phenotypic haloadaptation of V. costicola, osmotic effects play a significant part in the sensing of and response to raised external salinity.