The effect of salinity on growth and lipid composition of a moderately halophilic Gram-negative bacterium HX

The effect of NaCl concentration on the growth rate and phospholipid composition of a moderately halophilic Gram-negative rod-shaped bacterium has been studied. This bacterium, designated HX, grows between 0.5 and 4.25 M NaCl at 20, 30, and 37 °C. It has a broad optimum salt concentration for growth of 1.0–2.0 M NaCl at 20 and 30 °C and 1.5–2.8 M NaCl at 37 °C. Its major lipids are phosphatidylethanolamine and phosphatidylglycerol, which constitutes >90% of the total lipid, together with smaller amounts of diphosphatidylglycerol; there are traces of an unidentified glycolipid, lysophosphatidylethanolamine, and phosphatidylserine. Lipid composition depends on NaCl concentration of the culture medium: at higher salinities the proportion of anionic phospholipids, mainly phosphatidylglycerol, is greater relative to the proportion of zwitterionic phospholipids, mainly phosphatidylethanolamine. The major fatty acids are 16:0, 16:1, 18:0, 18:1, 17:0cyc, and 19:0cyc; the proportions of the cyclopropane fatty acids are greater in cultures grown at higher salinities and this increase is due to NaCl concentration, not growth rate. Following a sudden increase in NaCl concentration of the culture medium (shift up), there are concomitant changes in growth rate and phospholipid composition. A lag period in growth is observed only when the magnitude of the salinity shift up is at least threefold. After a salinity shift up, there is a rapid decrease in the ratio of phosphatidylethanolamine to phosphatidylglycerol. It appears that the alterations in growth rate and lipid composition are a physiological response for adaptation to a change in salinity.Key words: halophilic bacteria, haloadaptation, membranes, lipids, salinity.