MODULATION OF MICROBIAL MEMBRANE LIPIDS IN RESPONSE TO VARYING SALT CONCENTRATIONS

Abstract

Cellular growth is generally inhibited by extreme environmental salinity, as well as other changes such as pH, temperature and other conditions in the surrounding habitat of the microorganism. Therefore, to thrive well in their habitat, an intelligent feedback system between the exterior membrane system and interior cytosolic matter will be necessary to achieve growth and maintain proper cellular functioning. This work was aimed at investigating the effect of sodium chloride (NaCl) concentrations on the growth pattern and membrane lipids of gram negative Escherichia coli and gram positive Staphylococcus aureus cells cultivated at 37 °C at three salt concentration; 0, 1.0 and 3.0 % (w/v). Standard culture of bacteria grown was monitored by measuring turbidity via spectrophotometer at hourly intervals. Changes in membrane structural make up; firstly, the lipid profiles were carried out by thin layer chromatography using solvent system, chloroform: methanol: diisobutyl ketone: glacial acetic acid: water (45:15:30:20:4 v/v/v/v/v), for polar lipids and diethyl ether: acetic acid (80:20:1 v/v/v), solvent system for neutral lipids. Secondly, the fatty acid constituents were resolved on gas chromatography/ mass spectrometry (gc/ms). Shorter lag phase was observed for S. aureus when compared to E. coli at all levels of salt stress. Also, the optimum growth performance was observed to be in a decreasing order for both E. coli (0.556, 0.534 and 0.486) and S. aureus (0.583, 0.553 and 0.495) at salt levels 0 %, 1.0 % and 3.0 % respectively. Polar lipids profile identified through thin layer chromatography were: Cardiolipin (CL), Phosphatidic acid (PA), Phosphatidylglycerol (PG), Phosphatidylethanolamine (PE) and Lysophosphatidylglycerol (LPG), while that of neutral lipids include: steryl esters (SE), triacylglycerols (TAG), diacylglycerols (DAG) or sterols and monoacylglycerols (MAG) for both organisms at the varying salt condition of growth, with an exception of S. aureus at 3% NaCl inclusion which had no triacylglycerol observed. However, fatty acids resolved by gc/ms varied amongst the experimental parameters considered. The fatty acids observed for E. coli were; C15:0, C16:0, C16:2 and C18:1 for salt stressed condition (3 % NaCl inclusion) while C16:2 was absent in control condition (0 % NaCl inclusion). Fatty acids observed for S. aureus were; C12:0, C13:0, C14:0, C15:0, C16:0, C16:1 and C18:1 for salt stressed condition while C12:0, C15:0 and C16:1 were absent in control condition. However, these fatty acids observed were present in different relative abundance. Therefore result obtained from this study showed that salt stress condition inhibited growth of both bacteria and had no major effect on the lipid profile of both microorganisms, while there was variation in the fatty acids content of the lipids been resolved.