Phospholipid and Lipopolysaccharide in Proteus mirabilis and Its Stable Protoplast l‐Form

Abstract

Cells of the stable protoplast L-form of P. mirabilis contain 1.5-2 times more extractable lipid, mostly phospholidid, per dry weight than cells of the bacterial form. Under identical cultivation conditions the qualitative and quantitative composition of the phospholipid is very similar in both cell forms. The range of individual phospholipid species is 76-80 for phosphatidylethanolamine, 10-13 for phosphatidylglycerol, 3.9-5.5 for diphosphatidylglycerol and 1.0-2.1 for lysophospholipid. All phospholipid species in the L-form differ from those of the bacterial form by a lower content of long-chain fatty acids and a higher content of short-chain fatty acids. Growth of the L-form in the presence of growth-stimulating horse serum results in a change of phospholipid composition accompanied by the uptake of phospholipid and fatty acids from the serum into L-form phospholipid. L-form protoplasts synthesize the same 2 types of lipopolysaccharide, I and II, that were previously identified in the bacterial form of P. mirabilis. Only small amounts of the more hydrophilic lipopolysaccharide II are present in the L-form. Lipopolysaccharides from both cell forms have virtually identical polysaccharide compositions but differ strikingly in the relative content of fatty acids in their lipid-A moieties. Molar ratios of tetradecanoic acids, hexadecanoic acid and 3-hydroxytetradecanoic acid are 5:1:6 in the bacterial form and 5:0.1:6 in the L-form grown in the serum-free medium. The observed differences between the bacterial form and the protoplast L-form indicate the adaptation of the L-form to life in the state lacking an envelope by formation of a physically more stable but sufficiently fluid protoplast membrane. A rapid method based on fatty acid analysis for the simultaneous quantitative determination of phospholipid and lipopolysaccharide content of whole cells is reported.