Metabolism and Function of Bacterial Lipids

Relatively little is known about the intermediary metabolism of bacterial phospholipids. Indeed, as Assilineau and Lederer have pointed out in their authoritative review (l), comparatively few bacterial phospholipids have been well characterized chemically by modern techniques. Interest in the chemistry and metabolism of bacterial lipids is, however, rapidly growing. In the past few years, publications from several laboratories have considerably enlarged our knowledge of the chemical structures of bacterial phospholipids, but detailed information on the metabolism of these substances is still lacking. The occurrence of phospholipids throughout nature suggests a general role of these substances in cell physiology, but it is by no means clear what this role may be. On the one hand, phospholipids may be considered to be purely structural elements of the cell, deriving their importance as essential cell constituents from their unusual physicochemical properties. On the other hand, various dynamic functions have been ascribed to phospholipids, amongst the most attractive of which has been a postulated role as carrier substances in the active transport of cations and other substances across cell membranes. Throughout nature, metabolically active membranes, such as those of mitochondria and the endoplasmic reticulum of animal tissues, and the protoplast membrane of bacteria, are especially rich in phospholipids. In animal tissues, phospholipids characteristically undergo a rapid rate of turnover in tissues such as liver, kidney, and intestinal mucosa, a fact which has suggested to some workers that the function of phospholipids must indeed be a dynamic one. Interpretation of turnover studies in animal tissues, however, is complicated by the presence of a diversity of cell types, and by the death and division of cells which may occur even in the tissues of adult animals. In bacteria, it has been shown that rate of turnover of cell constituents such as protein is a function of the conditions of culture (2). There is virtually no turnover of protein in Escherichia coli during the exponential phase of growth whereas the protein of resting cells undergoes an appreciable turnover. Corresponding studies on the turnover of well defined phospholipids have not to our knowledge been reported. In earlier studies such as those of Mitchell and Moyle (3) on phosphate uptake and turnover in Micrococcus pyogenes, the uptake of labeled phosphate into crude, unfractionated lipid